Corrdination Chemistry _GSR
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Transcript of Corrdination Chemistry _GSR
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 1
CHEMISTRY OF TRANSITION ELEMENTS
CO-ORDINATION CHEMISTRY
INTRODUCTION :
The branch of inorganic chemistry that deals with the study of coordination compounds
is called coordination chemistry.
A coordination compound is a compound of a metal with a certain number of species
called ligands bound to the metal. An example is [Ni(CO)4].
A coordination compound is the product of a Lewis acid-base reaction in which
neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by
coordinate covalent bonds.
Ligands are Lewis bases- they contain at least one pair of electrons to donate to a
metal atom/ion. Ligands are also called complexing agents.
Metal atoms/ions are Lewis acids - they can accept pairs of electrons from Lewis bases.
Within a ligand, the atom that is directly bonded to the metal atom/ion is called the
donor atom.
A coordinate covalent bond is a covalent bond in which one atom (i.e., the donor atom)
supplies both electrons. This type of bonding is different from a normal covalent bond
in which each atom supplies one electron.
If the coordination complex carries a net charge, the complex is called a complex ion.
Compounds that contain a coordination complex are called coordination compounds.
The coordination sphere of a coordination compound or complex consists of the central
metal atom/ion plus its attached ligands. The coordination sphere is usually enclosed in
brackets when written in a formula.
The coordination number is the number of donor atoms of ligands bonded to the central
metal atom/ion.
COORDINATION NUMBER (C.N.)
The maximum number of atoms, ions or molecules that are directly linked to central
metal atom in complex is called as the coordination number of the metal. It is different for
different metals. The geometry of the complex depends upon the co-ordination number of
metal.
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Chemistry of Transition Elements 2
Coordination Number Geometry
2 Linear
3 Triangular or Trigonal planar
4 Tetrahedral or square planar
5 Trigonal bipyramidal
6 Octahedral
Co-ordination number is the characteristic property of the metal. It takes the values
from 2 to 8, where 4 and 6 are the most common coordination numbers of coordination
compounds.
LIGANDS
Ligand is an ion or neutral molecule attached to the central metal ion in a coordination
compound. Within a ligand, the atom that is directly bonded to the metal atom/ion is called
donor atom. Each ligand has filled p orbital that bonds with the metal.
CLASSIFICATION OF LIGANDS
According to the number of bonds a ligand makes with a metal on distinguishes
monodentate ligands (e.g. ammonia NH3) and polydentate ligands. Polydentate ligands are
called
bidenatate, if they interact with a metal through two donor atoms,
tridentate, if they interact with a metal through three donor atoms,
quadridentate, if they interact with a metal through four donor atoms,
pentadentate, if they interact with a metal through five donor atoms,
hexadentate, if they interact with a metal through six donor atoms.
Ligands interacting with one metal through more than six donor atoms are rare.
The examples of all above types are given below :
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Chemistry of Transition Elements 3
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Chemistry of Transition Elements 4
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Chemistry of Transition Elements 5
Ligands can also be classified as shown below :
Classification of Ligands
Monodentate a Lewis base which can form only one name means one tooth
bond to a central metal atom.
Has more than one donor atom than can
form more than one coordinate covalent
bond to the same metal ion
Classified according to the number of
Donor atoms correctly positioned for
Potential binding to a central metal atom.
Chelating non-linear, often with 2 or 3
atoms separating the donor
atoms
bridging can donate more than one pair of
electrons to more than one metal atom
simultaneously
ambidentate has more than one element that can possesses bridging
serve as a donor atom capability but tends to
be monodentate; often
linear in geometry
macrocyclic large ring compound with several donor an example is 18crown6
atoms that can bind a central metal atom
inside the ring
pi-donor donates electrons from a pi bond to a
metal ion
Chelating, bridging and ambidentate ligands are described in detail.
A chelating ligand has several donor atoms arranged in such a way that they can
interact with one metal center. In the following example, the two nirogen atoms of 1,2-diaminoethane (= ethylenediamine, abbreviation en) are bound to the metal. Together with
the metal the ligand forms a five membered chelate ring.
NH2
NH2bidentate
NH2
NH
NH2
tridentate
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Chemistry of Transition Elements 6
NH2
NH2
LL
L
L
H2C
CH2
M
A bridging ligand acts as a bridge between two or more metal centers. In di
mhydroxo-bis (tetraaquairon (III), (it may also be called octoaqua-di mhyroxo-
diiron(III)), two hydroxyls bridge the two irons. Bridging ligands are preceded by m.
Important bridging ligands are: OH-, S2-, CO32-, PO4
3-, NH2-.
An ambidentate ligand has two donor atoms but their geometrical arrangement does
not allow them to bind to the same metal, i.e. they cannot form a chelate ring. These ligands
are responsible for linkage isomerism. Examples of ambidentate ligands are CN-, CO, SCN,
(CH3)2SO (dimetylsulfoxide = DMSO), HCON(CH3)2 (dimetylformamide= DMF)
HISTORICAL DEVELOPMENT IN COORDINATION CHEMISTRY
The exact date of preparation of the first co-ordination compound is not exactly
known. The discovery of hexaamminecobalt (III)chloride, CoCl3.6NH3, by Tassaert, in 1798,
is generally regarded as the beginning of co-ordination chemistry. The formation of this orange
coloured comound by Tassaert was quite intriguing in face of valency considerations. It was
difficult for chemists to understand as to why two stable molecules such as COCl3 and NH3could combine to form another stable molecule. Later on, many such compound were prepared
and their properties were studied.
COORDINATION COMPOUNDS OF TRANSITION METALS
Most of the metals form the co-ordination compounds. The two important conditions
that a metal should exhibit to form coordination compounds is
1) small size and
2) ability to exhibit variable oxidation states.
These two conditions are generally met with d block or transition metals. Hence, most
co-ordination compounds are formed by the transition metals.
The d block elements consist of three rows called first, second and third transition
series. The electronic configurations of these elements is shown below :
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Chemistry of Transition Elements 7
First Second Third
Sc 21 3d14s2 Y 39 4d15s2 La 57 5d16s2
Ti 22 3d24s2 Zr 40 4d25s2 Hf 72 5d26s2
V 23 3d34s2 Nb 41 4d45s1 Ta 73 5d36s2
Cr 24 3d54s1 Mo 42 4d55s1 W 74 5d46s2
Mn 25 3d54s2 Tc 43 4d55s2 Re 75 5d56s2
Fe 26 3d64s2 Ru 44 4d75s1 Os 76 5d66s2
Co 27 3d74s2 Rh 45 4d85s1 Ir 77 5d76s2
Ni 28 3d84s2 Pd 46 4d105s0 Pt 78 5d106s0
Cu 29 3d104s1 Ag 47 4d105s1 Au 79 5d106s1
Zn 30 3d104s2 Cd 48 4d105s2 Hg 80 5d106s2
NOMENCLATURE OF CO-ORDINATION COMPOUNDS
Introduction :
Thousands of coordination compounds are known. Nomenclature is important in
coordination chemistry because it gives us basic information about the structure of acoordination compound. IUPAC has recommanded certain rules for the nomenclature of
coordination compounds which are discussed below.Coordination Compound
A complex is formed by the interaction of metal atom and ligands. The ligand in acomplex is said to be coordinated to the metal atom or ion that is the center of the coordination
compound. Any neutral compound that contain a metal atom and its associated ligands iscalled a coordination compound. Such a compound may be formed between a complex ion
and other ions, for example [Ag(NH3)2]+Cl or K+2[Pt(NO2)4]
2 or the complex itself maybe neutral, for example, [Pt(NH3)2 (NO2)2]. The formula of the complex is usually enclosed
in square brackets.Rules for Nomenclature of Coordination Compounds
Rule 1In naming a coordination compound, the name of the cation is given first followed by
the name of the anion. This is illustrated by considering the names of the following compouds
K+ [Pt(NH3)Cl5] Potassium amminepentachloroplatinate (IV)
cation Anion
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Chemistry of Transition Elements 8
[Co(NH3)4 SO4] + NO3 Tetramminesulfatocobalt (III) nitrate.
Cation Anion
Rule II : Naming coordination SphereWhile naming a coordinatio sphere, ligands are named first and then the metal atom
along with its oxidation number in parentheses.
A) Naming LigandsNames of some common Negative Ligands
Symbol Name Charge Symbol Name ChargeBr bromo 1 SO4 sulphato 2
Cl chloro 1 H hydrido 1
I Iodo 1 NO2 nitrito 1CO3 carbonato 2 ONO nitrito-O 1
CN cyano 1 SCN thiocyanato 1OH hydroxo 1 NCS thiocyanato-N 1
C2O4 oxalato 2 SO3 Sulfito 2O oxo 2 S2O3 thiosulfato 2
N3 azido 1 N nitrido 1NO3 nitrato 1 C6H5 Phenyl 1
O2 Peroxo 2 NH2 amido 1NH imido 2
Names of Neutral LigandsSymbol Name Charge Symbol Name Charge
NH3 ammine 0 H2O aqua 0CO carbonyl 0 NO nitrosyl 0
(Ph3)P triphenylphosphine 0 C2H4 ethylene 0
CH3NH2 methyl amine 0 en ethylenediamine 0N2 dinitrogen 0 C6H6 benzene 0
Rule No. 1 Naming Ligands
Various ligands that are coordinated to the metal ion are listed in alphabetical order. Certain ligands such as SCN, NO2 contain two atoms that can coordinate with the metal
ion. In such cases the symbol of atom that is coordinated to the metal ion is mentionedafter the name of ligand separated by hyphen.
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Chemistry of Transition Elements 9
Rule No. 2 : Indicating Number of Ligands
If the same ligand is present more than once then the number of ligands is indicatedby prefixing words such as di, tri, tetra etc to the name of ligands.
[Co(NH3)6]+3 hexaammine cobalt (III)
K2 [Pt Cl6] Potassium hexachloroplatinate (IV)
[Pt(NH3)4Cl2]Cl2 tetraamminedichloroplatinum (IV) Chloride.K [Pt(NH3)Cl5] Potassium amminepentachloroplatinate (IV)
(Always remember that when ligands are listed alphabetically, spellings (alphabet) of
their proper names are taken into consideration and not the spellings of the prefixes of di, trietc. Thus diammine should be listed under a and not under d)
Use of bis, tris, tetrakis etc.
Words such as bis, tris, tetrakis etc are used to denote the number of those ligands. Whosename already includes a number
Eg : ethylene diammine, triphenyl phosphineMany a times ambiguity is created due to use of words such as di, tri etc to indicate
the number of ligands. For example, when two methyl amine molecules are coordinated tometal atom then using di gives dimethylamine. This confuses us whether dimethylamine means
two molecules of methyl amine or one molecule of dimethyl amine. In such cases words suchas bis, tris, tetrakis etc are used to denote numbers of such ligands. Some of such ligands
whose number is denoted by using bis, tris etc are listed below.Benzene
Pyridinemethyl amine, thiosulphato
Whenever the words bis, tris, tetrakis etc are used to specify the number of ligands,the name of the ligand is written in parentheses.
[Co(en)3] Cl3tris (ethylenediamine) cobalt (III) chloride.Na3 [Ag(S2O3)2]
Sodium bis (thiosulfato) argentate (I)[CuCl2(CH3NH2)2]
dichlorobis(methyl amine) copper (II)B) Naming Metal Atom/Ion
Name of the central metal atom is written after the names of ligands. The oxidationnumber of the metal atom is indicated by Roman numerals in parentheses after the name of
the metal atom.
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Chemistry of Transition Elements 10
Determining Oxidation State of Metal Atom
Oxidation number of central atom is determined by usual method.Eg. In K4[Fe(CN)6], the oxidation number of Fe is :
4XK +XFe + 6XCN = 0 where X = oxidation number4(+1) + XFe + 6(1) = 0
XFe = +2Eg. In [Cr(en)3]Cl3, the oxidation number of Cr is :
Xcr + 3Xen + 3XCl = 0Xcr + 3(0) + 3(1) = 0
XCr = +3Eg. In [Fe(CN)6]
4, the oxidation number of Fe is :
XFe + 6XCN = 4
XFe + 6(1) = 4XFe = +2
Naming Metal AtomThe name of metal atom depends upon the charge on coordination sphere.
Neutral or Cationic Coordination SphereWhen the coordination sphere is either a cation or a neutral molecule, the name of
the central atom remains as such.Eg : [Co(NH3)6]Cl3
In the above complex Co is in +3 oxidation state and coordination sphere in theabove complex is cationic.
[Co(NH3)6]Charge = XCo + 6XNH3
= +3+0= +3
Thus the coordination sphere bears +3 charge and hence is cationic. Thus the name
of the metal should be written as Cobalt.[Co(NH3)6]Cl3Hexaamminecobalt (III) Chloride.
[Pt(NH3)2 Cl4] Diamminetetrachloroplatinum (IV)[Cr(en)3]Cl3 Tris(ethylenediamine)chromium (III) chloride.
[Pt Cl2(NH3)2] Diamminedichloroplatinum (II)
Anionic Coordination SphereIf the coordination sphere is anion or bears a negative charge then the name of the
central metal atom ends in ATE. (ium of the name of metal atom is replaced by ate).
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Chemistry of Transition Elements 11
Eg K3[Co(NO2)6] Potassium hexanitritocobaltate (III).
K2[Pt Cl6] Potassium hexachloroplatinate (IV)Na[AlCl4] Sodium tetrachloroaluminate (III)
Na4[Fe(CN)6] Sodium hexacyanoferrate (II)Li [AlH4] Lithium tetrahydridoaluminate (III)
Ba [BrF4] Barium tetrafluorobromate (III)When there is a Latin name for the metal, it is used to name the metal atom in
negatively charged coordination sphere.
English name Latin name anion nameCopper Cuprum Cuprate
Gold Aurum Aurate
Iron Ferrum FerrateLead Plumbum Plumbate
Tin Stannum Stannate
Names of some Metal Atoms In Negatively Charged Coordination SphereMn Manganate
Fe FerrateCu Cuperate
Co CobaltateZn Zincate
Mo MolybdateSb antimonate
K[Ag (CN)2] Potassium dicyanoargentate (I)
K2[OsCl5N] Potassium pentachloronitridoosmate (VI)
Nomenclature of Complexes Containing Bridging Ligands
For ligands which act as bridge between two metal atoms, the greek letter u is prefixed
to their names. If a coordination compound contains more than one bridging ligand then theprefix m is repeated before the name of each kind of bridging ligand.
Bridging ligands are mentioned alphabetically before the other ligands. This may beillustrated by considering the following examples.
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Chemistry of Transition Elements 12
N
N
(NH3)4Co Co (NH3)4 (NO3)4
H2
O2
m -amido- m -nitrito-N octamminedicobalt (III) nitrate
O
O
(H2O)4Fe Fe 4(H2O) (SO4)2
H
H
di- m -hydroxo-octaaquodiiron (III) sulphate
NH
OH
( en)2Co Co ( en)2
3+
m -hydroxo- m -imido-tetrakis (ethylenediamine) dicobalt (III) ion.
Nomenclature of Some Coordination Compounds
Formula Name[Co(CO3)(NH3)4]Cl Pentaamminecarbonatocobalt(III) chloride
K4[Fe(CN)6] Potassium hexacyanoferrate(II)[Co(NH3)6]Cl3 Hexaamminecobalt(III)chloride
Na3[Co(NO2)6] Sodium hexanitrito-Ncobaltate(III)[PtCl4(NH3)2] Diamminetetrachloroplatnium(IV)
[Co(NO2)3(NH3)3] Triamminetrinitrito-Ncobalt(III)
[CoCl(ONO)(en)2]+ Chlorobis(ethylenediammine)nitrito-Ocobalt(III)
[Ag(CN)2]1 Dicyanoargentate(I)
[CoCl2(en)2]SO4 Dichlorobis(ethylenediamine)cobalt(III)sulphate
N
(NH3)4Co Co (NH3)4
2+H
O2m -amido- m -superoxo-octamminedicobalt(III)
Na2[CrOF4] Sodium tetrafluoroxochromate(IV)
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Chemistry of Transition Elements 13
[Cu(NH3)4]SO4 Tetraamminecopper(II)sulphate
[Cr(H2O)6]Cl3 Hexaaquachromium(III)chlorideNa2[SiF6] Sodium hexafluorosilicate(IV)
K3[Fe(CN)6] Potassium hexacyanoferrate(III)K4[Mo(CN)8] Potassium octacyanomolybdate(IV)
K3[Fe(CN)5NO] Potassium pentacyanonitrosylferrate(II)[PdI2(NOO)2(H2O)2] Diaquadiiododinitrito-Npalladium(IV)
[Co(en)3]2(SO4)3 Tris(ethylenediamine)cobalt(III)sulphate
NATURE OF METAL-LIGAND BONDING IN COORDINATION COMPOUNDS
Various theories have been proposed to explain various features such as metal-ligand
bonding, colour, geometry and magnetic properties of transition metal complexes. These are
1. The valence bond theory (VBT)
2. The crystal field theory (CFT)
3. The molecular orbital theory (MOT)
All the above theories have been discussed below with appropriate details.
VALENCE BOND THEORY
The valence bond theory was developed by Prof. Linus Pauling. It deals with the
electronic structure of the central metal atom in its ground state and is concerned mainly with
the study of:
1. the kind of bonding,
2. geometry,
3. the gross magnetic properties of the metal complexes.
ASSUMPTIONS OF VALENCE BOND THEORY
This theory involves the following assumptions:
1) The Central metal atom makes available a number of vacant orbitals equal to its
coordination number for the formation of covalent bonds with the ligand orbitals.*1
2) These vacant orbitals hybridize together to form hybrid orbitals *2 which are the same
in number as the atomic orbitals hybridizing together. These hybrid orbitals are vacant,
equivalent in energy and have definite geometry.
3) The ligands have at least one s -orbital containing a lone pair of electrons.
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Chemistry of Transition Elements 14
4) Vacant hybrid orbitals of the metal atom or ion overlap with filled s -orbitals of the
ligands to form ligand metal s bond. This type of bond is known as co-ordinate
covalent bond.
5) In addition to the s bond, there is the possibility of a p bond formation due to the
side-ways overlapping of a filled metal orbital with a suitable vacant ligand.
[*1 : - The rearrangement of non bonding electrons of the metal atom or ion takes
place in the following way while making available the empty orbitals for the ligands;
A) The rearrangement of non bonding electrons takes place according to
Hunds rule when the ligands are WEAK.
B) Under the influence of a strong ligand, the electrons can be forced to
pair up against the Hunds rule of maximum multiplicity.]
[*2 : - Numerous combinations of s, p and d orbitals are possible for hybridisation. The
type of hybridisation that the empty orbitals of metal atom undergo decides the geometry
of the resulting complex. Though there are numerous hybridisations possible, in practice
only a few are encountered in metal complexes. The following table gives the co-
ordination number, orbital hybridisation, spatial geometry and examples associated with
each.]
Hybridisation and Geometry
CN Hybridisation Molecular geometry Examples
2 sp Linear [Ag(NH3)2]+, [Ag(CN)2]
3 sp2 Trigonal [HgI3]
4 dsp2 square planar [Ni(CN)4]2 , [Pt(NH3)4]
2+
[PdCl4]2 , [Cu(NH3)4]
2+
sp3 Tetrahedral [Ni(CO)4], [Zn(NH3)4]2+
[NiCl4]2, [Cu(CN)4]
3
5 sp3d Trigonal bipyramidal [Fe(CO)5], [CuCl5]3
dsp3 square pyramidal [SbF5]2, [Ni(CN)5]
3
6 sp3d2 Octahedral [CoF6]3 [Cr(H2O)6]
2+
[Cr(NH3)6]2+ [FeF6]
3
[Fe(H2O)6]3+ [Fe(NH3)6]
2+
7 d2sp3 Octahedral [Cr(CN)6]3 , [CrF6]
3
[Cr(CO)6], [Mn(CN)6]5
[Fe(CN)6]4, [PtCl6]
2
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Chemistry of Transition Elements 15
REPRESENTATION OF COMPLEXES BY VBT
We will now discuss a few examples of complex formation to illustrate the norms of VBT.
1) Complexes with co-ordination number = 2
Any complex involving coordination number 2 involves sp hybridisation and has linear
geometry. Consider an example of [Ag(CN)2]-. Its formation by using VBT is shown below:
e configuration of Ag+
4d10 5s0 6p
e configuration of Ag+
during approach of two 4d10 5sO 6p
strong CN ligands
e configuration of Ag+
in [Ag(CN)2] 4d sp hybridisation
[Ag(CN)2] involves sp hybridisation and thus has linear geometry. Since all electrons
in the electronic configuration of [Ag(CN)2] are paired, it is diamangetic.
Examples to solve :
Q.1 : Describe the hybridisation and geometry of a) [CuCl2]-2 , b) [Cu(NH3)2]
+
Hint : Use following steps.
i) Determine the oxidation state of the central metal ion.
ii) Write the electronic configuration of metal atom in that particular oxidation state.
iii) Rearrange the electrons of central metal ion using the hints given in *1. (Cl is a weak
ligand while NH3 is a strong ligand)
iv) Follow the method illustrated for [Ag(CN)2] to determine the hybridisation and geometry.
2) Complexes with the co-ordination number 3 :
The complexes with co-ordination number 3 involve sp2 hybridisation and have trigonal
geometry. An example of such a complex is [HgI3]. Its formation on the basis of VBT is
illustrated below;
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Chemistry of Transition Elements 16
e configuration of Hg+2
5d10 6sO 6pO
e configuration of Hg+2
during approach of 3I 5d10 6s 6p
weak lignads.
e configuration of Hg+2
in [HgI3] 5d10 sp2 hybridization
Since [HgI3] involves sp2 hybridisation, it has trigonal geometry.
Magnetic nature- Since all the electrons in [HgI3] are paired, it is diamagnetic.
3) Complexes with the co-ordination number 4 :
There are two possible configurations for metal complexes with co-ordination number
four. These are tetrahedral and square planar. Tetrahedral structure arises from sp3 hybridisation
while the square planar structure is the result of dsp2 hybridisation.
A) Tetrahedral Complexes :
Here we shall discuss the structures of some complexes which have tetrahedral
geometry. Consider [Ni(CO)4] in which Ni is in zero oxidation state*3. Its valence shell
configuration is 3d84s2. The formation of [Ni(CO)4] as per the norms of VBT is explained
below :
e configuration of Ni3d8 4s2
e configuration of Ni
during approach of 3d10 4s 4p
strong CO ligands
e configuration of Ni(CO)4
3d10 sp3 hybridisation
Since [Ni(CO)4] involves sp3 hybridisation, and it has tetrahedral geometry.
Since all the electrons is [Ni(CO)4] are paired, it is diamagnetic.
[*3 : - See the nomenclature of the complexes to find out oxidation state of the metal
atom.]
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Chemistry of Transition Elements 17
Consider the other example of [FeCl4] in which Fe is in +3 oxidation state. Its
formation is illustrated below :
e- configuration of Fe+3
e- configuration of Fe+3 during approach ofweak Cl- ligands
e- configuration of Fe+3 in[FeCl4]
-2
3d5 4s 4p
sp3 hybridisation
Cl- Cl- Cl- Cl-
sp3 hybridisation
Thus, [FeCl4]-2 involves sp3 hybridisation and has tetrahedral geometry.
B) Square planar complexes :
Another possible geometry for the 4-coordinated complex is the square planar
involving dsp2 hybridisation. Some examples involving square planar geometries are discussed
below :
Consider [Ni(CN)4]-2 in which Ni is in +2 oxidation state. The formation of
[Ni(CN)4]-2 as per the norms VBT is explained below;
e configuration of Ni3d8 4s2
e configuration of Ni
during approach of 3d10 4s 4p
strong CO ligands
e configuration of Ni(CO)4
3d10 sp3 hybridisation
Since [Ni(CO)4] involves sp3 hybridisation, and it has tetrahedral geometry.
Since all the electrons is [Ni(CO)4] are paired, it is diamagnetic.
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Chemistry of Transition Elements 18
Consider the other example of [Pt(NH3)4]2+ in which Pt is in +2 oxidation state and
has valence shell configuration of 5d8. The square planar geometry of [Pt(NH3)4]+2 is explained
by using VBT as
e- configuration of Pt+2
e- configuration of Pt+2 during approach of fourNH3 strong ligands
e- configuration of Pt2- in[Pt(NH3)4]
+2 after gainingfour pairs of electrons from4Cl- ions.
5d 6s 6p
dsp2 hybridisation
NH3 NH3 NH3 NH3
dsp2 hybridisation
Thus, since [Pt(NH3)4]+2 involves dsp2 hybridisation, it has square planar geometry.
Examples to solve :
Q.2 : Predict the hybridisation and geometries of the following complexes by using
VBT.
1) [MnCl4]-2
2) [FeCl4]-2
3) [CoCl4]-2
Complexes with co-ordination number 5 :
There are two possible configurations for metal complexes with co-ordination number
five. These are trigonal bipyramidal and square pyramidal.
Trigonal bipyramidal structure arises form sp3d hybridisation while the square pyramidal
structure is the result of dsp3 hybridisation.
Consider the example of [Fe(CO)5] in which Fe is in zero oxidation state W and
has configuration 3d64s2. The TBP geometry of [Fe(CO)5] is explained by using VBT.
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Chemistry of Transition Elements 19
e configuration of Fe
3d6 4s2 5p0
e configuration of Fe
during approach of 3d8 4s0 5p0
during approach of strong
CO lignads
e configuration of Fe
in Fe(CO)5 dsp3 hybridisation
Since [Fe(CO)5] involves dsp3 hybridisation, it is square pyramidal. Since complex
contains paired electrons, it is diamagnetic.
Consider the other example of [Ni(CN)5]3- in which Ni is in +2 oxidation state
and has electronic configuration 3d8. [Ni(CN)5]-3 involves dsp3 hybridisation and has
square pyramidal geometry which is explained by using VBT.
e- configuration of Ni+2
e- configuration of Ni+2 during approach of 5strong CO ligands
e- configuration of Ni+2 in[Ni(CO)5] after gaining5 electron pairs fromCO ligands
3d 4s 4p
dsp3 hybridisation
CO
dsp3 hybridisation
CO CO CO CO
Complexes with the co-ordination number 6 :
Complexes with co-ordination number six are most exclusive and have been studied
on large scale. The complexes with the co-ordination number six involve either d2sp3 or sp3d2
hybridisation. Both the hybridisation give octahedral geometry. The hybridisation and geometry
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Chemistry of Transition Elements 20
of some complexes with coordination number 6 is discussed below :
Consider an example of [Fe(CN)6]-3 in which Fe is in +3 oxidation state & has
valence shell configuration of 3d5. The hybridisation and geomery of [Fe(CN)6]-3 is discussed
below by using VBT :
e configuration of Fe+3
3d5 4s0
e configuration of Fe+3
during approach of six strong 3d5 4s0 5p0
CN ligands
e configuration of Fe+3
in [Fe(CN)6]3 d2sp3
Since [Fe(CN)6]3 involves d2 sp3 hybridisation, it has octahedral geometry.
Since the complex contains unpaired electron, it is paramagnetic.
Consider the other example of [FeCl6]-3 in which Fe is in +3 oxidation state and has
valence shell configuration of 3d5. The hybridisation and geometry of [FeCl6]-3 is discussed
by using VBT :
e- configuration of Fe+3
e- configuration of Fe+3 during approach of 6weak Cl- ligands
e- configuration of Fe+3 in[FeCl6]
-3 after gaining6 electron pairs from6 Cl- ligands
3d 4s 4p
sp3d2 hybridisation
4d
sp3d2 hybridisation
Cl- Cl- Cl- Cl- Cl- Cl-
Thus, [Fe(Cl)6]-3 involves sp3d2 hybridisation and has octahedral geometry.
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 21
INNER ORBITAL AND OUTER ORBITAL OCTAHEDRAL COMPLEXES
Since two d-orbitals used in d2sp3 hybridisation belong to inner shell [i.e. (n-l)th shell],
the octahedral complex compounds resulted from d2sp3 hybridisation are called inner orbital
octahedral complexes. Since these complexes have comparatively lesser number of unpaired
electrons than the outer orbital octahedral complexes (see later on), these complexes are also
called low spin or spin paired octahedral complexes. It is due to the presence of strong
ligands in inner-orbital octahedral complexes of 3d transition series that the electrons present
in 3dz2. and 3dx
2-y
2 orbitals (eg set) are forced to occupy 3dxy, 3dyz and 3dxz orbitals
(t2g set) and thus 3d orbitals of eg set become vacant and hence can be used in 3dx2-y
2,
3dz2, 4s, 4px 4py 4pz (d
2sp3) hybridisation.
Since two d-orbitals are from the outer shell (i.e. nth shell), the octahedral complexes
resulted from sp3d2 hybridisation are called outer orbital octahedral complexes. Since these
complexes have comparatively greater number of unpaired electrons than the inner orbital
octahedral complexes, these are also called high spin or spin free octahedral complexes.
MAGNETISM
Movement of an electrical charge generates a magnetic field in a material. Magnetism
is therefore a characteristic property of all materials that contain electrically charged particles
and for most purposes can be considered to be entirely of electronic origin. In an atom, the
magnetic field is due to the coupled spin and orbital magnetic moments associated with the
motion of electrons. The spin magnetic moment is due to the precession of the electrons about
their own axes whereas the orbital magnetic moment is due to the motion of electrons around
the nucleus. The resultant combination of the spin and orbital magnetic moments of the
constituent atoms of a material gives rise to the observed magnetic properties.
Transition metal complexes are broadly classified as paramagnetic and diamagnetic
on the basis of magnetic properties.
Paramagnetism derives from the spin and orbital angular momenta of electrons. This
type of magnetism occurs only in compounds containing unpaired electrons.
electron spinning on its axis or
gives the spin magnetic moment
electron moving in its orbital creates
an additional magnetic field, leading
to the orbital magnetic moment
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 22
Diamagnetism arises when the compound contains no unpaired electrons as the spin
and orbital angular momenta is cancelled out when the electrons exists in pairs.
The value of the magnetic moment associated with paramagnetic substances can be measured
experimentally as well as can be calculated theoretically.
The value of the magnetic moment is calculated experimentally by using Gouy balance.
Gouy balance is used to measure the mass of a sample first in the absence of a magnetic
field, and then when the magnetic field is switched on. The difference in mass can be used
to calculate the magnetic susceptibility of the sample, and from the magnetic susceptibility the
magnetic moment can be obtained.
M2.84 Tm = c
m = magnetic moment in Bohr magnetons (B.M.)
Mc = magnetic susceptibility
T = absolute temperature.
The value of the magnetic moment is theoretically calculated as follows :
The spin and the orbital motion of the electrons are the sources of magnetic moment.
Thus, m is given by the expression :
S L 4S(S 1) L(L 1)+m = + + +
For the 3d transition metal complexes, the orbital moment is not important because
the ligand field quenches the orbital contribution. This can be more easily understood from
the following explanation that comes from CFT.
In order for an electron to contribute to the orbital in which it resides must be able
to transform into an exactly identical and degenerate orbital by a simple rotation (it is the
rotation of the electrons which induces the orbital contribution). For example, in an octahedral
complex, the degenerate t2g set of orbitals (dxz, dyx, dyz) can be inter converted by a 900
rotation. However the orbitals in the eg subset (dz2,dx2-y2) cannot be interconverted by
rotation about any axis as the orbital shapes are different; therefore an electron in the eg set
does not contribute to the orbital angular momentum and is said to be quenched. In the free
icon case the electrons can be transformed between any of the orbitals as they are all
degenerate, but there will still be partial orbital quenching as the orbitals are not identical.
Electrons in the t2g set do not always caontribute to the orbital angular moment. For
example in the d3, t2g3 case, an electron in the dxz orbital cannot be rotation be placed in
the dyz orbital as the orbital already has electron of the same spin. This process is also called
quenching.
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 23
Tetrahedral complexes can be treated in a similar way with the exception that we fill
the e orbitals first, and the electrons in these do not contribute to the orbital angular momentum.
Thus, for 3d complexes, the magnetic moment (m s) can be calculated by the following
spin formula:
S 4S(S 1) 2 S(S 1)m = + = +
Where S is the total spin of the complex. In the ground state, S is one-half the number
of unpaired electrons, n.
Therefore spin-only magnetic moment S n(n 2)m = +
Units of the m s is Bohr Magneton (B.M).
Thus, spin only formula can be used to calculate the magnetic moment from the value
of number of spin-free (unpaired ) electrons in the complex.
Number of unpaired electrons Spin-only magnetic moment, B.M.
1 1.7
2 2.8
3 3.9
4 4.9
5 5.9
Solved example :
Q.3 Calculate the magnetic moment of [Fe(CN)6]3-
.
Ans. : The electronic configuration of Fe+3 in [Fe(CN)6]-3 is
d2sp3 hybrid orbitals
3d 4s 4p 4d
CN CN CN CN CN CN
[Fe(CN)6]-3 has one unpaired electron hence the magnetic moment of [Fe(CN)6]
-3 is
S n(n 2) 1(1 1) 1.7 BMm = + = + =
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 24
Example to Solve :
Q.4) [CoF6]-4 is
a) outer orbital and diamagnetic
b) inner orbital and paramagnetic
c) inner orbital and diamagnetic
d) outer orbital and paramagnetic
Q.5) Ni(CO)4 is
a) square planar and paramagnetic
b) tetrahedral and diamagnetic
c) square planar and diamagnetic
d) tetrahedral and paramagnetic
DRAWBACKS OF VALENCE BOND THEORY
1. The valence bond theory does not take into account the splitting of the metals d energy
levels.
2. It is unable to account for or predict the relative energies of the different alternative
structures for a complex.
3. It is not helpful in the interpretation of the spectra of complexes.
4. If fails to explain the reaction rates and mechanisms of reactions with complexes.
5. It does not indicate why certain ligands form outer-orbital complexes whereas some
others form inner-orbital complexes.
6. It does not explain why certain 4- coordinated complexes are tetrahedral whereas others
are square-planar.
7. This theory does not account for the detailed magnetic properties of certain complexes.
For these complexes, experimentally determined magnetic moments are slightly higher
than the values theoretically calculated from the spin-only formula. This deviation is due
to the orbital contribution to the magnetic moment, which is not explained by this theory.
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 25
SOLVED EXAMPLES ON VBT
Q.6) On the basis of VBT, answer the following questions for 4-coordinated
complexes: [NiCl4]-2, [Ni(CN)4]
-2.
(i) What is the O.S. of the central metal atom/ion.
(ii) What type of hybridisation is involed?
(iii) What is the geometry and magnetic behaviour of the complex?
(iv) Calculate the value of magnetic moment?
Ans. : [NiCl4 ]-2
Since Cl- ion is mononegatively charged, the oxidation state of Ni is +2.
The hybridisation and geometry of [NiCl4]-2 is accounted on the basis of VBT as
follows:
e- configuration of Ni++
e- configuration of Ni++ during approach of 4Cl-weak ligands
e- configuration of Ni++ in[NiCl4]
-2 after gaining4 electron pairs fromCl- ligands
3d 4s 4p
sp3 hybridisation
sp3 hybridisation
Cl- Cl- Cl- Cl-
Thus, [NiCl4]-2 involves sp3 hybridisation and has tetrahedral geometry.
Since [NiCl4]-2 has two unpaired electrons, it is paramagnetic and its magnetic moment is
( )n n 2m = +
( )2 2 2 8 2.828 BM= + = =[Ni(CN)4 ]
-2
Since CN- is mono negatively charged ligand, O.S. of Ni atom is +2.
Thy hybridisation and geometry of [Ni(CN)4]-2 is accounted on the basis of VBT as
follows:
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 26
e- configuration of Ni++
e- configuration of Ni++ during approach of fourstrong CN- ligands
e- configuration of Ni++ in[Ni(CN)4]
-2 after gaining4 electron pairs fromCN ligands
3d 4s 4p
dsp2 hybridisation
dsp2 hybridisation
CN- CN- CN- CN-
Thus, [Ni(CN)4]-2 involves dsp2 hybridisation and has square planar geometry. Since
all the electrons in [Ni(CN)4]-2 are paired. it is diamagnetic.
Example to Solve :
Q.7] On the basis of VBT answer the following questions for the co-ordination
complexes.
A) [Ag(CN)2]- B) [HgI3]
- C) [Zn(NH)4]+2
D) [CoCl4]-2 E) [CoBr4]
-2 F) [MnCl4]-2
G) [Ni(CN)4]-2 H) [Fe(CN)6]
-3 I) [FeCl6]-3
J) [Fe(NH3)6]+2 K) [Mn(CN)6]
4- L) [Cr(CO)6]
M) [Co(CN)6]3- N) [Mn(H2O)6]
+2 O) [FeF6]-3
P) [CoF6]-3 Q) [MnF6]
3- R) [Cu(NH3)6]+2
S) [Zn(NH3)6]+2
i) What is the o.s. of the central metal atom.
ii) What type of hybridisation is involved.
iii) What is the geometry and magnetic behaviour of the complexes?
Q.8] Determine the hybridisation and geometry of [Ti(bpy)3]-.
Ans. : bpy is a strong, neutral bidentate ligand. Ti is present in the form Ti in the complex
and has configuration 3d3 4s2. The hybridisation and geometry of [Ti(bpy)3] is deduced by
using VBT as follows:
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 27
e- configuration of Ti-1
e- configuration of Ti-1 during approach of threebidentate strong (bpy) ligands
e- configuration of Ti- in[Ti(bpy)3]
- after gainingsix electron pairs from3 bpy ligands
3d 4s 4p
d2sp2 hybridisation
bpy bpy bpy
Since [Ti(bpy)3]- involves d2sp3 byubridisation, it has octahedral geometry.
Ti
bpy
bpy
bpy
-1
Example to solve :
Q.9] Determine the hybridisation and geometry of following complexes that are
formed from polydentate ligands.
A) [Fe(en)3]+2 B) [Ni(DMG)2] C) [Pt(gly)2]
[Hint - en, DMG and gly are strong bidentate ligands.]
Q.10] Determine the hybridisation and geometry of [Fe(H2O)5(NO)]+2
Ans. : In this complex, since NO which acts as ligand is present as NO+ ion, the central
metal atom is present as Fe+. This electronic configuration of Fe+ is 3d6 4s1=3d7. The
hybridisation and geometry of [Fe(H2O)5(NO)]+2 is deduced by using VBT as follows;
e- configuration of Fe+
e- configuration of Fe+ during approach of 5H2Oand one NO ligands
3d 4s 4p
sp3d2 hybridisation
4d
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 28
e- configuration of Fe+ in[Fe(H2O)5(NO)]
+2 after gaining six electron pairs fromfive H2O ligands and one NO ligand
sp3d2 hybridisation
Thus, [Fe(H2O)5(NO)]+ involes sp3d2 hybridisation and has octahedral geometry.
Fe
+
H2O
H2OH2O
H2O
H2O
NO
(Since H2O and NO+ are weak ligands, the distributionof 3d7 electrons in five 3d orbitals in
[Fe(H2O)5(NO)]+2 remains the same as it is for Fe+ ion in free state.)
Example to Solve :
Q.11] Determine the hybridisation and geometry of the following complexes.
[Fe(CN)5(NO)]-2
[Co(ONO)(NH3)5]+2
[Co(NH3)4Cl2]+1
[Cr(NH3)2(SCN)4]+1
[Hint : If the complex contains at least one strong ligand then the e- distribution will be against
the Hunds rule of maximum multiplicity. See*1 for more details.]
Q.12) The magnetic moment value of [Mn(CN)6]3- ion is 2.8 BM. Predict the type
of hybridisation and geometry of the ion.
Ans. : We know that m is given by:
( )n n 2m = +
or ( )2.8 n n 2= +or (2.8)2 = n(n+2)
or n2+2n - 7.84 = 0
or n2+2n - 8 = 0
or (n+4)(n-2) = 0
\ n = 4, +2
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 29
Thus, [Mn(CN)6]3- ion has two unpaired electrons (n=2). In the given complex ion,
Mn is present as Mn+3 which is a 3d4 ion. Since C.N. of Mn3+ = 6, [Mn(CN)6]3- ion has
octahedral geometry which may arise either from d2sp3 hybridisation (inner orbital) or from
sp3d2 hybridisation (Outer orbital) as shown in Fig.1. Now since d2sp3 hybridisation gives
n = 2 and sp3d2 hybridisation gives n=4, [Mn(CN)6]3- ion has inner orbital octahedral
geometry which results from d2sp3 hyubridisation.
Mn3+ ion (3d44s04p04d0)
[Mn(CN)6]3- ion (d2sp3)
[Mn(CN)6]3- ion (sp3d2)
3d 4s 4p
d2sp3 hybridisation : Innerorbital octahedral geometry
4d
sp3d2 hybridisation: Outerorbital octahedral geometry
(n=4)
(n=2)CN- CN- CN- CN- CN- CN-
CN- CN- CN- CN- CN- CN-(n=4)
Fig. 1. d2sp3 and sp3d2 hybridisation of Mn3+ ion in [Mn(CN)6 ]3- ion.
Q.13) Magnetic moment value of [MnBr4]2- ion is 5.9 B.M. On the basis of VBT,
predict the type of hybridisation and geometry of the ion.
Ans. : We know that,
( )n n 2 B.Mm = +If we put n=5 in the above equation, we get m = 35 B.M. = 5.91 B.M. Thus
[Mn Br4]2- ion has five unpaired electrons (n=5). In [MnBr4]
2- ion, the central atom is Mn2+
ion which is 3d5 ion. Now since C.N. of Mn2+ = 4, [MnBr4]2- ion may have either square
planar (dsp2 hybridisation) or tetrahedral (sp3 hybridisation) geometry as shown below;
Mn2+ ion (3d54s04p0)
3d 4s 4p
(n = 5)
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Notes for NET & SET - Chemical Sciences
Chemistry of Transition Elements 30
[MnBr4]2-
(Square planar)
[MnBr4]2-
(Tetrahedral)
dsp2 (square planar)
Br- Br- Br- Br-
Br- Br- Br- Br-
sp3 (tetrahedral)
(n = 3)
(n = 5)
Square planar and tetrahedral geometries of [MnBr4 ]2- ion.
Since dsp2 hybridisation (square planar) gives n=3 and sp3 hybridisation (tetrahedral)
gives n=5, [MnBr4]2- ion has tetrahedral geometry and not square planar. Alternatively, since
Br- given is a weak field ligand, [MnBr4]-2 ion is tetrahedral in geometry. Tetrahedral
complexes are given by weak field ligands, since these are HS complexes.
Q.14) Explain : [Co(NH3)6]+3 is diamagnetic while [CoF6]
-3 is strongly paramagnetic.
Ans. : [Co(NH3)6]+3 has d2sp3 hybridisation giving octahedral configuration where all the
electrons are paired so that the molecule is diamagnetic. In case of [CoF6]-3 , F is a weak
ligand so that all the electrons cannot be paired and we find four unpaired electrons so that
the complex is strongly paramagnetic. It is a case of outer orbital complex since the outer
4d orbital is involved in bybridisation.
[Co(NH3)6]+3
[CoF6]-3
3d 4s 4p
sp3d2 hybridisation
d2sp3 hybridisation
3d 4s 4p 4d