JSS INTERNATIONAL SCHOOL, DUBAI

COORDINATION COMPOUNDS – NOTES

Types of salts:

a.   Simple salts: Salts formed by the neutralization of an acid with a base.

They contain one type of anion and one type of cation.
Eg: NaCl

b.   Double salts: These are molecular compounds which are formed by the
evaporation of solution containing two (or) more salts in stoichiometric
proportions.
Double salts retain their properties only in solid state. They are also called as
lattice compounds.
Example K2 SO4 . Al2 (SO4)3 . 24H2O - Potash alum
FeSO4 . (NH4)2 SO4 . 6H2O - Mohr’s salt
K2 SO4 . Al2 (SO4)3 . 24H2O → 2K+ + 2Al3+ + 4SO4 2- + 24H2O. Double salts give
the test of all their constituent ions in solution.

c.   Complex salts or Coordination compounds:

A compound in which a central metal atom or ion is bonded to anions, cations or
neutral molecules capable of donating one or more pairs of electrons by
coordinate bonds is known as coordinate compound.
Eg: Fe(CN)2 + 4KCN → K4 [Fe(CN)6]
Here, Fe is bonded to the six cyanide ions by coordinate bonds. When dissolved in
water, the compound forms four K+ and one [Fe(CN)6]4-, ferrocyanide ions. The
ferrocyanide ion doesn’t further ionize. Hence, the solution tests negative for
both ferrous and cyanide ions.

Terms used in coordination compounds:

a.   Central metal atom or ion: The atom or ion that is capable of accepting one or
more electron pairs from an electron rich species is called the central metal
atom or ion.
Generally, d-block metal atoms or ions act as central metal atoms/ions as they
have vacant d-orbitals that can accommodate the electron pairs.
They are Lewis acids as they accept electron pairs.

b.   Ligand: An ion or a neutral molecule capable of donating one or more electron

pairs to the central metal atom or ion is called a ligand.
Types of ligands:

i.   Based on the nature :

1.   Anionic ligands: These are negatively charged ions.
Eg: Cl-, C2O42-, CH3COO-, NO2-etc.
2.   Cationic ligands: These are positively charged ions capable of donating
electron pairs.
Eg: NO+
3.   Neutral ligands: Neutral molecules with one or more lone pairs of
electrons.
Eg: H2O, NH3, CO, ethylene diamine etc

ii.   Based on the number of electron pairs donated:

1.   Monodentate ligands:
Ligands capable of donating one pair of electrons to the central metal
atom or ion.
Eg: Cl-, H2O, NH3, NO+ etc
2.   Bidentate ligands:
Ligands capable of donating two pairs of electrons to the central metal
atom or ion.
Eg: C2O42-, ethylene diamine etc
3.   Polydentate ligands:
Ligands capable of donating more than two pairs of electrons to the
central metal atom or ion.

Eg: Tridentate
 Coordination sphere:
The central metal atom or ion and the ligands, enclosed with in square bracket is
called as coordination sphere. This represents a single constituent unit. The
ionisable species are placed outside the square brackets.

Coordination number:
 The coordination number of a metal ion in a complex can be defined as the
number of monodentate ligands to which the metal is directly bonded by
coordinate bonds.
Numerically coordination number represents the total number of the chemical
bonds formed between the central metal ion and the donor atoms of the ligands.
For example in K4 [Fe(CN)6], the coordination number of Fe(II) is 6 and in
[Cu(NH3) 4 ]SO4 the coordination number of Cu(II) is 4. Charge on the complex
ion Charge on the complex ion is equal to the sum of the charges on the metal ion
and their ligands.

Types of complexes:
1.   Anionic complex: Complexes in which the anion is a complex ion.
Eg: K4 [Fe(CN)6]
2.   Cationic complex: Complexes in which the cation is a complex ion.
Eg: [Cu(NH3) 4 ]SO4
3.   Neutral complex
Eg: [Ni(CO)4]

IUPAC nomenclature of coordination compounds:

Naming ligands:

Names of anionic ligands end with ‘o’

 Names of cationic ligands end with ‘ium’.

Eg: NO+ - nitrosonium

There is no specific way for naming neutral ligands.

Rules for naming complexes:

1.   The cation is named first, followed by the name of the anion.

2.   In the name of the complex ion, the ligands are named first, followed by the
name of the central metal atom.
3.   The number of ligands is represented by mono, di, tri etc for simplex ligands and
mono, bis, tris, tetrakis etc (1,2,3,4…) for complex ligands.
4.   If more than one ligand is present, the ligands are arranged in the alphabetic
order of their names.
5.   The whole of the complex ion’s name is written as one word in small letters.
6.   If the complex is anionic, Latin name is used for the metal if any, and the name
ends with ‘ate’. Common names are used for cationic complexes.
7.   The oxidation number of the metal is indicated in roman numerals after the
name of the metal in parenthesis. ( )

Eg: K4 [Fe(CN)6] – potassium hexacyanoferrate(II)

[Co(en)3]Cl3 - tris(ethylenediamine)cobalt(III) chloride

[Co(NH3)3(NO2)3] - triamminetrinitrocobalt(III)

[Co (NH3)5Cl]2+ - pentaamminechlorocobalt(III) ion

[Pt(NH3)4] [CuCl4] - tetraammineplatinum(II) tetrachlorocuprate (II)

Isomerism in coordination compounds:

1.   Ionization isomerism:
The phenomenon by which two coordination compounds have the same molecular
formula but form different ions in solution is called ionization isomerism and the
compounds are called ionization isomers.

Example:
[Co(NH3)5Br]SO4 à [Co(NH3)5Br]+2 + SO4-2
pentaamminebromocobalt(III) sulphate

(Gives test for sulphate – white ppt with BaCl2 solution)

[Co(NH3)5SO4]Br à [Co(NH3)5 SO4]+ + Br-

pentaamminesulphatocobalt(III) bromide

(Gives test for bromide – pale yellow ppt with AgNO3 solution)

2.   Hydrate isomerism:
The phenomenon by which two coordination compounds have the same molecular
formula but have different number of water molecules as ligands and water of
hydration is called hydrate isomerism and the compounds are called hydrate
isomers.

Example:
[Cr(H2O)4Cl2]Cl.2H2O - Bright green
Tetraaquadichlorochromium(III) chloride dehydrate

[Cr(H2O)5Cl]Cl2.H2O - grey-green
Pentaaquachlorochromium(III) chloride monohydrate

[Cr(H2O)6]Cl3 - Violet
Hexaaquachromium(III) chloride

3.   Linkage isomerism:
The phenomenon by which two coordination compounds have the same
molecular formula and same ligands but differ in the ligating atom is called
linkage isomerism and the compounds are called linkage isomers.
This type of isomerism is found in complexes of ambidentate ligands.

Example:
[Co(NH3)5ONO]Cl2 - the nitrito isomer - red colour
pentaamminenitritocobalt(III) chloride - O is the ligating atom

[Co(NH3)5 NO2]Cl2 - the nitro isomer - yellow colour

pentaamminenitrocobalt(III) chloride - N is the ligating atom
 4.   Coordination isomerism:
This type of isomerism is seen in bimetallic complexes, where both anionic and
cation parts are complex ions. The distribution of ligands between the two
complex ions varies.
Example:
1.   [Co(NH3)6] [Cr(CN)6] - hexamminecobalt(III) hexacyanochromate(III) and
[Cr(NH3)6] [Co(CN)6] - hexamminechromium(III) hexacyanocobaltate(III)

2.   [Pt(NH3)4] [CuCl4] - tetraammineplatinum II) tetrachlorocuprate(II) and

[Cu(NH3)4] [PtCl4] - tetraamminecopper(II) tetrachloroplatinate(II)

Werner’s Theory
Alfred Werner (1866-1919) French born Swiss chemist founded the modern
theory on coordination compounds. He won the Nobel Prize for chemistry in
1913for his pioneering work in coordination chemistry. He is considered as the
“Father of coordination chemistry”.

Postulates of Werner’s theory

1) Every metal atom has two types of valencies
i) Primary valency
ii) Secondary valency
2) The primary valency corresponds to the oxidation state of the metal ion.
The primary valency of the metal ion is ionisable and is always satisfied by
negative ions. It is non-directional.
3) Secondary valency corresponds to the coordination number of the metal ion or
atom. The secondary valency is non ionisable and may be satisfied by either
negative ions or neutral molecules.
4) The molecules or ions that satisfy secondary valencies are called ligands. The
ligands are oriented in definite directions in space. So the secondary valency is
directional.
5) Sometimes, the same ion may satisfy both primary and secondary valency.

Werner’s representation
Werner represented the first member of the series [Co(NH3)6]Cl3 as follows.
In this representation, the primary valency (dotted lines) is satisfied by the three
chloride ions. The six secondary valencies (solid lines) are satisfied by the six
ammonia molecules.
 Valence bond theory:
1.   [Fe(CN)6]3-
Fe – At. No. 26 – [Ar]3d64s2
Fe3+ - [Ar] 3d5

3d 4s 4p
When the strong ligand CN approaches the ferric ion, the unpaired electrons in
-

the 3d orbitals pair up against Hund’s rule.

3d 4s 4p

d2sp3 hybridization
The two empty 3d, one 4s and three 4p orbitals undergo d2sp3 hybridization. The
orbitals of the ligands containing lone pair of electrons overlap with the empty
hybrid orbitals of the metal ion forming coordinate bonds.
Geometry – Octahedral
Magnetic character – Since it has an unpaired electron, it is paramagnetic in
nature.

Structure

CN 3-
NC CN

Fe

NC CN
CN

Since the inner d-orbitals are involved in hybridization, it is an inner orbital complex.

2.   [Co(NH3)6]3+
Co – At. No. 27 – [Ar]3d74s2
Co3+ - [Ar] 3d6

3d 4s 4p
 When the strong ligand NH3 approaches, the unpaired electrons in the 3d
orbitals pair up against Hund’s rule.

3d 4s 4p

d2sp3 hybridization
The two empty 3d, one 4s and three 4p orbitals undergo d2sp3 hybridization. The
orbitals of the ligands containing lone pair of electrons overlap with the empty
hybrid orbitals of the metal ion forming coordinate bonds.
Geometry – Octahedral
Magnetic character – Since it has no unpaired electron, it is diamagnetic in
nature.

NH3 3+
H3 N NH3

Co

H3 N NH3
NH3

Since the inner d-orbitals are involved in hybridization, it is an inner orbital complex.

3.   [Co(F)6]3-
Co – At. No. 27 – [Ar]3d74s2
Co3+ - [Ar] 3d6

3d 4s 4p

The weak ligand F- cannot pair the 3d electrons against Hund’s rule.

3d 4s 4p 4d

sp3d2 hybridization
The one empty 4s, three 4p and two 4d orbitals undergo sp3d2 hybridization. The
orbitals of the ligands containing lone pair of electrons overlap with the empty
hybrid orbitals of the metal ion forming coordinate bonds.
 Geometry – Octahedral
Magnetic character – Since it has unpaired electrons, it is paramagnetic in
nature.

F 3-
F F

Co

F F
F

Since the outer d-orbitals are involved in hybridization, it is an outer orbital complex.
Since it has many unpaired electrons, it is highly paramagnetic and is called a high
spin complex.

4.   Ni(CO)4
Ni – At. No. 28 – [Ar]3d84s2

3d 4s 4p
When the strong ligand CO approaches, the 4s electrons shift to 3d orbitals and
pair up against Hund’s rule.

3d 4s 4p

sp3 hybridization
The empty 4s and three 4p orbitals undergo sp3 hybridization. The orbitals of the
ligands containing lone pair of electrons overlap with the empty hybrid orbitals
of the metal forming coordinate bonds.
Geometry – Tetrahedral
Magnetic character – Since it has no unpaired electron, it is diamagnetic in
nature.
CO

Ni
CO
OC
CO
Crystal Field Theory:

Developed by Bethe and Van Vleck (1929 – 32).

This model explains the bonding interaction between the metal atom or ion and the
ligands. When the ligands approach the metal ion/atom, the degeneracy of the d-
orbitals is broken.

The electrons in the d orbitals of the central metal ion and those in the ligand repel
each other due to repulsion between like charges. Therefore, the d electrons closer to
the ligands will have a higher energy than those further away, which results in the d
orbitals splitting in energy. This splitting is called crystal field splitting and is affected
by:
•   the nature of the metal ion
•   the metal's oxidation state (a higher oxidation state leads to a larger
splitting)
•   the arrangement of the ligands around the metal ion
•   the nature of the ligands surrounding the metal ion

Based on the orientation of ligands, two types of splitting is possible.

Octahedral splitting:

In octahedral complexes, the dx2-y2 and dz2 experience greater repulsion than the dxy, dyz
and dzx orbitals. Therefore, the energy of dx2-y2 and dz2 increases forming the eg set and
that of dxy, dyz and dzx orbitals decreases, giving rise to the t2g orbitals. The difference in
energies of the two sets is called crystal field stabilization energy and is equal to 10Dq.
It is denoted as Δo.
 Tetrahedral splitting:

In tetrahedral complexes, the dxy, dyz and dzx orbitals experience greater repulsion than
the dx2-y2 and dz2 orbitals. Therefore, the energy of dxy, dyz and dzx orbitals increases
giving rise to the t2g orbitals and that of dx2-y2 and dz2 decreases, forming the eg set. The
difference in energies of the two sets is called crystal field stabilization energy and is
equal to 10Dq. It is denoted as Δt.

Colour of complexes based on crystal field theory:

The colour of a complex is due to the d-d transitions. When light is incident on a
coordination compound, light of certain wavelength is absorbed and the electron from
the lower d-orbitals jumps to the higher d-orbitals. The wavelength of absorbed light
depends on the energy difference between the two energy levels. The rest of the light is
transmitted and is called the complementary colour of the absorbed colour.

Eg: Copper sulphate is blue because it absorbs red colour from white light.

If the central metal atom or ion has no unpaired electrons, or has d0 or d10
configuration, d-d transitions are not possible. Hence, such complexes are colourless.
High spin and low spin complexes:

The magnitudes of crystal field splitting energy and pairing energy determine whether
a complex is high spin or low spin.

If crystal field splitting energy is less than pairing energy, ie., Δ<P, the complex will be
a high spin complex. Weak ligands form high spin complexes.

If Δ>P, the complex will be low spin. Strong ligands form low spin complexes.

Stability constant:

It is defined as the measure of resistance to the replacement of a ligand by another.

Consider the example of [Cu(NH3)4]2+. The equilibrium between the undissociated ions
and dissociated components can be expressed as follows.

[Cu(NH3)4]2+ Cu2+ + 4NH3

The equilibrium constant, called instability constant is given by

!"#$ ]['()]*
Ki =
[+, -.) /]#$ ]

The reciprocal of this is called stability constant, for the equilibrium

Cu2+ + 4NH3 [Cu(NH3)4]2+

is given as,
[+, -.) /]#$
stability constant, K =
!"#$   ]['()]*

Greater the stability constant, more stable is the complex.