Transition Metal Chemistry and Coordination Compounds

Lecture 18-20

The Transition Metals

Oxidation States of the 1st Row Transition Metals(most stable oxidation numbers are shown in red)

Aqueous oxoanions of transition elements

Mn(II) Mn(VI) Mn(VII)

V(V)Cr(VI)

Mn(VII)

One of the most characteristic chemical properties of these elements is the occurrence of multiple oxidation states.

Effects of the metal oxidation state and of ligand identity on color

[V(H2O)6]2+ [V(H2O)6]3+

[Cr(NH3)6]3+ [Cr(NH3)5Cl ]2+

Ionization Energies for the 1st Row Transition Metals

Scandium Titanium Vanadium

Chromium Manganese Iron

Cobalt Nickel Copper

Coordination Compounds

A coordination compound typically consists of a complex ion and a counter ion.

A complex ion contains a central metal cation bonded to one or more molecules or ions.

The molecules or ions that surround the metal in a complex ion are called ligands.

A ligand has at least one unshared pair of valence electrons

H

O

H

• ••• • •

H

N

HH• •

••Cl• •

••

-

••C O••

22.3

Coordination Compounds

The atom in a ligand that is bound directly to the metal atom is the donor atom.

H

O

H

• ••• • •

H

N

HH

The number of donor atoms surrounding the central metal atom in a complex ion is the coordination number.

Ligands with:

one donor atom monodentate

two donor atoms bidentate

three or more donor atoms polydentate

H2O, NH3, Cl-

ethylenediamine

EDTA

Coordination Compounds

H2N CH2 CH2 NH2

• • • •

bidentate ligand polydentate ligand(EDTA)

Bidentate and polydentate ligands are called chelating agents

EDTA Complex of Lead

What are the oxidation numbers of the metals in K[Au(OH)4] and [Cr(NH3)6](NO3)3 ?

OH- has charge of -1

K+ has charge of +1

? Au + 1 + 4x(-1) = 0

Au = +3

NO3- has charge of -1

NH3 has no charge

? Cr + 6x(0) + 3x(-1) = 0

Cr = +3

Naming Coordination Compounds

22.3

• The cation is named before the anion.

• Within a complex ion, the ligands are named first in alphabetical order and the metal atom is named last.

• The names of anionic ligands end with the letter o. Neutral ligands are usually called by the name of the molecule. The exceptions are H2O (aqua), CO (carbonyl), and NH3 (ammine).

• When several ligands of a particular kind are present, the Greek prefixes di-, tri-, tetra-, penta-, and hexa- are used to indicate the number. If the ligand contains a Greek prefix, use the prefixes bis, tris, and tetrakis to indicate the number.

• The oxidation number of the metal is written in Roman numerals following the name of the metal.

• If the complex is an anion, its name ends in –ate.

What is the systematic name of [Cr(H2O)4Cl2]Cl ?

tetraaquodichlorochromium(III) chloride

Write the formula of tris(ethylenediamine)cobalt(II) sulfate

[Co(en)3]SO4

Crystal-Field Theory• Model explaining bonding for transition Model explaining bonding for transition

metal complexesmetal complexes– • Originally developed to explain properties for

crystalline material– • Basic idea:– Electrostatic interaction between lone-pair

electrons result in coordination.

Energetics• CFT - Electrostatic between CFT - Electrostatic between

metal ion and donor atommetal ion and donor atom

i) Separate metal and ligand high energy

ii) Coordinated Metal - ligand stabilized

iii) Destabilization due to ligand -d electron repulsion

iv) Splitting due to octahedral field.

i

ii

iii

iv

d-Orbitals and Ligand Interaction(Octahedral Field)

•Ligands approach metal

d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction

d-orbitals pointing directly at axis are affected most by electrostatic interaction

Ligand-Metal Interaction•Crystal Field Theory - Describes bonding in Metal Complexes• Basic Assumption in CFT:• Electrostatic interaction between ligand and metal

d-orbitals align along the octahedral axis d-orbitals align along the octahedral axis will be affected the most.will be affected the most.

More directly the ligand attacks the More directly the ligand attacks the metal orbital, the higher the energy of metal orbital, the higher the energy of the d-orbital.the d-orbital.

In an octahedral field the degeneracy of In an octahedral field the degeneracy of the five d-orbitals is liftedthe five d-orbitals is lifted

Splitting of the d-Orbitals• Octahedral field Splitting Pattern:•

The energy gap is The energy gap is referred to as referred to as (10 Dq) (10 Dq) , the , the crystal field crystal field splitting energy.splitting energy.

The dThe dzz22 and d and dxx

22yy

22 orbitals lie on the same axes as negative charges.orbitals lie on the same axes as negative charges.

Therefore, there is a large, unfavorable interaction between ligand (-) orbitals.Therefore, there is a large, unfavorable interaction between ligand (-) orbitals.

These orbitals form the degenerate high energy pair of energy levels.These orbitals form the degenerate high energy pair of energy levels.

The dThe dxyxy , d , dyxyx and d and dxzxz orbitals bisect the negative charges. orbitals bisect the negative charges.

Therefore, there is a smaller repulsion between ligand & metal for these orbitals.Therefore, there is a smaller repulsion between ligand & metal for these orbitals.

These orbitals form the degenerate low energy set of energy levels.These orbitals form the degenerate low energy set of energy levels.

o2/5 o

3/5 o

o is the crystal field splitting

t2g

eg

E(t2g) = -0.4o x 3 = -1.2o

E(eg) = +0.6o x 2 = +1.2o

Splitting of d orbitals in an octahedral field

The magnitude of the splitting(ligand effect)

Strongfield

Weakfield

The spectrochemical series

CO, CN- > phen > NO2- > en > NH3 > NCS- > H2O > F- > RCO2

- > OH- > Cl- > Br- > I-

The magnitude of the splitting(metal ion effect)

Strongfield

Weakfield

increases with increasing formal charge on the metal ion

increases on going down the periodic table

The spectrochemical series

•For a given ligand, the color depends on the oxidation state of the metal ion.

•For a given metal ion, the color depends on the ligand.

I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2- < CN- < CO

WEAKER FIELD STRONGER FIELD

LARGER SMALLER

LONGER SHORTER

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Electron Configuration in Octahedral Field

• Electron configuration of metal Electron configuration of metal ion:ion:

• s-electrons are lost first. s-electrons are lost first. • TiTi3+3+ is a d is a d11, V, V3+3+ is d is d22 , and , and

CrCr3+3+ is d is d33 • Hund's rule:Hund's rule:• First three electrons are in First three electrons are in

separate d orbitals with their separate d orbitals with their spins parallel.spins parallel.

• Fourth e- has choice:Fourth e- has choice:• Higher orbital if Higher orbital if is small; is small;

High spinHigh spin• Lower orbital if Lower orbital if is large: is large:

Low spin.Low spin.• Weak field ligandsWeak field ligands• Small Small , High spin complex , High spin complex• Strong field LigandsStrong field Ligands• Large Large , Low spin complex , Low spin complex

d1 d2

d3 d4

Placing electrons in d orbitalsStrong field Weak field Strong field Weak field

d4

Strong field =Low spin

(2 unpaired)

Weak field =High spin

(4 unpaired)

< o > o

When the 4th electron is assigned it will either go into the higher energy eg orbital at an energy cost of 0 or be paired at an energy cost of , the pairing energy.

Pairing Energy,

The pairing energy, , is made up of two parts.

1) Coulombic repulsion energy caused by having two electrons in same orbital. Destabilizing energy contribution of c for each doubly occupied orbital.

2) Exchange stabilizing energy for each pair of electrons having the same spin and same energy. Stabilizing contribution of e for each pair having same spin and same energy

= sum of all c and e interactions

Placing electrons in d orbitals

1 u.e. 5 u.e.

d5

0 u.e. 4 u.e.

d6

1 u.e. 3 u.e.

d7

2 u.e. 2 u.e.

d8

1 u.e. 1 u.e.

d9

0 u.e. 0 u.e.

d10

To sum up•Electron Configuration for Octahedral complexes of metal ion Electron Configuration for Octahedral complexes of metal ion having dhaving d11 to d to d1010 configuration [M(H configuration [M(H22O)O)66]]+n+n. . •Only the dOnly the d44 through d through d77 cases have both high-spin and low spin configuration cases have both high-spin and low spin configuration..

Electron configurations for octahedral complexes of metal ions having from d1 to d10 configurations. Only the d4 through d7 cases have both high-spin and low-spin configurations.

Colour in Coordination Compounds

22.5

E = h

The absorption maximum for the complex ion [Co(NH3)6]3+ occurs at 470 nm. What is the color of the complex and what is the crystal field splitting in kJ/mol?

Absorbs blue, will appear orange.

= hhc=

(6.63 x 10-34 J s) x (3.00 x 108 m s-1)

470 x 10-9 m= = 4.23 x 10-19 J

(kJ/mol) = 4.23 x 10-19 J/atom x 6.022 x 1023 atoms/mol

= 255 kJ/mol

22.5

Color Absorption of Co3+ Complexes

• The Colors of Some Complexes of the CoThe Colors of Some Complexes of the Co3+ 3+ IonIon

The complex with fluoride ion, [CoFThe complex with fluoride ion, [CoF66]]3+3+ , is high spin and has one absorption band. , is high spin and has one absorption band.

The other complexes are low spin and have two absorption bands. In all but one The other complexes are low spin and have two absorption bands. In all but one case, one of these absorptionsis in the visible region of the spectrum. The case, one of these absorptionsis in the visible region of the spectrum. The wavelengths refer to the center of that absorption band.wavelengths refer to the center of that absorption band.

Complex IonComplex Ion Wavelength of Wavelength of Color of Light Color of Light Color of ComplexColor of Complex light absorbed light absorbed Absorbed Absorbed

[CoF[CoF66] ] 3+3+ 700 (nm)700 (nm) RedRed GreenGreen

[Co(C[Co(C22OO44))33] ] 3+3+ 600, 420600, 420 Yellow, violetYellow, violet Dark greenDark green

[Co(H[Co(H22O)O)66] ] 3+3+ 600, 400600, 400 Yellow, violetYellow, violet Blue-greenBlue-green

[Co(NH[Co(NH33))66] ] 3+3+ 475, 340475, 340 Blue, violetBlue, violet Yellow-orangeYellow-orange

[Co(en)[Co(en)33] ] 3+3+ 470, 340470, 340 Blue, ultraviolet Blue, ultraviolet Yellow-orangeYellow-orange

[Co(CN)[Co(CN)66] ] 3+3+ 310310 Ultraviolet Ultraviolet Pale YellowPale Yellow

Colors & How We Perceive it

800

430

650 580

560

490

Artist color wheelArtist color wheelshowing the colors whichshowing the colors whichare complementary to oneare complementary to oneanother and the wavelengthanother and the wavelengthrange of each color.range of each color.

400

Complex Influence on Color•Compounds of Transition metal complexes solution.Compounds of Transition metal complexes solution.

[Fe(H2O)6]3+

[Co(H2O)6]2+

[Ni(H2O)6]2+

[Cu(H2O)6]2+

[Zn(H2O)6]2+

800

430

650 580

560

490

400