Coordination Compounds

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Coordination CompoundsCoordination Compounds

Backgrounds………Backgrounds………

Why atom combines to form chemical bonds?Why atom combines to form chemical bonds?

Octet Rule, Pauli Exclusion Principle, Hund’s rule and Aufbau PrincipleOctet Rule, Pauli Exclusion Principle, Hund’s rule and Aufbau Principle

ReasonReason: minimum energy and maximum stability/symmetry.: minimum energy and maximum stability/symmetry.

Why, it essential to learn Coordination Compounds?Why, it essential to learn Coordination Compounds?

Biological systemsBiological systems- e.g. Haemoglobin (Fe-porphyrin, - e.g. Haemoglobin (Fe-porphyrin, redred), ),

Chlorophyll (Mg- porphyrin, Chlorophyll (Mg- porphyrin, greengreen), Vitamins B), Vitamins B1212 (Co-complex), Cytochromes and (Co-complex), Cytochromes and

Oxydase enzymes (Fe-Cu-complex), metalloenzymes, DNA…etc. Oxydase enzymes (Fe-Cu-complex), metalloenzymes, DNA…etc.

Alternatively, adrenaline, citric acid and cortisone complex with metals (e.g.Alternatively, adrenaline, citric acid and cortisone complex with metals (e.g.

Pb,Cu, Fe, CrPb,Cu, Fe, Cr), which gave ), which gave metal poisoningmetal poisoning and EDTA-M complexes used in and EDTA-M complexes used in

treating metal poisoningtreating metal poisoning..

[Cu(NH[Cu(NH33))44]]2+2+ ionion inhibits the growth of fungi and bacteria. inhibits the growth of fungi and bacteria.

[RhI[RhI22(CO)(CO)22]]-- ion ion is used as a catalyst in the " is used as a catalyst in the "Monsanto ProcessMonsanto Process" for making" for making

acetic acid, the active ingredient in vinegar.acetic acid, the active ingredient in vinegar.

Na-EDTA complexesNa-EDTA complexes: Soap, beer , mayonnaise. : Soap, beer , mayonnaise.

EDTAEDTA4-4- is used to "trap" trace amounts of transition metals that could is used to "trap" trace amounts of transition metals that could

potentially catalyze the decomposition of the product. potentially catalyze the decomposition of the product.

Colors on a computer Screen, DVD, CD, camera, electronic goods, etc. Colors on a computer Screen, DVD, CD, camera, electronic goods, etc.

Preparation:Preparation: (A) Simple salts (A) Simple salts:: NaOH + HCl----------> NaCl + HNaOH + HCl----------> NaCl + H22O O

(B) Molecular or Addition Compound(B) Molecular or Addition Compound:: Stoichiometric amounts of two or moreStoichiometric amounts of two or more

stable compounds join together. E.g. Fe(CN)stable compounds join together. E.g. Fe(CN)22.4KCN (Pot. ferrocyanide), .4KCN (Pot. ferrocyanide),

and FeSOand FeSO44.(NH.(NH44))22SOSO44.6H.6H22O (Mohr’s salt), etc. O (Mohr’s salt), etc.

Types of AddTypes of Addnn. Compounds. Compounds: (i) : (i) Double salts or Lattice compounds:Double salts or Lattice compounds:

(ii)(ii) Coordination or Complex compounds Coordination or Complex compounds: : M-atom / ion + ligands (atoms, ions, M-atom / ion + ligands (atoms, ions,

or molecules) bonded Coordination bond.or molecules) bonded Coordination bond. Explanation is complexExplanation is complex, because , because

each coordination compound has slightly variable chemical, structural and each coordination compound has slightly variable chemical, structural and

physical behavior depending on M-ion and ligands. physical behavior depending on M-ion and ligands.

Ligands (complexing agents):Ligands (complexing agents): Lewis bases - Lewis bases - donor atomdonor atom. .

Ligands may be +ve (NOLigands may be +ve (NO++), -ve (X), -ve (X--) or neutral (NH) or neutral (NH33). In the case of mixed ligands, ). In the case of mixed ligands,

complex ions gave isomeric structure and geometric shapes. complex ions gave isomeric structure and geometric shapes. Bidentate ligands Bidentate ligands

gave optically active isomers. gave optically active isomers.

(i) (i) Monodentate Ligands:Monodentate Ligands:

(ii) (ii) Bidentate ligandsBidentate ligands: :

(iii) (iii) AmbidentateAmbidentate: more than one donor atoms in the same molecule.: more than one donor atoms in the same molecule.

(iv) (iv) Polydentate LigandsPolydentate Ligands: Having more than two donating sites.: Having more than two donating sites.

Some Monodentate LigandsSome Monodentate Ligands

ligandligand namename ligandligand namename

FF-- fluoride ionfluoride ion ClCl-- chloride ionchloride ion

BrBr-- bromide ionbromide ion II-- iodide ioniodide ion

HH22OO waterwater NHNH33 ammoniaammonia

OHOH-- hydroxide hydroxide ionion COCO carbon carbon

monoxidemonoxide

CNCN-- cyanide ioncyanide ion SCNSCN-- thiocyanate ionthiocyanate ion

Methods of Studying Coordinate compoundsMethods of Studying Coordinate compounds

1.1. Electrical conductivityElectrical conductivity: depends on concentrations and no. of charges on it. : depends on concentrations and no. of charges on it.

2.2. Cryoscopic measurementCryoscopic measurement: freezing point changes of a liquid.: freezing point changes of a liquid.

3. 3. Magnetic moment/propertiesMagnetic moment/properties: gave no. of unpaired es.: gave no. of unpaired es.

4. 4. Dipole momentDipole moment: structural information for non-ionic complexes.: structural information for non-ionic complexes.

5. 5. Electronic Spectra (UV-Vis):Electronic Spectra (UV-Vis): for energy of orbitals and shape of complex. for energy of orbitals and shape of complex.

6. 6. X-ray studyX-ray study::

Structure of Coordination CompoundsStructure of Coordination Compounds

The arrangement of ligands determine their structure, physical and The arrangement of ligands determine their structure, physical and

chemical propertieschemical properties

e.g. [CoCle.g. [CoCl44]]2- 2- structure might be:structure might be:

a. Sq. planar-ligands present at the corner of a squarea. Sq. planar-ligands present at the corner of a square

b. Tb. Thh-ligands present at the corner of T-ligands present at the corner of Th h

c. Something else? Experimentally found Tc. Something else? Experimentally found Th h

1. 1. Werner’s Coordination Theory (1893)Werner’s Coordination Theory (1893)

Alfred WernerAlfred Werner (1866-1919) (1866-1919)

1893, age 26: Coordination theory1893, age 26: Coordination theory

Nobel prize for Chemistry, 1913Nobel prize for Chemistry, 1913

Addition of 6 mol NHAddition of 6 mol NH33 to CoCl to CoCl33(aq)(aq)

Conductivity studiesConductivity studies

Precipitation with AgNOPrecipitation with AgNO33

Compound Moles of ions Moles of AgCl(s)

“CoCl3.6NH3”

“CoCl3.5NH3”

“CoCl3.4NH3”

“CoCl3.3NH3”

4 3

3

2

0

2

1

0

Co

NH3

NH3

NH3

Cl

NH3 NH3 NH3 Cl

Cl

Cl– attached to NH3 may be dissociated

Proposed six ammonia molecules to covalently bond to CoProposed six ammonia molecules to covalently bond to Co3+3+

Compound Moles of ions Moles of AgCl(s)

[Co(NH3)6]Cl3

[Co(NH3)5Cl]Cl2

[Co(NH3)4Cl2]Cl

[Co(NH3)3Cl3]

4 3

3

2

0

2

1

0

NH3

Co

NH3

H3N NH3

NH3H3N

3+

3Cl–

H

N

HH

M

ligand

N forms a coordinate covalent bond to the metal

(coordination sphere)

(counter-ion)

Coordination compounds structure:Coordination compounds structure:

(i)(i) 11o o ValencyValency: Ionizable bonds, (ii) : Ionizable bonds, (ii) 22oo Valency Valency: Non-Ionizable bonds e.g.: Non-Ionizable bonds e.g.

[Co(NH[Co(NH33))66]Cl]Cl3 3

Generally, CN varies from 1-12. However, 2, 4 and 6 are the most common. Generally, CN varies from 1-12. However, 2, 4 and 6 are the most common.

CN=2 linear structureCN=2 linear structure

CN=4 Sq. planar or TCN=4 Sq. planar or Thh structure structure

CN=6 Octahedral structureCN=6 Octahedral structure

Why Transition metals form Coordination Complexes?Why Transition metals form Coordination Complexes?

Sidgwick (EAN rule)-Sidgwick (EAN rule)- Because TM has vacant d-orbitals, which can Because TM has vacant d-orbitals, which can

accommodate electron pairs to gain stability like next noble gas configuration.accommodate electron pairs to gain stability like next noble gas configuration.

e.g.,Ke.g.,K44[Fe(CN)[Fe(CN)66], EAN=36(Kr); ], EAN=36(Kr);

[Cu(CN)[Cu(CN)44]]-3-3, EAN=36(Kr); , EAN=36(Kr);

[Ni(CO)[Ni(CO)44], EAN=36(Kr);], EAN=36(Kr);

[PtCl[PtCl66]]-2-2, EAN=86(Rn)., EAN=86(Rn).

ExceptionsExceptions: [Fe(CN): [Fe(CN)66]]-3-3, EAN=35; [Cr(NH, EAN=35; [Cr(NH33))66]]+3+3, EAN=33, EAN=33

Role of d-orbitals in the Complex formation:Role of d-orbitals in the Complex formation:

d-orbitals shape and degeneracy: d-orbitals shape and degeneracy: In case of In case of isolatedisolated gaseousgaseous and and free free

metal ionmetal ion all the five d-orbitals are degenerate. all the five d-orbitals are degenerate. These orbitals are These orbitals are

oriented in space as shown below:oriented in space as shown below:

Shape of d-OrbitalsShape of d-Orbitals

Bonding in TM ComplexesBonding in TM Complexes

Theories for Metal to Ligand bonding in complexes: Theories for Metal to Ligand bonding in complexes:

Valence bond Theory (L. Pauling, 1930)Valence bond Theory (L. Pauling, 1930)

A complex involves reaction between Lewis bases (Ls) and a Lewis acid (M or M-A complex involves reaction between Lewis bases (Ls) and a Lewis acid (M or M-

ion) through coordinate covalent or dative bond. ion) through coordinate covalent or dative bond.

Assumptions:Assumptions:

1. Central metal atom or ion have a number of empty s, p, and d orbitals. On 1. Central metal atom or ion have a number of empty s, p, and d orbitals. On

hybridization, gave hybrid orbitals. hybridization, gave hybrid orbitals. These are vacant, equivalent in energy These are vacant, equivalent in energy

and have definite geometryand have definite geometry..

2. The ligands have at least one 2. The ligands have at least one σσ-orbital containing a lone pair of electrons.-orbital containing a lone pair of electrons.

3. Hybrid orbitals of the metal atom or ion overlap with the filled 3. Hybrid orbitals of the metal atom or ion overlap with the filled σσ-orbitals of -orbitals of

the ligands to form ligand→metal the ligands to form ligand→metal σσ-bond. -bond.

This coordinate bond is a special type of covalent bond shows the This coordinate bond is a special type of covalent bond shows the

characteristics of both the overlapping orbitals and Polar in nature due to characteristics of both the overlapping orbitals and Polar in nature due to

donation.donation. Pauling Pauling measured magnetic moment to find out the number of measured magnetic moment to find out the number of

unpaired electrons in a complex and the geometries of the complex ions unpaired electrons in a complex and the geometries of the complex ions

having the central metal ion with configurations dhaving the central metal ion with configurations d11 to d to d99..

Metal or metal ionMetal or metal ion: Lewis acid: Lewis acid

LigandLigand: Lewis base: Lewis base

Hybridization of Hybridization of ss, , pp, , dd orbitals results: orbitals results:

C.N.C.N. GeometryGeometry

44 tetrahedraltetrahedral

55

66

44

HybridsHybrids

spsp33

square planarsquare planar dspdsp22

trigonal bipyramidaltrigonal bipyramidal dspdsp33 or or spsp33dd

octahedraloctahedral dd22spsp33 or or spsp33dd22

Example 1Example 1: [CoF: [CoF66]]33––

Co [Ar] 3Co [Ar] 3d d 77 4 4ss22

CoCo3+3+ [Ar] 3 [Ar] 3d d 6 6

complex is paramagneticcomplex is paramagnetic

3d 4s 4p 4d

4sp3d2

octahedraloctahedral

Example 2: [Co(NH3)6]3+

Co [Ar] 3d7 4s2

Co3+ [Ar] 3d6

3d 4s 4p

complex is diamagneticcomplex is diamagnetic

4d

d2sp3

octahedraloctahedral

Limitations of VBT:Limitations of VBT:

1. (a) 1. (a) Oh Oh (d2sp3 or sp3d2), (d2sp3 or sp3d2), tetrahedraltetrahedral (sp3) and (sp3) and square planarsquare planar (dsp2) (dsp2)

complexes of complexes of d1(1 unpaired electronsd1(1 unpaired electrons for Oh, Th or Sq. planar), d2 (2 for Oh, Th or Sq. planar), d2 (2

unpaired electronsunpaired electrons for Oh, Th or Sq. planar), d3 (3 unpaired electronsfor Oh, Th or Sq. planar), d3 (3 unpaired electrons for for

Oh, Th or Sq. planar) and d9 ionOh, Th or Sq. planar) and d9 ion same as d1same as d1 and hence cannot be and hence cannot be

distinguished from each other merely on the basis of the number of distinguished from each other merely on the basis of the number of

unpaired electrons (b) unpaired electrons (b) Outer-orbital Oh and Th complexesOuter-orbital Oh and Th complexes of all the ions of all the ions

viz.viz. d1 - d9d1 - d9 which have the same number of unpaired electrons cannot be which have the same number of unpaired electrons cannot be

distinguished from each other.distinguished from each other.

2. 2. Color and magnetic moments of complexes are due to d-orbital electrons.Color and magnetic moments of complexes are due to d-orbital electrons.

There must be a quantitative connection between spectra and magnetic There must be a quantitative connection between spectra and magnetic

moment. This is not revealed in VBT and moment. This is not revealed in VBT and consequently magnetic and consequently magnetic and

spectral properties could not be explained by this theoryspectral properties could not be explained by this theory..

3. VBT does not explain the behavior of complexes having 3. VBT does not explain the behavior of complexes having d8 iond8 ion (e.g. Ni+2, (e.g. Ni+2,

Pb+2, Au+3, etc.) Pb+2, Au+3, etc.) in forming 5-coordinated complexesin forming 5-coordinated complexes. Also, VBT. Also, VBT prefers prefers

only square planar geometry of complexes not Th or trigonal bipyramidal.only square planar geometry of complexes not Th or trigonal bipyramidal.

4. The metal ion has much importance while ligand is not properly stressed.4. The metal ion has much importance while ligand is not properly stressed.

5. VBT cannot explain reaction rates and mechanism of reactions.5. VBT cannot explain reaction rates and mechanism of reactions.

An artist’s paint pigment shop in Venice – An artist’s paint pigment shop in Venice – been around since the 1600’s ... been around since the 1600’s ...

A ruby and an emerald (No you can’t A ruby and an emerald (No you can’t have them ..).have them ..).

Why TM compounds (mainly oxides and sulfides) and complexes have Why TM compounds (mainly oxides and sulfides) and complexes have

characteristic colors – e.g. Cr3+ is green, Co3+ characteristic colors – e.g. Cr3+ is green, Co3+ blueblue, CdS is , CdS is yellowyellow, HgS , HgS orange orange

redred, etc...? , etc...?

What is the connection between color and electronic structure ? What is the connection between color and electronic structure ?

2. 2. Crystal Field Theory (CFT) and their Historical Crystal Field Theory (CFT) and their Historical

Background Background (H. Bethe, L. Orgel and V. Bleck, 1935)(H. Bethe, L. Orgel and V. Bleck, 1935)

Mainly applied for Mainly applied for ionic crystals. ionic crystals. TThe bonding between thehe bonding between the metal metal

atom/atom/ion and the ligandsion and the ligands ( (–vely point charge or point charge for –vely point charge or point charge for

anionic Ls and point dipole/dipole for neutral moleculesanionic Ls and point dipole/dipole for neutral molecules). ). This explainThis explain

the d-orbitals splitting into groups as a result of electrostatic the d-orbitals splitting into groups as a result of electrostatic

interactionsinteractions. . CFT is very useful to understand, interpret and predict CFT is very useful to understand, interpret and predict

the the magnetic behaviormagnetic behavior, , colorscolors and and some structuressome structures of coordination of coordination

complexes.complexes.

Bethe Bethe et al.et al. investigated, how the strength of a crystal field affect the investigated, how the strength of a crystal field affect the

electronic levels of the gaseous metal electronic levels of the gaseous metal atoms/atoms/ions.ions.

First timeFirst time, C.F. theory was developed by considering two compounds: , C.F. theory was developed by considering two compounds:

MnMnIIIIO, and CuO, and CuIICl.Cl.

How does How does wewe describe and characterize the bonding between describe and characterize the bonding between M M ion ion

and ligands in terms of and ligands in terms of thisthis electronic theory? electronic theory?

Crystal field around metal s-, p- & d-orbitals and splitting of Crystal field around metal s-, p- & d-orbitals and splitting of

d-orbitals in Od-orbitals in Ohh::

Basic concept: Basic concept: purely electrostatic interaction between Mpurely electrostatic interaction between M+n +n – Ls – Ls

((bonding attractionbonding attraction) and d-es repel Ls electrons, but unequal that ) and d-es repel Ls electrons, but unequal that

causes splitting of d-orbitals.causes splitting of d-orbitals.

i. Spherical field:i. Spherical field: s-, p- and s-, p- and d-d-orbitals degenerate (orbitals degenerate (remain unchanged)remain unchanged)..

ii. Unsymmetrical field: ii. Unsymmetrical field: What will happen?What will happen?

Three interactions are possible between the Three interactions are possible between the ligand fieldligand field with the with the metallic metallic

atom: atom: (i) Ligand field with (i) Ligand field with s s orbital (ii) Ligand field with orbital (ii) Ligand field with p p orbital (iii) orbital (iii)

Ligand field with Ligand field with d d orbital orbital

CFT emphasizes the CFT emphasizes the electrostatic attractionelectrostatic attraction of the chemical bond in a of the chemical bond in a

complex of the complex of the metallic ion (0/+ve ox. state)metallic ion (0/+ve ox. state) with the with the electrons (-ve ox. electrons (-ve ox.

state)state) coming from the ligands. coming from the ligands.

Bonding in, and Electronic Structure of, Transition Metal ComplexesBonding in, and Electronic Structure of, Transition Metal Complexes

tt2g2g-orbitals (stabilize) e-orbitals (stabilize) egg-orbitals (destabilize)-orbitals (destabilize)

The energy difference between eThe energy difference between egg and t and t2g2g-orbitals in the crystal field is -orbitals in the crystal field is

known as known as crystal field splitting (crystal field splitting (CFS)CFS), 10Dq or Δ, 10Dq or Δoo. . At hypothetical At hypothetical

degenerate d-orbitals, no splitting state assumed called degenerate d-orbitals, no splitting state assumed called BarycenterBarycenter, ,

CFSECFSE =0. From =0. From conservation of energy statesconservation of energy states t t2g2g orbitals lie at -0.4Δ orbitals lie at -0.4Δoo and and

the ethe egg orbitals lie at +0.6Δ orbitals lie at +0.6Δoo. .

((CFSEoCFSEohh= -0.4nt= -0.4nt2g2g + 0.6ne + 0.6negg), (), (n= no. of electronsn= no. of electrons))

Pairing energy? Pairing energy? CFSEoCFSEohh= -0.4nt= -0.4nt2g2g + 0.6ne + 0.6negg + mP ( + mP (m= no. of paired esm= no. of paired es))

Crystal field splitting diagramsCrystal field splitting diagrams

(i) (i) Octahedral complexOctahedral complex: :

eg

t2g

crystal field stabilization crystal field stabilization energy (CFSE)energy (CFSE)

lamda = lamda = hc/hc/ΔΔoo

Summary:Summary:

CFSE in OCFSE in Ohh complexes: complexes:

The stability depends upon Δo and spin pairing energy (P)The stability depends upon Δo and spin pairing energy (P)..

Importance of d-orbitals splitting

In In octahedral complexoctahedral complex, for example , for example dd1 complex1 complex: electrons place themselves at : electrons place themselves at

tt2g2g orbital which is at the energy level Δ orbital which is at the energy level Δoo less than the energy level of less than the energy level of d d orbital orbital

which does not undergo splitting.which does not undergo splitting.

• • The The additional stabilizationadditional stabilization caused by the splitting of d orbital is known as the caused by the splitting of d orbital is known as the

crystal field crystal field stabilizationstabilization energy energy ((CFSECFSE).).

• • Every electron in the tEvery electron in the t2g2g orbital set contribute -Δ orbital set contribute -Δoo for ( for (CFSECFSE). However, ). However,

electrons in the eelectrons in the egg orbital set resides at the higher energy level from orbital set resides at the higher energy level from d d orbital orbital

which does not undergo splitting and every electron contributes Δwhich does not undergo splitting and every electron contributes Δoo to CFSE. to CFSE.

CFSE importance:CFSE importance:

CFSE = +ve, unfavorableCFSE = +ve, unfavorable

CFSE = 0 (no change in stability)CFSE = 0 (no change in stability)

CFSE = -ve (gain stability)CFSE = -ve (gain stability)

d2 d3

How is a How is a dd44 configuration distributed? configuration distributed?

CFSE = [-0.4xnCFSE = [-0.4xn11+0.6xn+0.6xn22] Δ] Δoo+mP+mP

For example, For example, dd55, , low-spinlow-spin ( ([Fe(NO[Fe(NO22))66]]3−3−). CFSE= 5 x 2/5 Δ). CFSE= 5 x 2/5 Δoo= 2Δ= 2Δoo. .

However, it is highly unfavorable condition due to the greatest loss of However, it is highly unfavorable condition due to the greatest loss of

exchange energyexchange energy. In . In high-spinhigh-spin ( ([FeBr[FeBr66]]3−3−), CFSE = 3 x 2/5 Δ), CFSE = 3 x 2/5 Δoo - 2 x 3/5 Δ - 2 x 3/5 Δoo

= 0. The stabilization generated by the electrons in the t= 0. The stabilization generated by the electrons in the t2g2g orbitals is orbitals is

canceled out by the destabilizing effect of the electrons in the ecanceled out by the destabilizing effect of the electrons in the egg

orbitals.orbitals.

d6d6- CFSE = -2.4Δ- CFSE = -2.4Δo o ((LSLS) and if consider pairing energy (P), 2.4Δ) and if consider pairing energy (P), 2.4Δoo- 3P. - 3P.

CFSE = -0.4ΔCFSE = -0.4Δoo ( (HSHS) and 0.4Δ) and 0.4Δoo-P. -P. d7d7- CFSE = -1.8Δ- CFSE = -1.8Δoo ( (LSLS) and 1.8Δ) and 1.8Δoo–3P & –3P &

-0.8Δ-0.8Δoo((HSHS) and 0.8Δ) and 0.8Δoo-2P. -2P. Similarly,d8, d9 and d10…..Similarly,d8, d9 and d10…..

Crystal Field stabilization is applicable to metal complexes of all Crystal Field stabilization is applicable to metal complexes of all

geometries, inluding square-planar (geometries, inluding square-planar (dd8 complexes) having very large 8 complexes) having very large

CFSE.CFSE.

Example 1Example 1: : For MnFor Mn3+ 3+ ion, the electron pairing energy, P is about 28000 ion, the electron pairing energy, P is about 28000

cmcm-1-1.. ΔΔoo values for the complexes [Mn(H values for the complexes [Mn(H22O)O)66]]3+3+ and [Mn(CN) and [Mn(CN)66]]3-3- are are

21000 cm21000 cm-1-1 and 38500 cm and 38500 cm-1-1 respectively. Do these complexes have respectively. Do these complexes have

HS or LS configuration? Also write down the configurations HS or LS configuration? Also write down the configurations

corresponding to these states? corresponding to these states?

Example 2Example 2:: Give the number of unpaired electrons for the [Fe(CN) Give the number of unpaired electrons for the [Fe(CN)66]]4-4-

and [Fe(CN)and [Fe(CN)66]]3-3- complexes. complexes.

High-spin (HS-) and low-spin (LS-) Oh complexesHigh-spin (HS-) and low-spin (LS-) Oh complexes

Ligands cause large Δ for Ligands cause large Δ for dd-orbitals are known as -orbitals are known as strong-field ligandsstrong-field ligands. .

e.g. CNe.g. CN−−, NO, NO22--, and CO, produce , and CO, produce low-spinlow-spin complexes and follow complexes and follow Aufbau Aufbau

principleprinciple. Conversely, ligands (e.g., I− & Br−) cause small Δ for . Conversely, ligands (e.g., I− & Br−) cause small Δ for dd-orbitals -orbitals

are known as are known as weak-field ligands weak-field ligands and produce and produce High-spinHigh-spin complexes and complexes and

follow follow Hund’s ruleHund’s rule..

Factors affecting the magnitude of ΔFactors affecting the magnitude of Δoo

1. 1. Oxidation state of the metal ionOxidation state of the metal ion: e.g., [Co(H: e.g., [Co(H22O)O)66]]+3+3; Δ; Δoo=18,600 =18,600

cmcm-1-1 and [Co(H and [Co(H22O)O)66]]+2+2; Δ; Δoo=9300 cm=9300 cm-1-1

2. 2. Nature of the metal ionNature of the metal ion: Δ increases (30-50%) from 3d to 4d to : Δ increases (30-50%) from 3d to 4d to

5d in the same oxidation state.5d in the same oxidation state.

3. 3. Number and geometry of the ligandsNumber and geometry of the ligands: e.g., Δ: e.g., Δt t = - 4/9 Δ= - 4/9 Δo o smaller smaller

than Oh complexes and so Δthan Oh complexes and so Δtt < P gave HS-complexes. < P gave HS-complexes.

4. 4. Nature of the ligandsNature of the ligands: Spectrochemical series (see below…).: Spectrochemical series (see below…).

THE THE SPECTROCHEMICAL SERIESSPECTROCHEMICAL SERIES (Tsuchida 1938) (Tsuchida 1938)

Based on factors affecting CFSE, Based on factors affecting CFSE, OO like: like:

i.i. the nature of the metal ion. the nature of the metal ion. ii.ii. the metal oxidation state. the metal oxidation state. iii.iii. the the

arrangement of the ligands around the metal ion. arrangement of the ligands around the metal ion. iv.iv. the nature of the the nature of the

ligands surrounding the metal ion. ligands surrounding the metal ion.

Tsuchida experimentally saw the effect of different metal oxidation state Tsuchida experimentally saw the effect of different metal oxidation state

and the ligands in the CFSE determination. The arrangement of metal and the ligands in the CFSE determination. The arrangement of metal

ions or ligands from higher to lower or vice versa is called ions or ligands from higher to lower or vice versa is called

spectrochemical seriesspectrochemical series. e.g.,. e.g.,

A. When the A. When the geometrygeometry and the and the ligandsligands are held constant, splitting are held constant, splitting

decreases in the following order:decreases in the following order: strong-field ions strong-field ions PtPt4+ 4+ >Ir>Ir3+ 3+ >Rh>Rh3+ 3+ >Co>Co3+ 3+

>Cr>Cr3+ 3+ >Fe>Fe3+ 3+ >Fe>Fe2+ 2+ >Co>Co2+ 2+ >Ni>Ni2+ 2+ >Mn>Mn2+2+ weak-field ions weak-field ions

B. When the B. When the geometrygeometry and the and the metalmetal are held constant, are held constant, dd orbitals orbitals

splitting decreases in the following order:splitting decreases in the following order: weak-field ligands weak-field ligands II−− < Br < Br−− < <

SS2−2− < SCN < SCN−− < Cl < Cl−− < NO < NO3−3− < N < N3−3− < F < F−− < OH < OH−− < C < C22OO442−2− < H < H22O < NCSO < NCS−− < CH < CH33CN < CN <

py < NHpy < NH33 < en < 2,2'-bipyridine < phen < NO < en < 2,2'-bipyridine < phen < NO22− − < PPh< PPh33 < CN < CN−− < CO < CO strong-strong-

field ligandsfield ligands

Question: Why do ligands F- and CN- that have negative charges are found in Question: Why do ligands F- and CN- that have negative charges are found in the position of the position of thethe weak and strong series? weak and strong series?

(a) For complex that contains M-F bond(a) For complex that contains M-F bond

(i) Ligands donate electrons to the central metal M (i) Ligands donate electrons to the central metal M

through π orbital on ligand F.through π orbital on ligand F.

(ii) Electrons from t(ii) Electrons from t2g2g orbitals (originated from the orbitals (originated from the

metallic ions) are filled in the anti-bonding π*- MO metallic ions) are filled in the anti-bonding π*- MO

which is at a higher energy level as compared to twhich is at a higher energy level as compared to t2g2g

(iii) The effect is the reduction of ∆.(iii) The effect is the reduction of ∆.

(b) For a complex which contains M-CN bond(b) For a complex which contains M-CN bond

(i) Ligands having high-energy π*-MOs, which is empty (e.g., CO, (i) Ligands having high-energy π*-MOs, which is empty (e.g., CO, CN and HCN and H22C=CHC=CH22))

(ii) As a result, the density of electrons from central metal M can (ii) As a result, the density of electrons from central metal M can

be donated to the ligands through the back-bonding.be donated to the ligands through the back-bonding.(iii) Electrons that are filled in π (t(iii) Electrons that are filled in π (t2g2g) orbital are from metal.) orbital are from metal.

(iv) The effect is the stabilization of t(iv) The effect is the stabilization of t2g2g orbitals. orbitals.

Pairing energy (P) vs. (Pairing energy (P) vs. (CFSE) ) OO

1. If 1. If OO < P, weak field; HS, e.g., < P, weak field; HS, e.g., [Cr(H[Cr(H22O)O)66]]2+2+

2. If 2. If OO > P, strong field; LS, e.g., > P, strong field; LS, e.g., [Cr(CN)[Cr(CN)66]]44––

3. If 3. If OO = P = P, ???; , ???; e.g., d4, d6 & d8. e.g., d4, d6 & d8.

Distribution of d-es in tDistribution of d-es in t2g2g- & e- & egg-sets either in -sets either in strong or weak Ostrong or weak Oh h fields are fields are

same for same for d1, d2 & d3 ionsd1, d2 & d3 ions. . Stronger fieldsStronger fields (Δo >P, (Δo >P, d4, d5, d6 & d7 ionsd4, d5, d6 & d7 ions) )

have electrons in thave electrons in t2g2g ( (low spin/spin pairedlow spin/spin paired and so lower resultant spin and so lower resultant spin

value). value). Weaker fieldsWeaker fields (Δo <P, (Δo <P, d4, d5, d6 & d7 ionsd4, d5, d6 & d7 ions) have in e) have in egg– electrons – electrons

((high spin/spin freehigh spin/spin free and so greater resultant spin value). and so greater resultant spin value). d8, d9 & d10d8, d9 & d10

stronger and weaker fields have same distributions in tstronger and weaker fields have same distributions in t2g2g and e and egg. .

e.g., LS- and HS-Oh complexes-

ddxx config.config.

examplesexamples P value P value (cm(cm-1-1))

Δo value Δo value (cm(cm-1-1))

spin statespin stateCFT CFT predictedpredicted

Observed Observed expt.expt.

Relative Relative magnitude magnitude of Δo & Pof Δo & P

d4d4 [Cr(H2O)6]2+[Cr(H2O)6]2+[Mn(H2O)6]3+[Mn(H2O)6]3+

23500235002800028000

13900139002100021000

HSHSHSHS

HSHSHSHS

Δo <PΔo <PΔo <PΔo <P

d5d5 [Mn(H2O)6]2+[Mn(H2O)6]2+[Fe(H2O)6]3+[Fe(H2O)6]3+

25500255003000030000

780078001370013700

HSHSHSHS

HSHSHSHS

Δo <PΔo <PΔo <PΔo <P

d6d6 [Fe(H2O)6]2+[Fe(H2O)6]2+[Fe(CN)6]4-[Fe(CN)6]4-[Co(NH3)6]3+[Co(NH3)6]3+[CoF6]3-[CoF6]3-

1760017600176001760021000210002100021000

1040010400330003300032000320001300013000

HSHSLSLSLSLSHSHS

HSHSLSLSLSLSHSHS

Δo <PΔo <PΔo >PΔo >PΔo >PΔo >PΔo <PΔo <P

d7d7 [Co(H2O)6]2+[Co(H2O)6]2+ 2250022500 93009300 HSHS HSHS Δo <PΔo <P

The pairing energy is constant along a period. The pairing energy is constant along a period. However, However, the bigger the the bigger the

period the lower the pairing energy (n = 4 period the lower the pairing energy (n = 4 <<<< n=3) so many heavy n=3) so many heavy

elements are low spin (Fe versus Os).elements are low spin (Fe versus Os).

If the If the P = 10Dq then the hs/lsP = 10Dq then the hs/ls determination is dependent on determination is dependent on temperaturetemperature. .

This complex is low spin at low temp, but changes to high spin at rt…. This complex is low spin at low temp, but changes to high spin at rt….

some very interesting materials!some very interesting materials!

Splitting in Tetrahedral geometrySplitting in Tetrahedral geometry: 4-coordinate complexes (: 4-coordinate complexes (TTh h & &

Sq. planar)Sq. planar).The CFSE is low and unable to force electrons to pair-.The CFSE is low and unable to force electrons to pair-HS HS

complexescomplexes result. 2.What will happen with strong field ligands? result. 2.What will happen with strong field ligands?

WhyWhy ΔΔt t = -0.45Δ= -0.45Δoo??

et2

d-electrons config. in Td-electrons config. in Thh- HS and LS-ligand fields: - HS and LS-ligand fields: CFSE = 0-(- 6Dq) = CFSE = 0-(- 6Dq) =

6Dq for 6Dq for dd11 system; n= no. of unpaired es. system; n= no. of unpaired es.

ddxx config. config.Weak field (HS-complexes)Weak field (HS-complexes) Strong field (LS-Strong field (LS-

complexes)complexes)

tt2 2 pp e e qq config. config. nn tt2 2

pp e e qq config. config. nn

dd11

dd22

tt2 2 00 e e 11

tt2 2 00 e e 22

1122

tt2 2 00 e e 11

tt2 2 00 e e 22

1122

dd33

dd44

dd55

dd66

tt2 2 11 e e 22

tt2 2 22 e e 22

tt2 2 33 e e 22

tt2 2 33 e e 33

33445544

tt2 2 00 e e 33

tt2 2 00 e e 44

tt2 2 11 e e 44

tt2 2 22 e e 44

11001122

dd77

dd88

dd99

dd1010

tt2 2 33 e e 44

tt2 2 44 e e 44

tt2 2 55 e e 44

tt2 2 66 e e 44

33221100

tt2 2 33 e e 44

tt2 2 44 e e 44

tt2 2 55 e e 44

tt2 2 66 e e 44

33221100

SP

Why ΔWhy Δspsp = 1.3Δ = 1.3Δoo??z out conditionz out condition Sq. planarSq. planar

M-ions (dM-ions (d88)-strong ligand field gave Sq. planar (LS) complexes. e.g., )-strong ligand field gave Sq. planar (LS) complexes. e.g.,

[Ni(CN)[Ni(CN)44]]-2-2, [Pt/PdCl, [Pt/PdCl44]]-2-2, [Pt(NH, [Pt(NH33))44]]+2+2 & [AuCl & [AuCl44]]-1-1 where d where dx2-y2x2-y2 remain always remain always

unoccupied.unoccupied.

Squar planarSquar planar::

Different Ways of d-d transitions

a) dza) dz22 -----d-----dxyxy Creates more repulsionCreates more repulsion

b) dzb) dz22 ------d------dxzxz Creates less repulsionCreates less repulsion

Selection Rules for Electronic Spectra of TMs- ComplexesSelection Rules for Electronic Spectra of TMs- Complexes

1.1. The Spin Rule,The Spin Rule, ΔS = 0 (ΔS = 0 (AllowedAllowed)- i.e., )- i.e., (electronic transition occurs from (electronic transition occurs from the ground state to the next excited states with the the ground state to the next excited states with the same multiplicitysame multiplicity or or no change in spin).no change in spin).

allowed transitions: singlet - singlet or triplet -- tripletforbidden transitions: singlet -- triplet or triplet -- singlet

• • Spin-forbidden transitionsSpin-forbidden transitions

– – Change in the spin state of the molecule Change in the spin state of the molecule

are forbiddenare forbidden

– – Strongly obeyed in lighter atomsStrongly obeyed in lighter atoms

– – Relaxed by effects that make spin a poorRelaxed by effects that make spin a poor

quantum number (heavy atoms)quantum number (heavy atoms)

For example,(d5) [Mn(H2O)6]+2 d-d transition forbidden-colorless/pale flesh.For example,(d5) [Mn(H2O)6]+2 d-d transition forbidden-colorless/pale flesh.

2. The Orbital Rule (Laporte),2. The Orbital Rule (Laporte), Δl = +/- 1, +/-2 (Δl = +/- 1, +/-2 (AllowedAllowed)- )- there must be a there must be a

change in the parity (symmetry) of the complex.change in the parity (symmetry) of the complex.

s2---s2---s1p1 s1p1 Δl = +1 (Laporte allowed, Δl = +1 (Laporte allowed, ε=5000-10000 unitε=5000-10000 unit), ), d---d---d,d, Δl = 0 Δl = 0

(Laporte forbidden, (Laporte forbidden, ε=5-101 unitε=5-101 unit). In TM-L complexes, due to ). In TM-L complexes, due to d & pd & p

orbitals mixing (orbitals mixing (not pure d-dnot pure d-d) in nature, e.g.,) in nature, e.g., (e.g.,T (e.g.,Thh- [MnBr- [MnBr44]]-2-2) and ) and

distorted Odistorted Ohh complexes [Co(NH complexes [Co(NH33))55Cl]Cl]+2+2-colored. -colored. No d & pNo d & p orbitals mixing orbitals mixing

in Oin Ohh [Co(NH [Co(NH33))66]]+3+3 & [Cu(H & [Cu(H22O)O)66]]+2+2 –colorless/fade. –colorless/fade.

Expected intensities of electronic transitions

Transition typeTransition type ExampleExample value of ε mvalue of ε m22 mol mol-1-1

Spin forbidden, Spin forbidden, Laporte forbiddenLaporte forbidden

[Mn(H[Mn(H22O)O)

66]]2+2+ 0.10.1

Spin allowed (Spin allowed (Oh complexOh complex), ), Laporte forbiddenLaporte forbidden

[Ti(H[Ti(H22O)O)

66]]3+3+ 11

Spin allowed (Spin allowed (Th complexTh complex), ), Laporte partially allowed Laporte partially allowed by d-p mixingby d-p mixing

[CoCl[CoCl44]]

2-2- 5050

Spin allowed, Spin allowed, Laporte allowed Laporte allowed e.g. charge transfer bandse.g. charge transfer bands

[TiCl[TiCl66]]

2-2- or MnO or MnO44-- 10001000

For M2+ complexes, expect Δ = 7500 - 12500 cm-1 or λ = 800 - 1350 nm. For M2+ complexes, expect Δ = 7500 - 12500 cm-1 or λ = 800 - 1350 nm. For M3+ complexes, expect Δ= 14000 - 25000 cm-1 or λ = 400 - 720 nm. For M3+ complexes, expect Δ= 14000 - 25000 cm-1 or λ = 400 - 720 nm.

Relaxation of the Laporte Rules can occur throughRelaxation of the Laporte Rules can occur through: :

a)a) Spin-Orbit couplingSpin-Orbit coupling - gives weak spin forbidden bands; - gives weak spin forbidden bands;

b)b) Vibronic couplingVibronic coupling - an oh complex may allowed vibrations when - an oh complex may allowed vibrations when

molecule is asymmetric so absorption of light is possible. molecule is asymmetric so absorption of light is possible.

c) π-acceptor & π-donor ligands mix with d-orbitals (geometry c) π-acceptor & π-donor ligands mix with d-orbitals (geometry

relaxation during transition) so transitions are no longer purely d-d.relaxation during transition) so transitions are no longer purely d-d.

Cu+2Ca+2 Sc+2 Ti+2V+2 Cr+2 Mn+2 Fe+2 Co+2 Ni+2 Zn+2

0.6

1.0

Å

Ioni

c ra

dii

no. of 3d-electrons

3d=0 3d=10

(O(Ohh ionic radii of M ionic radii of M+2+2 for 1 for 1stst row transition metals) row transition metals)

For MFor M+3+3 ions trends and explanations are same. ions trends and explanations are same.

Sc+2-unstableSc+2-unstable

CFSE= - ve CFSE= - ve (more –ve values, more stable complexes) (more –ve values, more stable complexes)

Applications of C.F. Theory

1.1. Ionic Radii Ionic Radii: MXs (O: MXs (Ohh))

CFSE=0 for d0, d5(HS) and d10 systemsCFSE=0 for d0, d5(HS) and d10 systems

CFSE=-12DqCFSE=-12Dq

XX

11stst row TM, ionic radii of M row TM, ionic radii of M+2+2-ions, e.g., metals halides (MX-ions, e.g., metals halides (MX22, X= F, Cl, Br, , X= F, Cl, Br,

I, Oh shape). I, Oh shape). Theoretical and Experimental values are sameTheoretical and Experimental values are same for for CaCa+2+2 (d (d00), ),

MnMn+2+2 (d (d55, HS), Zn, HS), Zn+2+2 (d (d1010), CFSE = 0, Two extreme cases, V), CFSE = 0, Two extreme cases, V2+2+ (d (d33) & Ni) & Ni2+2+ (d (d88) )

in a weak ligand fields (Xs), CFSE = 1.2Δin a weak ligand fields (Xs), CFSE = 1.2Δoo, others (d, others (d22, d, d44, d, d77 & d & d99), CFSE = ), CFSE =

0.6 - 0.8Δ0.6 - 0.8Δo. o. Why? Why? FF-->Cl>Cl-->Br>Br-->I>I-- in electronegativity in electronegativity, , What will be the ionic What will be the ionic

radii radii trendtrend in 1 in 1stst TM? TM?

Strong ligands?Strong ligands? Ionic radii decreases for the strong field case until the Ionic radii decreases for the strong field case until the

tt2g2g66 config. is reached, due to increasing nuclear charge and poor config. is reached, due to increasing nuclear charge and poor

shielding by tshielding by t2g2g d-electrons. At this point the next electron enters the e d-electrons. At this point the next electron enters the egg--

orbital directed at the ligands, repellingorbital directed at the ligands, repelling them and causing an increase in them and causing an increase in

the effective radius of the metal-ligand.the effective radius of the metal-ligand.

Hydration enthalpy/stability of complexes/L.E.Hydration enthalpy/stability of complexes/L.E.


350

400

450

500

heat

s of

hyd

ratio

n(k

cal/m

ol)

xx

xx

xx

xx

experimental values of CFSE

Calculated values of CFSE


700

800

900

1000

heat

s of

hyd

rati

on(k

cal/

mol

)

xx

x

xx

x

x

experimental values of CFSE

Calculated values of CFSE

CFSECFSE value for Heat of hydration of Mvalue for Heat of hydration of M+2+2 and M and M+3+3-ions of 1-ions of 1stst-row TM, a -row TM, a

similar trends is found for the lattice energy similar trends is found for the lattice energy vsvs TM-ions. TM-ions. Higher the heat Higher the heat

of hydration or lattice energy-more stable compounds (CFSE?)of hydration or lattice energy-more stable compounds (CFSE?). .

(Hydration enthalpy of M(Hydration enthalpy of M+2+2 and and MM+3+3 for 1for 1stst row TMs in row TMs in Oh complexesOh complexes))

F- >> Cl- > Br- > I-F- >> Cl- > Br- > I-

Here, the experimental values increasing irregularly, Here, the experimental values increasing irregularly, maxima at V+2 (d3 maxima at V+2 (d3

ion) and Ni+2 (d8) and minima at Ca+2(d0 ion), Mn+2 (d5) and Zn+2 (d10).ion) and Ni+2 (d8) and minima at Ca+2(d0 ion), Mn+2 (d5) and Zn+2 (d10).

The unexpected maxima and minima can be explained on the basis of The unexpected maxima and minima can be explained on the basis of

CFSE concept. [M(H2O)6]+2 are high-spin Oh complexes and for high-CFSE concept. [M(H2O)6]+2 are high-spin Oh complexes and for high-

spin complexes, CFSE is minimum (zero) for d0 (Ca+2), d5 (Mn+2) and spin complexes, CFSE is minimum (zero) for d0 (Ca+2), d5 (Mn+2) and

d10 (Zn+2) ions and maximum (=1.2 Δo) for d3 (V+2) and d8 (Ni+2) ions. d10 (Zn+2) ions and maximum (=1.2 Δo) for d3 (V+2) and d8 (Ni+2) ions.

In totoIn toto, Ca+2/+3 to Zn+2/+3 ionic radii decrease-hydration energy , Ca+2/+3 to Zn+2/+3 ionic radii decrease-hydration energy

increase.increase.

Color of ComplexesColor of Complexes

The bright colors exhibited by many coordination compounds can be The bright colors exhibited by many coordination compounds can be

explained by C.F. Theory. explained by C.F. Theory.

When white light is allowed to fall on a complex, the following things When white light is allowed to fall on a complex, the following things

may occur:may occur:

i. The complex may i. The complex may absorbed theabsorbed the whole white lightwhole white light. Thus complex . Thus complex

appears appears blackblack..

ii. The complex may ii. The complex may reflect (or transmit) the whole lightreflect (or transmit) the whole light. In this case it . In this case it

appears appears whitewhite..

iii. The complex may iii. The complex may absorb some of it and may reflect (or transmit) the absorb some of it and may reflect (or transmit) the

remaining lightremaining light. In this case the complex has some color, i.e. . In this case the complex has some color, i.e. it is it is

coloredcolored. The absorption of light by the complexes takes place in the . The absorption of light by the complexes takes place in the

visible region of the spectrumvisible region of the spectrum ( (4000Ǻ to 7000Ǻ wavelength)4000Ǻ to 7000Ǻ wavelength). .

High---------------------------------decreasing energy-----------------------------------------→Low

Color absorbed Violet Blue Green-blue

Blue-green

Green Yellow-green

Yellow Orange Red

λ of the absorbed

4000Å 4350 Å

4800 Å

4900 Å

5000 Å 5600 Å 5800 Å 5900 Å 6050-7000 Å

Low -----------------------------increasing wavelength--------------------------------------------→High

Color transmitted (color of the complex

Yellow-green

Yellow Orange Red Purple Violet Blue Green Blue-green

[Cu(H[Cu(H22O)O)44]]2+2+ ions--hydrated cupric sulphate (blue), [Ti(H ions--hydrated cupric sulphate (blue), [Ti(H22O)O)66]]3+3+ ions (purple) ions (purple)

The color of the absorbed light is different from that of the transmitted The color of the absorbed light is different from that of the transmitted

light called light called complementary colorcomplementary color. The relation between the colors of . The relation between the colors of

the absorbed and reflected light is as below:the absorbed and reflected light is as below:

The complex ions absorb light in the The complex ions absorb light in the IR (λ> 7000 Å)IR (λ> 7000 Å) or or UV (λ< 4000 Å)UV (λ< 4000 Å)

are colorless, e.g. (i) anhydrous cupric sulphate is colorless, IR are colorless, e.g. (i) anhydrous cupric sulphate is colorless, IR

region. (ii) [Cu(CN)region. (ii) [Cu(CN)44]]2-2- ion UV region, colorless. ion UV region, colorless.

Wave no. (ν) = 1/ λ(cm) =1/ Å x 10Wave no. (ν) = 1/ λ(cm) =1/ Å x 10-8-8 cm =x cm cm =x cm-1-1 (1 cm (1 cm-1-1=2.85 x 10=2.85 x 10-3-3

kcal/mole or 350 cmkcal/mole or 350 cm-1-1 = 1.0 kcal/mole). = 1.0 kcal/mole).

Note: Note: Rarely, the energy of the photon absorbed corresponds exactly Rarely, the energy of the photon absorbed corresponds exactly

to the size of the gap Δ; Other factors (such as to the size of the gap Δ; Other factors (such as electron-electron electron-electron

repulsionrepulsion and Jahn-Teller effects) that also affect the energy and Jahn-Teller effects) that also affect the energy

difference between the ground and excited states.difference between the ground and excited states.

Mol

ar a

bsor

ptan

ce

30000 20000

(cm-1)

10000

Å= 5000

(Visible absorption spectrum of [Ti(H(Visible absorption spectrum of [Ti(H22O)O)66]]3+3+

ion. Peak of the curve shows the maximum ion. Peak of the curve shows the maximum absorption)absorption)

Visible absorption spectrum of a complex ion is useful in predicting the Visible absorption spectrum of a complex ion is useful in predicting the

color of the complex. e. g., [Ti(Hcolor of the complex. e. g., [Ti(H22O)O)66]]3+3+ ion shows absorption maxima at a ion shows absorption maxima at a

wavelength of about wavelength of about λλ- 5000Ǻ- 5000Ǻ. . νν = 1/ = 1/ λλ (in cm). 5000Ǻ- green color (in cm). 5000Ǻ- green color

absorbed. Transmitted light is purple (Δo, E= 57 kcal/mole), absorbed. Transmitted light is purple (Δo, E= 57 kcal/mole),

corresponding to tcorresponding to t2g2g1 1 eegg

00------t------t2g2g0 0 eegg

1 1 called called d-d or ligand field transitiond-d or ligand field transition..

Magnetism of ComplexesMagnetism of Complexes

dx config. Ions n µs µexp

d1d2d3d4d5d6d7d8d9

Ti+3Ti+2, V+2V+2 Cr+3

Cr+2 Mn+3Mn+2 Fe+3Fe+2 Co+3

Co+2Ni+2Cu+2

123454321

1.732.833.874.905.924.903.872.831.73

1.73-1.852.75-2.853.80-3.904.75-4.905.65-6.105.10-5.704.30-5.202.80-4.001.70-2.20

µµss = √ n(n+2) BM. (BM- Bohr Magneton). = √ n(n+2) BM. (BM- Bohr Magneton). Spin only formula is valid in 1Spin only formula is valid in 1stst

row TMsrow TMs. . [CoCl[CoCl44]]2-2- & [MnF & [MnF44]]2-2- gave 1.73 and 5.9 BM respectively, predict gave 1.73 and 5.9 BM respectively, predict

the geometry? the geometry?

Magnetic property of coordination compounds gave:Magnetic property of coordination compounds gave:

• Unpaired electrons in d-orbitalsUnpaired electrons in d-orbitals and the magnetic momentand the magnetic moment

• Transition metal complexes are Paramagnetic or Diamagnetic. Transition metal complexes are Paramagnetic or Diamagnetic.

• The crystal field splitting diagram as well as strong or weak field The crystal field splitting diagram as well as strong or weak field

ligands and the way of the ligand field splitting parameter changes ligands and the way of the ligand field splitting parameter changes

with the nature and oxidation state of a transition element (HS and with the nature and oxidation state of a transition element (HS and

LS complexes).LS complexes).

Distortion of ODistortion of Ohh Complexes Complexes

In OIn Ohh-complexes, when all -complexes, when all ligand electron clouds and metal ion are at the ligand electron clouds and metal ion are at the

same length called regular (i.e. symmetrical) Oh complexessame length called regular (i.e. symmetrical) Oh complexes. Conversely, . Conversely,

unequal length Oh areunequal length Oh are called called distorted Oh complexesdistorted Oh complexes. The change in . The change in

shape is called shape is called distortiondistortion..

Distorted in Oh complexes may be of the following three types:Distorted in Oh complexes may be of the following three types:

(i) Diagonally distorted Oh complexes: Distortion of a regular Oh along (i) Diagonally distorted Oh complexes: Distortion of a regular Oh along

two-fold axis.two-fold axis.

(ii) Trigonally distorted Oh complexes: Distortion along a three-fold axis. (ii) Trigonally distorted Oh complexes: Distortion along a three-fold axis.

(iii) (iii) Tetragonally distorted OTetragonally distorted Ohh complexes complexes: Distortion along four-fold axis. : Distortion along four-fold axis.

For example: (a) For example: (a) Elongation along z-axis (Z-out condition):Elongation along z-axis (Z-out condition): two long two long

bonds (z-axis) and four short bonds (xy plane). e.g., bonds (z-axis) and four short bonds (xy plane). e.g.,

i. CuCli. CuCl22: four at 2.30 Å and two at 2.95 Å bond length.: four at 2.30 Å and two at 2.95 Å bond length.

ii. CuFii. CuF22: four at 1.93 Å and two at 2.27 Å bond length.: four at 1.93 Å and two at 2.27 Å bond length.

iii. LS-Oh complexes of Ni+2, Pd+2 and Pt+2 (iii. LS-Oh complexes of Ni+2, Pd+2 and Pt+2 (all d8 ionsall d8 ions) - strong ) - strong

distortion gave distortion gave square planar geometrysquare planar geometry..

(b) (b) Compression along z-axis (Z-in condition)Compression along z-axis (Z-in condition):: two short bonds (z axis) two short bonds (z axis)

and four long bonds (xy plane). e.g., (i) Kand four long bonds (xy plane). e.g., (i) K22CuFCuF44: two at 1.95 Å and four at : two at 1.95 Å and four at

2.08 Å. (ii) FeF2.08 Å. (ii) FeF22: two at 1.99 Å and four at 3.12 Å. : two at 1.99 Å and four at 3.12 Å.

(conditions (a) & (b) gave Tetragonal distortion geometry) shown below:(conditions (a) & (b) gave Tetragonal distortion geometry) shown below:

Tetrahedral

x2-y2 z2 eg

xy yz xz t2g

-0.6 t

+0.4 tt = 0.45 o

t2g + eg

xy yz xz x2-y2 z2

degenerate d-orbitals on Mn+

x2-y2 z2

xy yz xz

eg

t2g

+6Dq

-4Dq

10Dq

Octahedral

x2-y2

z2

xy

yz xz

Tetragonal

x2-y2

xy

z2

yz xz

Sq. Planar

12

3sp

(z-elongation)

No of d No of d

electronselectrons

HS-Oh complexes: weak ligand field HS-Oh complexes: weak ligand field LS-Oh complexes: strong ligand fieldLS-Oh complexes: strong ligand field

Distribution of es- in t2g and eg-Distribution of es- in t2g and eg-

orbitalsorbitals

Predicted Predicted

DistortionDistortion

Distribution of es- in t2g and Distribution of es- in t2g and

eg-orbitalseg-orbitals

Predicted Predicted

DistortionDistortion

d0d0

d1d1

d2d2

d3d3

d4d4

d5d5

d6d6

d7d7

d8d8

d9d9

d10d10

tt2g2g00(sym)-e(sym)-e

gg00(sym)(sym)

tt2g2g11(unsym)-e(unsym)-e

gg00(sym)(sym)


gg00(sym)(sym)


gg00(sym)(sym)


gg11(unsym)(unsym)


gg22(sym)(sym)


gg22(sym)(sym)


gg22(sym)(sym)


gg22(sym)(sym)


gg33(unsym)(unsym)


gg44(sym)(sym)

No distor.No distor.

Slight DSlight D

Slight DSlight D


Strong DStrong D


Slight DSlight D

Slight DSlight D


Strong DStrong D



gg00(sym)(sym)


gg00(sym)(sym)


gg00(sym(sym))


gg00(sym)(sym)


gg00(sym)(sym)


gg00(sym)(sym)


gg00(sym)(sym)


gg11(unsym)(unsym)


gg22(unsym)(unsym)


gg33(unsym)(unsym)


gg44(sym)(sym)


Slight DSlight D

Slight DSlight D


Slight DSlight D

Slight DSlight D


Strong DStrong D

Strong Distor. Strong Distor.

leads sq. leads sq.

planarplanar

Strong DStrong D

No distor. No distor.

No Distortion ConditionNo Distortion Condition: Both t: Both t2g2g and e and egg-sets as symmetrical orbitals -sets as symmetrical orbitals

lead to perfectly symmetrical (regular) Oh complexes. lead to perfectly symmetrical (regular) Oh complexes.

Condition for slight DistortionCondition for slight Distortion: d-orbitals of O: d-orbitals of Ohh complexes have complexes have tt2g2g--

orbitals as unsym. orbitals gave slight distortion in the complexorbitals as unsym. orbitals gave slight distortion in the complex, ,

because they do not come directly in front of the ligands approaching because they do not come directly in front of the ligands approaching

for metal ionfor metal ion. . Only slight distortions from the regular Oh was observed. Only slight distortions from the regular Oh was observed.

However, not experimentally detected in Oh.However, not experimentally detected in Oh.

Condition for strong DistortionCondition for strong Distortion: e: egg orbitals pointing directly towards the orbitals pointing directly towards the

ligands are unsym.(i.e. 1, 3 & 2 es) (ligands are unsym.(i.e. 1, 3 & 2 es) (only in LS-complexesonly in LS-complexes), gave strong ), gave strong

distortions, leading to tetragonal and even to distortions, leading to tetragonal and even to sq. planar complexessq. planar complexes..

Jahn-Teller Effect/ Theorem (1937)Jahn-Teller Effect/ Theorem (1937): Explain, why certain complexes : Explain, why certain complexes

undergo distortion?undergo distortion? (i.e. (i.e. OOhh --- ---tetragonaltetragonal))

StatementStatement “ “any non-linear molecular system possessing degenerate any non-linear molecular system possessing degenerate

electronic state will be unstable and will undergo distortion to form a electronic state will be unstable and will undergo distortion to form a

system of system of lower symmetry lower symmetry and and lower energy lower energy and thus will remove and thus will remove

degeneracydegeneracy.” .” Jahn-Teller distortion is automatic for the non-linear Jahn-Teller distortion is automatic for the non-linear

molecular systems.molecular systems.

It does not predict the nature or its magnitude of distortion. However, It does not predict the nature or its magnitude of distortion. However,

always occurs in a manner which decrease in the energy of the system.always occurs in a manner which decrease in the energy of the system.

SymmetricalSymmetrical: : tt2g2g-orbitals-orbitals: t: t2g2g00, t, t2g2g

33, t, t2g2g66; ; eegg-orbitals-orbitals: e: egg

00, e, egg44 and e and egg

22 in HS- in HS-

complexes (dcomplexes (dxx2-2-yy2)2)11 (d (dzz2)2)1 1 ((No J-T distortion observedNo J-T distortion observed).).

UnsymmetricalUnsymmetrical: : tt2g2g-orbitals-orbitals: t: t2g2g11, t, t2g2g

22, t, t2g2g44, t, t2g2g

55; ; eegg-orbitals-orbitals: e: egg

11, e, egg33 and e and egg

22 in LS- in LS-

complexes (dcomplexes (dxx22-y-y2)2)00 (d (dzz2)2)22 ((J-T distortion observedJ-T distortion observed).).

Cause of distortion with some complexesCause of distortion with some complexes: e.g., (i) : e.g., (i) dd44 ion ion (HS): two possible (HS): two possible

config. of es in tconfig. of es in t2g2g- & e- & egg-orbitals: -orbitals: 11. t. t2g2g33 d dzz2211 d dxx22-y-y220 0 22. t. t2g2g

33 d dzz2200 d dxx22-y-y2211

case 1case 1, M, M++-L interaction along the z-axis is more than along the x- and y-axes, -L interaction along the z-axis is more than along the x- and y-axes,

leads to a larger inter-ionic distance (leads to a larger inter-ionic distance (called z out condition or z-elongationcalled z out condition or z-elongation). ).

(ii) (ii) dd99 ion ion (Cu (Cu2+ 2+ complex) e.g., aq. soln. of [Cu(NHcomplex) e.g., aq. soln. of [Cu(NH33))44]]2+2+ - a large - a large tetragonal tetragonal

distortion (sq. planar complex). M-ion has config. tdistortion (sq. planar complex). M-ion has config. t2g2g66eegg

33 in both the fields (LS in both the fields (LS

& HS). The two possible arrangements of electrons in t& HS). The two possible arrangements of electrons in t2g2g- and e- and egg-orbitals are: -orbitals are:

1. t1. t2g2g66 d dzz2222 d dxx22-y-y2211 and 2. t and 2. t2g2g

66 d dzz2211 d dxx22-y-y2222

Thus, a large distortion/Asymmetry is only due to eThus, a large distortion/Asymmetry is only due to egg incomplete orbitals. incomplete orbitals.

Cause of DistortionCause of Distortion: Strong distortion mainly due to repulsion of M-Ls : Strong distortion mainly due to repulsion of M-Ls

electrons in eelectrons in egg-orbitals. E.g., in -orbitals. E.g., in dd44 ion ion (HS) and (HS) and dd99 ion (Cu ion (Cu+2+2 ion, LS & HS ion, LS & HS), ),

Case 1.Case 1. d-electron charge density is higher in z direction than x and y. d-electron charge density is higher in z direction than x and y.

Thus dThus dzz2 orbital (2 orbital (half filled (dhalf filled (d44) & completely filled(d) & completely filled(d99)))) offers greater offers greater

shielding of the Cushielding of the Cu+2+2 nucleus than the half filled d nucleus than the half filled dxx2-2-yy2 orbital. Same 2 orbital. Same

time, the ligands on the x- and y-axes experience a higher effective time, the ligands on the x- and y-axes experience a higher effective

nuclear charge. So, the ligands on the x- and y-axes are drawn in closer nuclear charge. So, the ligands on the x- and y-axes are drawn in closer

to the Cuto the Cu+2+2 nucleus and those on the z-axis move further out ( nucleus and those on the z-axis move further out (called z called z

out condition or z-elongationout condition or z-elongation). ).

Thus, the Oh-complex will distorted to tetragonal geometry which is Thus, the Oh-complex will distorted to tetragonal geometry which is

elongated along z direction and compressed along x and y directions. elongated along z direction and compressed along x and y directions.

Since the distortion to tetragonal geometry is automatic (without Since the distortion to tetragonal geometry is automatic (without

supplying energy from outside), the overall energy of the unsplit esupplying energy from outside), the overall energy of the unsplit egg

orbitals are zero.orbitals are zero. Therefore, sum of energy after splitting must be zero.Therefore, sum of energy after splitting must be zero.

ConclusionConclusion: whenever there are more electrons in d: whenever there are more electrons in dzz2 orbital than in d2 orbital than in dxx2-2-

yy2 orbital (O2 orbital (Ohh complex) of any M-ion distortion follow the above rule and complex) of any M-ion distortion follow the above rule and

maintain the centre of gravity rule. maintain the centre of gravity rule.

Consider configuration 2Consider configuration 2: (d: (dzz2)2)11 (d (dxx22-y-y2)2)22 t t2g2g66, , explanation for this explanation for this

observation is exactly the sameobservation is exactly the same. However, d. However, dxx2-2-yy2 will be higher in 2 will be higher in

energy. The resulting distortion will be called energy. The resulting distortion will be called Z-in conditionZ-in condition. . How then to How then to

decide which of the two possible Oh distortion config.(1. (ddecide which of the two possible Oh distortion config.(1. (dzz2)2)22 (d (dxx2-2-yy2)2)1 1

tt2g2g66 and 2. t and 2. t2g2g

66 (d (dzz2)2)11 (d (dxx2-2-yy2)2)22 ) would yield the more stable complex? ) would yield the more stable complex?

CFT offers no way of deciding itCFT offers no way of deciding it. . Experimental results, however, show Experimental results, however, show

that it is that it is Z out conditionZ out condition Oh distortion configuration with two long and Oh distortion configuration with two long and

four short bonds which is more stablefour short bonds which is more stable. There is no theoretical . There is no theoretical

explanation for the instability of structure corresponding to explanation for the instability of structure corresponding to Z-in Z-in

conditioncondition having four long and two short bonds. having four long and two short bonds.

tt2g2g11 configuration in Oh complexes configuration in Oh complexes: Single electron can occupy any t: Single electron can occupy any t2g2g orbitals. If orbitals. If

the electron is in dthe electron is in dxyxy orbital, it would screen the M-ion nucleus orbital, it would screen the M-ion nucleus more effectively in more effectively in

xy plane than in xz & yz planes. This would reduce the attraction between M-L in xy plane than in xz & yz planes. This would reduce the attraction between M-L in

xy plane. So, the Oh geometry of the complex would get distorted by the xy plane. So, the Oh geometry of the complex would get distorted by the

elongation of the M-L bonds in xy plane. Repulsion increased and energy elongation of the M-L bonds in xy plane. Repulsion increased and energy

decreased in the same plane as compared to yz & xz planes. decreased in the same plane as compared to yz & xz planes.

On the other hand, if the electron is present in either dOn the other hand, if the electron is present in either dxzxz or d or dyzyz orbital, it will orbital, it will

screen M-ion nucleusscreen M-ion nucleus more effectively in the xz and yz planes compared to the more effectively in the xz and yz planes compared to the

xy plane, and decrease attraction between M-L along the z axis. Thus, the bonds xy plane, and decrease attraction between M-L along the z axis. Thus, the bonds

along the z direction elongated and get distorted to tetragonal geometry.along the z direction elongated and get distorted to tetragonal geometry.

The energies of both dThe energies of both dxyxy and d and dxx2-2-yy2 orbitals alter in a similar manner due to 2 orbitals alter in a similar manner due to

distortion of Oh geometry by Jahn-Tellar effect. The change in energies of distortion of Oh geometry by Jahn-Tellar effect. The change in energies of

these orbitals will again obey the centre of gravity rule. these orbitals will again obey the centre of gravity rule.

The J-T effect shown by tThe J-T effect shown by t2g2g orbitals is much weaker than e orbitals is much weaker than eg g orbitals. Because in orbitals. Because in

tt2g2g orbitals the charge density lie in between the x, y and z directions, while in orbitals the charge density lie in between the x, y and z directions, while in

eegg orbitals, the charge density lie directly in the directions along which the orbitals, the charge density lie directly in the directions along which the

ligands are placed. ligands are placed.

Thus, an electron in any of the tThus, an electron in any of the t2g2g orbitals would shield the positive charge of orbitals would shield the positive charge of

M-ion much less effectively in the x, y and z directions, i.e. along the directions M-ion much less effectively in the x, y and z directions, i.e. along the directions

of the ligands than the electron placed in any of the eof the ligands than the electron placed in any of the egg orbitals. orbitals.

The magnitude of J-T effectThe magnitude of J-T effect is related to the screening of the nuclear charge of is related to the screening of the nuclear charge of

the M-ion by the d electrons in the directions of the ligands. the M-ion by the d electrons in the directions of the ligands. It is smaller in Oh It is smaller in Oh

complexes with ground state configurations tcomplexes with ground state configurations t2g2g11, t, t2g2g

22, t, t2g2g44 and t and t2g2g

55 than in ground than in ground

state configurations tstate configurations t2g2g6 6 eegg

11, t, t2g2g6 6 eegg

33, t, t2g2g3 3 eegg

11, etc, etc. .

In fact, it has not been possible to detect J-T effect in Oh complexes of M-ions In fact, it has not been possible to detect J-T effect in Oh complexes of M-ions

with ground state configuration twith ground state configuration t2g2g11, t, t2g2g

22, t, t2g2g44 and t and t2g2g

55 (except from indirect (except from indirect

spectroscopic evidence) because the magnitude of the effect is comparatively spectroscopic evidence) because the magnitude of the effect is comparatively

very small and these complexes get little bid more stabilized due to J-T effect. very small and these complexes get little bid more stabilized due to J-T effect.

Jahn-Teller effect on Electronic spectra of ComplexesJahn-Teller effect on Electronic spectra of Complexes

Electronic absorption bands in the spectra of coordination complexes Electronic absorption bands in the spectra of coordination complexes

are associated with d-d transitions (are associated with d-d transitions (tt2g2g --- --- e egg ). For example, ). For example,

[Ti(H[Ti(H22O)O)66]]3+3+- - Oh complexOh complex, (d, (d11 system), absorbs around system), absorbs around λλ= = 5000 Ǻ. 5000 Ǻ. This This

should give rise to a single symmetrical absorption bandshould give rise to a single symmetrical absorption band. . But is not so,But is not so,

absorption band is unsymmetrical, which result of overlapping of more absorption band is unsymmetrical, which result of overlapping of more

than one absorption bands. than one absorption bands. This can be accounted for on the basis of J-This can be accounted for on the basis of J-

T effect as followsT effect as follows: :

% A

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(Unsymmetrical absorption band (Unsymmetrical absorption band observed in the spectrum of [Ti(Hobserved in the spectrum of [Ti(H22O)O)66]]+3+3).).

SupposeSuppose, electron is in d, electron is in dxyxy orbital ( orbital (slightly stable complexslightly stable complex). The ). The

complex distorted to tetragonal geometry in which the complex distorted to tetragonal geometry in which the Ti-OHTi-OH22 bonds bonds

along z axis are shorter along z axis are shorter than x and y axes. The energy of dthan x and y axes. The energy of dxyxy orbital gets orbital gets

lower than dlower than dxzxz and d and dyzyz orbitals and energy gap δ orbitals and energy gap δ22 between d between dxyxy and (d and (dxzxz, ,

ddyzyz) is much smaller than the average width of an electronic absorption ) is much smaller than the average width of an electronic absorption

band (band (δδ22 - neither account for absorption of EMR (Vis- region) nor - neither account for absorption of EMR (Vis- region) nor

asymmetry in the electronic absorption band of [Ti(Hasymmetry in the electronic absorption band of [Ti(H22O)O)66]]+3+3). ).

Thus, dThus, dxyxy electron on excitation may occupy either (i) d electron on excitation may occupy either (i) dxx2-2-yy2 orbital or (ii) 2 orbital or (ii)

ddzz2 orbital. If the electron occupies d2 orbital. If the electron occupies dxx2-2-yy2 orbital in the excited state, 2 orbital in the excited state,

charge polarized more in the xy plane and if in dcharge polarized more in the xy plane and if in dzz2 orbital it will be along 2 orbital it will be along

the z direction. This results in two electronic sets in the excited state the z direction. This results in two electronic sets in the excited state

having different energy. having different energy.

E

dz2

dx2-y

2

dxz, dyz

dxy

E1 E2 (Splitting of the excited state energy levels due to J-T (Splitting of the excited state energy levels due to J-T effect in [Ti(Heffect in [Ti(H22O)O)66]]+3+3 in the distorted geometry making in the distorted geometry making

possible two electronic transitions).possible two electronic transitions).

Lowering of the symmetry of [Ti(HLowering of the symmetry of [Ti(H22O)O)66]]+3+3 from O from Oh h to tetragonal due to J-to tetragonal due to J-

T effect, splits the excited state and makes possible two electronic T effect, splits the excited state and makes possible two electronic

transitions or electronic jumps, viz. dtransitions or electronic jumps, viz. dxyxy11 → d → dxx2-2-yy2211 and d and dxyxy

11 → d → dzz2211, ,

which should give rise to two absorption bands. which should give rise to two absorption bands. Since the energy Since the energy

difference δdifference δ22 between d between dxyxy and d and dxzxz, d, dyzyz is very small, the population is very small, the population

difference between ddifference between dxyxy and d and dxzxz, d, dyzyz levels is also very small. levels is also very small. Therefore, Therefore,

the ground state of [Ti(Hthe ground state of [Ti(H22O)O)66]]+3+3 is often written as tis often written as t2g2g11 and transition as and transition as

tt2g2g11 → d → dxx2-2-yy2211 and t and t2g2g

11 → d → dzz2211. .

ConverselyConversely, , suppose Ti-OHsuppose Ti-OH22 bonds along z axis are longer than bonds bonds along z axis are longer than bonds

along x and y directions. i.e., along x and y directions. i.e., (d(dxzxz, d, dyzyz))11 --- ---ddzz2211 (lower energy) than d (lower energy) than dxx2-2-

yy2211. Thus, two transitions; t. Thus, two transitions; t2g2g11 → d → dxx2-2-yy2211 & t & t2g2g

11 → d → dzz2211. .

The distorted geometry of Ti(HThe distorted geometry of Ti(H22O)O)66]]+3+3 obtained by compression of Ti-OH obtained by compression of Ti-OH22

bonds along z axis is stable by bonds along z axis is stable by δδ22/3 over its distorted geometry obtained /3 over its distorted geometry obtained

by elongation of Ti-OHby elongation of Ti-OH22 bonds along z axis bonds along z axis. But the difference δ. But the difference δ22/3 in the /3 in the

energies of the two distorted geometries of Ti(Henergies of the two distorted geometries of Ti(H22O)O)66]]+3+3 is very small and is very small and

the thermal energy available at rt to these complex ions in the distorted the thermal energy available at rt to these complex ions in the distorted

geometry is sufficiently enough to allow these ions to change their geometry is sufficiently enough to allow these ions to change their

geometry.geometry.

Since ΔESince ΔE11 (Ed (Edxx2-2-yy2 - Ed2 - Edxzxz, d, dyzyz) is close to ΔE) is close to ΔE22 (Edz (Edz22 – Ed – Edxzxz, d, dyzyz) , the two ) , the two

transitions will again give rise to two overlapping bands (unsymmetrical transitions will again give rise to two overlapping bands (unsymmetrical

band). Thus, an unsymmetric absorption band will be observed whether band). Thus, an unsymmetric absorption band will be observed whether

the complex ion [Ti(Hthe complex ion [Ti(H22O)O)66]]+3+3 has the distorted geometries and an has the distorted geometries and an

equilibrium exists between them due to equilibrium exists between them due to dynamic J-T effectdynamic J-T effect. .

(Splitting of the excited state energy levels due to J-T effect (Splitting of the excited state energy levels due to J-T effect in [Ti(Hin [Ti(H22O)O)66]]+3+3 in the distorted geometry making possible two in the distorted geometry making possible two

electronic transitions).electronic transitions).

E

dz2

dx2

-y2

dxz, dyz

dxy

E1 E2

Factors determine the geometry of the complex ionsFactors determine the geometry of the complex ions::

1. J-T effect favor the distorted Ti(H1. J-T effect favor the distorted Ti(H22O)O)66]]+3+3 in which d in which dxyxy (lowest energy). (lowest energy).

2. The thermal energy available to the complex tends to equalize the population 2. The thermal energy available to the complex tends to equalize the population

of ions in the two distorted geometries. Thus, an equilibrium condition between of ions in the two distorted geometries. Thus, an equilibrium condition between

the populations of the two distorted geometries exists. This is known as the populations of the two distorted geometries exists. This is known as

dynamic J-T effectdynamic J-T effect (dynamic J-T effect, energy gap of t (dynamic J-T effect, energy gap of t2g2g & e & egg should be small). should be small).

3. 3. In Oh complexIn Oh complex, the energy gap between t, the energy gap between t2g2g & e & egg is quite high. The thermal is quite high. The thermal

energy available at rt is not enough to influence the population of the ions in any energy available at rt is not enough to influence the population of the ions in any

distorted geometries. Hence, J-T effect will solely determine the stable distorted distorted geometries. Hence, J-T effect will solely determine the stable distorted

geometry in which the complex will acquire. Such distorted geometry is known geometry in which the complex will acquire. Such distorted geometry is known

as as static J-T effectstatic J-T effect. .

Consider, CuConsider, Cu+2+2:: vizviz. [Cu(H. [Cu(H22O)O)66]]+2+2. Mostly Oh complexes of Cu. Mostly Oh complexes of Cu+2+2 exhibit exhibit

tetragonal distortion because of J-T effect in which Cu-L bonds along z tetragonal distortion because of J-T effect in which Cu-L bonds along z

axis are axis are elongatedelongated compared to the bonds in the xy plane. compared to the bonds in the xy plane.

t2g6

eg3

dxy

dxz, dyz

dz2

dx2-y2

3 12

(Possible electronic transition in tetragonal [Cu(H(Possible electronic transition in tetragonal [Cu(H22O)O)66]]+2+2). ).

The electronic absorption spectrum of CuThe electronic absorption spectrum of Cu+2:+2: splitting of e splitting of egg orbitals in Oh orbitals in Oh

CuCu+2+2 complexes gave the energy of the d complexes gave the energy of the dzz2 orbital decreased close to the 2 orbital decreased close to the

energies of denergies of dxyxy & (d & (dxzxz, d, dyzyz) orbitals (J-T effect). Therefore, the energy ) orbitals (J-T effect). Therefore, the energy

required to excite an electron from any one of these orbitals to the required to excite an electron from any one of these orbitals to the

partially vacant dpartially vacant dxx2-2-yy2 orbital is nearly of the same magnitude. Thus, the 2 orbital is nearly of the same magnitude. Thus, the

following three electronic transitions are possible in electronic spectra following three electronic transitions are possible in electronic spectra

of [Cu(Hof [Cu(H22O)O)66]]+2+2 : :

(i) Transition from d(i) Transition from dzz2 ---- d2 ---- dxx2-2-yy2 orbital by absorbing energy hν2 orbital by absorbing energy hν11..

(ii) Transition from d(ii) Transition from dxyxy ---- d ---- dxx2-2-yy2 orbital by absorbing energy hν2 orbital by absorbing energy hν22..

(iii) Transition from d(iii) Transition from dxzxz ---- d ---- dxx2-2-yy2 orbital by absorbing energy hν2 orbital by absorbing energy hν33. .

The three transitions of close energy would give rise to three absorption The three transitions of close energy would give rise to three absorption

bands at close frequencies νbands at close frequencies ν11, ν, ν22 and ν and ν33 which overlap to give a which overlap to give a

composite, unsymmetrical absorption band as shown:composite, unsymmetrical absorption band as shown:

% A

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5000 10000 15000

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If no J-T distortionIf no J-T distortion, the spectrum would have consisted of a single , the spectrum would have consisted of a single

symmetric band corresponding to the transition tsymmetric band corresponding to the transition t2g2g66eegg

33 → t → t2g2g55eegg

44. The . The

observed spectrum of the complex explained only on the basis of the J-observed spectrum of the complex explained only on the basis of the J-

T effect.T effect.

Limitations of CFTLimitations of CFT

(i) CFT considers only the M-ion d-orbitals and gives no consideration to other (i) CFT considers only the M-ion d-orbitals and gives no consideration to other

M-orbitals as s-, p-orbitals or ligand M-orbitals as s-, p-orbitals or ligand ππ-orbitals. Thus, to explain all the properties -orbitals. Thus, to explain all the properties

of the complexes dependent on the of the complexes dependent on the ππ-ligand orbitals will be outside the scope of -ligand orbitals will be outside the scope of

CFT. This does not consider the formation of CFT. This does not consider the formation of ππ-bonding in complexes.-bonding in complexes.

(ii) CFT is unable to account the relative strength of ligands. It gives no (ii) CFT is unable to account the relative strength of ligands. It gives no

explanation, why Hexplanation, why H22O is a stronger ligand than OHO is a stronger ligand than OH-- in the spectrochemical series in the spectrochemical series

(iii) According to CFT, the bond between the M-Ls are purely ionic. It gives no (iii) According to CFT, the bond between the M-Ls are purely ionic. It gives no

account of the partly covalent nature of the M-ligand bonds and the effects account of the partly covalent nature of the M-ligand bonds and the effects

directly dependent on covalently cannot be explained by CFT. directly dependent on covalently cannot be explained by CFT.

In quantum mechanics, spin multiplicity denotes the number of possible quantum states of a system with given principal spin quantum number S. The different states are distinguished by the spin projection quantum number Sz, which can take the values -S, -S+1, ..., S-1, S. Therefore, the multiplicity is 2S+1, where S is the number of singly occupied electrons multipled by the Electron Spin Quantum Number, ms.A system with S=0 has exactly one possible state; it is therefore in a singlet state. A system with S=1/2 is a doublet; S=1 is a triplet, and so on.The most important application is to electrons. A single electron (as in a free radical) has S=1/2; it is therefore always in a doublet state. Two electrons (as in a diradical) can pair up in a singlet or in a triplet state. Normally the singlet is the ground state.For example, oxygen has two singly occupied electrons which could have spin multiplicity of 3. This means that the spins could be up up or up down or down down, total 3 possibilities. Using the formula, Spin Multiplicity of oxygen = 2S+1 = 2(2*1/2)+1 = 3, where S= two singly occupied electrons*ms. (ms always equal to 1/2)

Subject: what is spin multiplicity in spin quantum no.??

S. R. or transition rule

Selection rules have been derived for electronic, vibrational and rotational transitions.

To get the selection rules for and , we shift gears, and treat light as an electromagnetic wave, not photons. Also, we consider the reverse process, where an atomic state absorbs energy from incident light. An electron is so small (point-like, so far as we know today) that it feels at any moment a uniform electric field, and a uniform magnetic field. Classical physics says that magnetic effects are much smaller than electric effects, so we will ignore the former. (In modern parlance, we are ``deriving'' the selection rule for electric dipole transitions, and ignoring magnetic dipole transitions.) The uniform electric field can shove the electron this way or that, but cannot flip it over. So S is unaffected, and hence . On the other hand, if the wavelength of light is short enough, the electric field will be shoving the electron one way in one part of its orbit, and another way for another part, and so may change its orbital angular momentum L. . We've just seen that S doesn't change, so L and J must change the same way. So the selection rules for and must be the same as those for and .

Coordination Compounds

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