2.
CENTRAL ATOM/ION :
COORDINATION OF In a co-ordination entity, the atom/ion to
COMPOUNDS which a fixed number of ions/neutral mole-
cules are attached is called the central atom
or ion.
3. LIGANDS :
Important Topics The ions or neutral molecules bound to the
• Werner's theory central atom/ion in the coordination entity
are called ligands.
• IUPAC nomenclature
For a species to act as ligand, it can donate
• Isomerism in Coordination Compounds atleast one pair of electron to the central
• VB theory atom.
• Magnetic properties and Shapes The atom of the ligand which is directly bond-
ed to the central atom is called donor atom.
• CFT
• Colour in coordination Compounds
• Metal carbonyls Types of ligands
Based on the number of donor atoms of the
ligand that binds to a metal ion or atom, the
Werner’s Theory of Coordination ligands are classified as,
Compounds i) Monodentate or unidentate ligand:
A ligand that binds to the central atom/
The main postulates are: ion through a single donor atom, is said to
be unidentate ligand.
1. In coordination compounds metals show
two types of valences -primary and sec- Eg: Cl-, Br-, I-, OH-, H2O, NH3, CN-, NC-, SCN- etc.
ondary. ii) Bidentate (Didentate) ligands :
2. The primary valences are normally ionisa- A ligand that binds to the central atom
ble and are satisfied by negative ions. through two donor atoms is called a bi-
3. The secondary valences are non ionisable. dentate ligand.
These are satisfied by neutral molecules Eg: Ethane-1,2-diamine or ethylenediamine
or negative ions. The secondary valence is 2-
(H2NCH2CH2NH2) Oxalate ion (C2O4 )
equal to the coordination number and is
fixed for a metal. iii) Polydentate ligand :
4. The ions/groups bound by the secondary A ligand that binds to the central atom
linkages to the metal have characteristic through more than two donor atoms is
spatial arrangements corresponding to dif- called polydentate ligand.
ferent coordination numbers. Eg: Triethylamine ammonia [N(CH2-CH2-NH2)3]
Ethylenediamine tetraacetate ion (EDTA)
Some definitions / Types of Ligands LIGANDS ARE ALSO CLASSIFIED AS:
i) Ambidentate ligands :
1. CO-ORDINATION ENTITY :
They are unidentate ligands which contain
The central metal atom or ion and ligands more than one donor atoms. They can co-
form a co-ordination entity. ordinate through two different atoms.
Eg:NO2-, ONO, CN-, NC-, SCN-, NCS-
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ii) Chelating Ligands : 8. Oxidation number of central atom :
Di or polydentate ligands can bind to the
The oxidation number of the central atom in
central atom through two or more donor at-
a complex is defined as the residual charge
oms and form ring complexes.
on it.
Such complexes are called chelates and
such types of ligands are said to be chelating 9. Homoleptic Complexes :
ligands.
Complexes which contain only one type of
Eg : Ethane-1,2-diamine, Oxalato
ligand are called homoleptic complexes.
Eg:[Co(NH3)6]3+, [Fe(CN)6]4+) etc.
10. Heteroleptic complexes :
Types of ligands Complexes which contain more than one
Monodentate Ambidentate Didentate lig- type of ligands are called heteroleptic com-
ligands ligands ands plexes.
Eg:[Co(NH3)4Cl2]+, [Cu(NH3)2Cl2] etc.
NH3 - ammine CN- - cyano en - ethane-1,2-
diamine
NC- - isocyano 2-
H2O - aqua C2O4 - Oxalato
-
CO - carbonyl NO2 - nitriti-N d block elements satisfies both primary and
CH3-NH2 - secondary Valances due to
ONO -nitrito-O
-
Polydentate
methylamine
ligands • small size of the metal ion
OH- - hydroxo SCN-- thiocyna-
EDTA - ethylen- • high ionic charges
to
Cl-- chlorido ediaminetetra • Availability of d orbital
NCS- - isothiocy- acetic acid
Br-- bromido
2-
anato
SO4 - sulphato
Nomenclature of Coordination
4. DENTICITY
Compounds
The number of donor atoms of a particular li-
gand that are directly bonded to the central
atom is called denticity. The following rules are used while naming
co-ordination compounds:
For unidentate ligands, the denticity is 1 , for di-
dentate ligands it is 2 and so on. (i) The cation is named first in both positive-
ly and negatively charged co-ordination
5. Co-ordination number : entities.
The co-ordination number (C.N) of a metal ion (ii) The ligands are named in alphabeti-
in a complex can be defined as the total num- cal order before the name of the central
ber of ligand donor atoms to which the metal atom/ion.
is directly bonded.
(iii) Prefixes mono, di, tri, etc., are used to indi-
6. Co-ordination sphere : cate the number of individual ligands.
The central atom/ion and the ligands attached (iv) Oxidation state of the metal is indicated
to it are enclosed in square bracket and is col- by Roman numeral in simple bracket.
lectively termed as the co-ordination sphere.
(v) If the complex ion is a cation, the central
7. Counter ion : atom is named same as the element. If
The ions outside the square bracket are called the complex ion is an anion, the name of
counter ion. the metal ends with the suffix-ate.
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Ionisation isomerism
It arises due to the inter change of ions between
Q) Write the IUPAC name of the following the inside and outside of co-ordination sphere.
They give different types of ions in aqueous
a) [Cr(NH3)4 (H2O)2]Cl3
solution.
b) K2[Fe(CN)4]
Eg: [Cr(NH3)5Cl]SO4 [Cr(NH3)5SO4]Cl
c) [Ni(CO)4]
Solvate/Hydrate isomerism
Solvate isomers differ in the no. of solvent mol-
ecule which are directly bonded to the metal
ion as ligand.
Eg: [Cr(H2O)6]Cl3 [Cr(H2O)5Cl]Cl2.H2O
Isomerism in Coordination Compounds
Compounds that have the same molecular stereo isomerism
formula but different structural formula or spa-
tial arrangement of atoms are called isomers
and the phenomenon is called isomerism.
Structural isomerism Sterio isomerism Square planer
Linkage isomerism Geometrical
isomerism
Coordination isomerism
Optical
Ionisation isomerism isomerism
Solvate isomerism Octahedral
Tetrahedral
Structural isomerism
Linkage isomerism Stereo isomerism
It arises in a co-ordination compound contain- Geometrical isomerism (In Octahedral com-
ing ambidentate ligand, which can bind to the plexes)[Ma3 b3]
central atom through more than one donor at-
oms. Facial (fac) Meridian (mer)
Eg: [Co(NH3)5NO2]Cl2 [Co(NH3)5ONO]Cl2 3 donor atoms of The position of 3 do-
same ligands occu- nor atoms of same
Coordination Isomerism py adjacent posi- ligands around the
tions at the corner meridian
If both anionic and cationic parts are complex-
es, the isomerism arises due to the interchange a b
of ligands between cationic and anionic enti- a b a a
ties. M M
a b a b
Eg: [Co(NH3)6][Cr(CN)6] [Cr(NH3 )6][Co(CN)6] b b
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Type of
Geometrical isomerism (In Octahedral com- C. No. Geometry
hybridisation
plexes) [Ma4b2]
2 sp Linear
cis trans
b b 3 sp2 Trigonal planar
a b a a
M M sp3 Tetrahedral
a a a a
a b
4
dsp2 Square planar
Geometrical isomerism (In Square planar
complexes) [Ma2b2] dsp3 Trigonal bipyramidal
cis trans sp3 d Square pyramidal
b a b a Octahedral
d2sp3
M M 6 (Inner orbital)
b a a b
Octahedral
sp3d2
(Outer orbital)
Sc Ti V Cr Mn Fe Co Ni Cu Zn
sterio isomerism 21 22 23 24 25 26 27 28 29 30
Optical isomerism
[FeF6]3-
[Ma2b4]
a a
b a a b
M M
b b b b
b b
Bonding in Coordination Compounds
[Fe(CN)6]3-
Valence Bond Theory
Hybridisation Strong field ligands
CO,NH3, CN-, NCS-,
Inner / Outer Configu-
ration EDTA, en
Geometry Weak field ligands
F-, Cl-Br-, 1-, SCN-,
Low / High spin
2-
C2O4
Magnetic properties
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[NiCl4]2-
CO OC CO
Ni Fe CO
OC CO CO OC CO
Ni(CO)4 Fe(CO)5
Tetrahedral Triginal bipyramidal
CO CO CO
CO CO CO CO
Cr OC Mn Mn CO
CO CO CO CO CO CO
CO
[Ni(CN)4]2-
Cr(CO)6 [Mn2(CO)10]
Octahedral Octahedral
OC CO CO
CO
Co Co
OC OC CO OC
[Co2(CO)8]
Metal Carbonyls
These are homoleptic carbonyls
Mononuclear carbonyls : Crystal Field splitting Theory (CFT)
(i) V(CO)5 → Pentacarbonylvanadium (0)
• The crystal field theory (CFT) is an electro-
(ii) Cr(CO)6 → Hexacarbonylchromium (0) static model which considers the metal-li-
(iii) Mo(CO)6 → Hexacarbonylmolybdenum (0) gand bond as purely ionic.
(iv) W(CO)6 → Hexacarbonyltungsten (0) • The ligands are treated as point charges in
case of anions or as dipoles in case of neu-
(v) Ni(CO)4 → Tetracarbonylnickel (0) tral molecules.
(vi) Fe(CO)5 → Pentacarbonyliron (0) • The five d-orbitals in an isolated gaseous
metal atom/ion have same energy, ie, they
Polynuclear carbonyls : are degenerate.
(i) Co2(CO)8 → Octacarbonyldicobalt (0) • But when the negative field is due to ligands
(ii) CO4(CO)12 → Dodecacarbonyltetracobalt (0) in a complex, the degeneracy of the d orbit-
als is lost.It results in splitting of the d orbitals.
(iii) Mn2(CO)10 → Decacarbonyldimanganese (0) This splitting of d-orbitals is termed as Crys-
tal field splitting.
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Crystal field splitting in Octahedral Spectrochemical series
Crystal field Ligands are arranged in the order of increasing
field strength called spectrochemical series.
2-
I- < Br- < SCN- < Cl- < S2- < F- < OH- < C2O4 < H2O
< NCS- < edta 4- < NH3 < en < NO2 - < CN- < CO
Strong
Weak field Increasing order of CFSE (∆0) field
ligands ligands
Limitations of Crystal Field Theory
CFSE ∆0 1. From the assumptions, that the ligands are
point charges, it follows that anionic ligands
should exert the greatest splitting effect. But
Crystal field splitting in Tetrahedral the anionic ligands actually are found at the
low end of the spectrochemical series.
Crystal field 2. It does not take into account the covalent
character of bonding between the ligand
and the central atom.
Difference between a Double salt
and a Complex
Both double salts as well as complexes are
formed by the combination of two or more sta-
ble compounds in stoichiometric ratio.
Double salts dissociate into simple ions com-
CFSE ∆t pletely when dissolved in water.
Eg : Carnallite -KCl.MgCl2 . 6H2O
Mohr’s salt - FeSO4.(NH4)2SO4 . 6H2O
Colour in Coordination compounds Potash alum - KAl(SO4)2.12H2O
Complex compounds do not dissociate into
The colour of the coordination compounds is
simple ions completely when dissolved in wa-
due to d-d transition of the electron.
ter.
In the absence of ligands, crystal field splitting
Eg: K4[Fe(CN)6]
does not occur and hence the substance is
colourless. [Co(NH3)5Cl]Cl2)
Colour of the Complex is the complementary
colour of the light absorbed by the complex.
3+
[Ti (H2O)6] - Violet
2+
[Cu (H2O)4] - Blue
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