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Coordination Compounds
Coordination compounds are those addition molecular compounds which retain their identity in
solid state as well as in dissolved state. In these compounds. the central metal atom or ion is
linked by ions or molecules with coordinate bonds. e.g., Potassium ferrocyanide, K4 [Fe(CN)6].

Double Salts

These are the addition molecular compounds which are stable in solid state but dissociate into
constituent ions in the solution. e.g., Mohr’s salt, [FeSO4·(NH4)2SO4.6H2O] get dissociated
into Fe2+, NH4+and SO42ꟷ
Terms Related to Coordination Compounds

1. Complex ion or Coordination Entity

It is an electrically charged species in which central metal atom or ion is surrounded by number
of ions or neutral molecules.

(i) Cationic complex entity It is the complex ion which carries positive charge. e.g.,
[Pt(NH3)4]2+

(ii) Anionic complex entity It is the complex ion which carries negative charge. e.g.,
[Fe(CN)6]4-

2. Central Atom or Ion

The atom or ion to which a fixed number of ions or groups are bound is called central atom
or ion. It is also referred as Lewis acid. e.g., in [NiCl2(H2O)4]. Ni is central metal atom. It is
generally transition element or inner-transition element.

3. Ligands
Ligands is electron donating species (ions or molecules) bound to the Central atom in the
coordination entity.

These may be charged or neutral. Ligands are of the following types:

(i) Unidentate: It is a ligand, which has one donor site, i.e., the ligand bound to a metal
ion through a single donor site. e.g., H2O, NH3, etc.
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(ii) Didentate: It is the ligand. which have two donor

sites.

(iii) Polydentate: It is the ligand, which have several

donor sites. e.g., [EDTA]4- is hexadentate ligand.

(iv) Ambidentate ligands: These are the monodentate ligands

which can ligate through two different sites, e.g., NO2ꟷ,
SCNꟷ, etc.

(v) Chelating ligands: Di or polydentate ligands cause cyclisation around the metal atom
which are known as chelate, such ligands use two or more donor atoms to bind a single metal
ion and are known as chelating ligands.

More the number of chelate rings, more is the stability of complex.

The stabilisation of coordination compounds due to chelation is known as chelate effect.

The number of ligating groups is called the denticity of the ligand.

4. Coordination Number

It is defined as the number of coordinate bonds formed by central metal atom, with the ligands.
e.g., in [PtCI6]2-, Pt has coordination number 6.

In case of monodentate ligands,

Coordination number = number of ligands

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in polydentate ligands.

Coordination number = number of ligands x denticity

5. Coordination Sphere

The central ion and the ligands attached to it are enclosed in square bracket which is known as
coordination sphere. The ionisable group written outside the bracket is known as counter ions.

6. Coordination Polyhedron

The spatial arrangement of the ligands which are directly attached to the central atom or ion, is
called coordination polyhedron around the central atom or ion.

7. Oxidation Number of Central Atom

The charge of the complex if all the ligands are removed along with the electron pairs that are
shared with the central atom, is called oxidation number of central atom.

e.g., [Cu(CN)4]3ꟷ , oxidation number of copper is +1, and represented as Cu(I).

Types of Complexes

1. Homoleptic complexes

Complexes in which the metal atom or ion is linked to only one kind of donor atoms, are called
homoleptic complexes e.g., [Co(NH3)6]3+

2. Heteroleptic complexes

Complexes in which the metal atom or ion is linked to more than one kind of donor atoms are
called heteroleptic complexes e.g., [Co(NH3)4CI2]+

3. Labile and Inert complexes

Complexes in which the ligand substitution is fast are known as labile complexes and in which
ligand substitution is slow, are known as inert complexes.

IUPAC Naming of Complex Compounds

Naming is based on set of rules given by IUPAC.

1. Name of the compound is written in two parts (i) name of cation, and (ii) name of anion.

2. The cation is named first in both positively and negatively charged coordination complexes.

3. The dissimilar ligands are named in an alphabetical order before the name of central metal
atom or ion.
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4. For more than one similar ligand. the prefixes di, tri, tetra, etc are added before its name. If
the di, tri, etc already appear in the complex then bis, tris, tetrakis are used.

5. If the complex part is anion, the name of the central metal ends with suffix ‘ate’.

6. Names of the anionic ligands end in ‘0’, names of positive ligands end with ‘ium’ and names
of neutral ligands remains as such. But exception are there as we use aqua for H2O, ammine for
NH3, carbonyl for CO and nitrosyl for NO.

7. Oxidation state for the metal in cation, anion or neutral coordination compounds is indicated
by Roman numeral in parentheses.

8. The name of the complex part is written as one word.

9. If the complex ion is a cation, the metal is named same as the element.

10. The neutral complex molecule is named similar to that of the complex cation.

Some examples are

(i) [Cr(NH3)3(H2O)3]Cl3

triamminetrichlorochromium(III) chloride

(ii) [Co(H2NCH2CH2NH2)3]2(SO4)3

tris(ethane-l,2-diamine)cobalt(III) sulphate

(iii) [Ag(NH3)2] [Ag(CN)2]

diamminesilver(I) dicyanoargentate(I)

(iv) K4 [Fe(CN)6]

potassium hexacyanoferrate(II)

Isomerism in Coordination Compounds

Coordination compounds exhibit the following types of isomerism:

1. Structural Isomerism

In this isomerism. isomers have different bonding pattern. Different types of structural isomers
are

(i) Linkage isomerism This type of isomerism is shown by the coordination compounds
having ambidentate ligands. e.g.,
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[Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl or pentaamminenitrito- N Cobalt (III) chloride

and pentaamminenitrito-O’Cobalt (III) chloride.

(ii) Coordination isomerism This type of isomerism arises from the interchange of ligands
between cationic and anionic complexes of different metal ions present in a complex, e.g.,

[Cr(NH3)6)] [CO(CN)6]and [CO(NH3)6] [Cr(CN)6]

(iii) Ionisation isomerism This isomerism arises due to exchange of ionisable anion
with anionic ligand. e.g.. [Co(NH3)5SO4]Br (red) and [Co(NH3)5Br]SO4 (violet)

(iv) Solvate isomerism This is also known as hydrate isomerism. In this isomerism, water is
taken as solvent. It has different number of water molecules in the coordination sphere and
outside it. e.g..
[Co(H2O)6]CI3, [Co(H2O)4C12]Cl·2H2O, [Co(H2O)3Cl3]. 3H2O

2. Stereoisomerism
Stereoisomers have the same chemical formula and chemical bonds but they have different
spatial arrangement. These are of two types :

(i) Geometrical isomerism Geometrical isomers are of two types i.e., cis and trans isomers.
This isomerism is common in complexes with coordination number 4 and 6.

Geometrical isomerism in complexes with coordination number 4

(i) Tetrahedral complexes do not show geometrical isomerism.

(ii) Square planar complexes of formula

[MX2L2] (X and L are unidentate) show
geometrical isomerism. The two X ligands may
be arranged adjacent to each other in a cis
isomer, or opposite to each other in a trans
isomer, e.g.,
(iii) Square planar complex of the type [MABXL] (where A, B, X, L, are unidentate ligands)
shows three isomers, two cis and one trans.

e.g., [Pt(NH3) (Br)(Cl)(Py)].

Geometrical isomerism in complexes with

coordination number 6
Octahedral complexes of formula [MX2L4], in which the
two X ligands may be oriented cis or trans to each other,
e.g., [Co(NH3)4Cl2)]+.
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Octahedral complexes of formula [MX2(A-A)2], where

X are unidentate ligands and (A-A) are bidentate ligand.
form cis and trans isomers, e.g., [CoC12(en)2]+

In octahedral complexes of formula [MA3X3], if three

donor atoms of the same ligands occupy adjacent
positions at the corners of an octahedral face. it is
known as facial (fac) isomer, when the positions are
around the meridian of the octahedron, it is known as
meridional (mer) isomer. e.g., [Co(NH3)3(NO2)3]

(ii) Optical isomerism These are the complexes which have chiral
structures. It arises when mirror images cannot be superimposed on one another. These
mirror imag es are called enantiomers. The two forms are called dextro (d) and laevo (l)
forms.

Tetrahedral complexes with formula [M(AB)2] show optical isomers and octahedral complexes
(cis form) exhibit optical isomerism.

Bonding in Coordination Compounds

Werner’s Theory

Metals exhibit two types of valencies in the formation of complexes.

These are primary valencies and secondary valencies.

1. Primary valencies correspond to oxidation number (ON) of the metal and are satisfied by
anions. These are ionisable and non-directional.

2. Secondary valencies correspond to coordination number (CN) of the metal atom and are
satisfied by ligands. These are non-ionisable and directional. Hence, geometry is decided by
these valencies.

Valence Bond Theory (VBT)

This theory was proposed by L. Pauling in 1930 s. According to this theory, when a complex is
formed, the metal ion/atom provides empty orbitals to the surrounding ligands. Coordination
number shows the number of such empty orbitals, i.e., number of empty orbitals is equal to the
coordination number. These empty orbitals hybridised
before participation in bonding and the nature of hybridisation depends on the nature of metal
and on the nature of approaching ligand.

Inner orbital complexes or outer orbital complexes

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When outer d-orbital are used in bonding, the complexes are called outer orbital complexes.
They are formed due to weak field ligands or high spin ligands and hybridisation is sp 3d2. They
have octahedral shape.
When d-orbitals of (n – 1) shell are used, these are known as inner orbital complex, they are
formed due to strong field ligands or low spin ligands and hybridisation is d 2sp3. They are also
octahedral in shape.

1. 6 – ligands (unidentate), octahedral entity.

(i) Inner orbital complex [Co(NH3)6]3+ Co z=27 [Ar] 3d7 4s2, Co3+ [Ar] 3d6 4s0 4p0

1. Hybridisation – d2sp3

2. Geometry – Octahedral

3. Magnetic properties –
Diamagnetic

All electrons are paired,

therefore complex will be
diamagnetic in nature.

(i) Outer orbital complex, [CoF6]3- Co z=27 [Ar] 3d7 4s2, Co3+ [Ar] 3d6 4s0 4p0 4d0

1. Hybridisation –sp3d2

2. Geometry –
Octahedral

3. Magnetic properties –
Paramagnetic
Complex has unpaired
electrons; therefore, it
will be paramagnetic in
nature.
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2. 4-ligands (unidentate) tetrahedral entity.

(ii)Inner orbital complex, [Ni(CN)4]2- Ni z=28 [Ar] 3d8 4s2, Ni2+ [Ar] 3d8 4s0 4p0

1. Hybridisation – dsp2

2. Geometry – Square Planar

3. Magnetic properties –
Diamagnetic

All electrons are paired so

complex will be diamagnetic
in nature.

(iii) Outer orbital complex, [NiCl4]2- Ni z=28 [Ar] 3d8 4s2, Ni2+ [Ar] 3d8 4s0 4p0

1. Hybridisation – sp3

2. Geometry – Tetrahedral
3. Magnetic properties –
Paramagnetic
Since, complex has unpaired
electrons. so it will be
paramagnetic in nature.
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Limitations of VBT

This theory could not explain the quantisation of the magnetic data, existence of inner orbital
and outer orbital complex, change of magnetic moment with temperature and colour of
complexes.

Crystal Field Theory (CFT)

This theory was proposed by H. Bethe and van Vleck. Orgel. in 1952, applied this theory to
coordination compounds. In this theory, ligands are treated as point charges in case of anions
and dipoles in case of neutral molecules.

The five d-orbitals are classified as

(i) Three d-orbitals i.e., dxy, dyz and dzx are oriented in between the coordinate axes and are
called t2g – orbitals.
(ii) The other two d-orbitals, i.e., d𝑥 2−𝑦2 𝑎𝑛𝑑 d𝑧 2 oriented along the x – y axes are called
x -y
eg – orbitals. Due to approach of ligands, the five-degenerate d-orbitals split. Splitting of
d-orbitals depends on the nature of the crystal field.
(iii)
[The energy difference between t2g and eg level is designated by Δ and is called crystal field
splitting energy.]
By using spectroscopic data for a number of coordination compounds, having the same metal
ions but different ligand, the crystal field splitting for each ligand has been calculated. A series
in which ligand are arranged in order of increasing magnitude of crystal field splitting, is
called spectrochemical series.

Spectrochemical series

Crystal field splitting in octahedral complexes

In case of octahedral complexes, energy separation is denoted by Δo (where subscript 0 is for

octahedral).

In octahedral complexes, the six-ligands approach the central metal ion along the axis of d x2-
2 2
y and d zorbitals.

Energy of eg set of orbitals > energy of t2g set of orbitals.

The energy of eg orbitals will increase by (3/5) Δo and t2g will decrease by (2/5) Δo.

If Δo < P, the fourth electron enters one of the eg orbitals giving the configuration t32g e1g.
Ligands for which Δo < P are known as weak field ligands and form high spin complexes.
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If Δo > P, it becomes more energetically favourable for the fourth electron to occupy a
t2g orbital with configuration t4 2g eog. (where, P = energy required for e- pairing in an orbital).
Ligands which produce this effect are known as strong field ligands and form low spin
complexes.

Crystal field splitting in tetrahedral complexes

In tetrahedral complexes, four ligands may be imagined to occupy the alternate comers of the
cube and the metal ion at the center of the cube.

Energy of t2g set of orbitals > Energy of eg set of orbitals.

In such complexes d – orbital splitting is inverted and is smaller as compared to the octahedral
field splitting.

Orbital splitting energies are so low that pairing of electrons are not possible so these are high
spin complexes.

Colour in Coordination Compounds

The crystal field theory attributes the colour of the coordination compounds to dod transition of
the electron, i.e., electron jump from t2g level to higher eg level.
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In the absence of ligands, crystal field splitting does not occur and hence the substance is
colourless.

Limitations of CFT

1. It does not consider the formation of 7t bonding in complexes.

2. It is also unable to account satisfactorily for the relative strengths of ligands e.g., it does not
explain why H2O is stronger ligand than OH-.
3. It gives no account of the partly covalent nature of metal-metal bonds.

Organometallic Compounds

Bonding in Metal Carbonyls:

Transition metals form a large number of homoleptic carbonyls.

Eg. [Ni(CO)4], [Fe(CO)5], [Cr(CO)6], [Co2(CO)8], [Mn2(CO)10]
etc. These carbonyls have simple, well defined structures.

The metal-carbon bond in metal carbonyls possess both s and p

character. The MC s bond is formed by the donation of lone
pair of electrons on the carbonyl carbon into a vacant orbital of
the metal. The MC p bond is formed by the donation of a pair
of electrons from a filled d orbital of metal into the vacant
anti-bonding p * orbital of carbon monoxide. Thus the metal to ligand bonding creates a
synergic effect which strengthens the bond between CO and the metal.

Application of Co-ordination Complexes

1. In Qualitative & Quantitative Analysis: Co-ordination compounds find use in
many qualitative and quantitative chemical analyses. For e.g. Ni2+ is detected and estimated by
the formation of a complex with Dimethyl Glyoxime (DMG). The brown ring test for the detection
of nitrate ion is due to the formation of the brown complex [Fe(H2O)5NO]2+. The Ca2+ and Mg2+
ions are estimated by the formation of stable complexes with EDTA.
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1. In water treatment: The Hardness of water is estimated by simple titration with Na2EDTA
(sodium salt of EDTA). The Ca2+ and Mg 2+ ions form stable complexes with EDTA. The hardness
of water can be removed by the formation of a complex with calgon (Sodium
polymetaphosphate)
2. In Metallurgy: Metals like silver and gold are extracted by the formation of complexes with CN-
ligands. Gold forms the complex [Au(CN)2]- and silver forms [Ag(CN)2]- which are separated with Zn.
Similarly, coordination compounds also find application in the refining of some metals. For
example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure nickel
3. Biological Applications: Coordination compounds are of great importance in biological
systems. Chlorophyll is a co-ordination compound of magnesium, Haemoglobin, is a co-
ordination compound of iron and Vitamin B12 (cyanocobalamine) is a co-ordination compound
of cobalt.
4. In Catalysis: Co-ordination compounds are used as catalysts for many industrial processes.
For e.g. Tris(triphenylphosphine)rhodiumchloride, [(Ph3P)3RhCl] (Wilkinson catalyst), is used
for the hydrogenation of alkenes.

5. In electroplating: Articles can be electroplated with silver and gold by using the solutions of the
complexes, [Ag(CN)2] — and [Au(CN)2] — respectively as electrolytes.
6. In photography: In black and white photography, the developed film is fixed by washing with
hypo solution which dissolves the undecomposed AgBr to form a complex ion [Ag(S2O3)2]3 –
7. In medicine: Cis-platin is used for the treatment of cancer. Excess of copper and iron in animal
or plant body are removed by the chelating ligands Dpenicillamine and desferrioxime B
through the formation of co-ordination compounds. EDTA is used in the treatment of lead
poisoning.
POSSIBLE QUESTIONS
1. Define the following terms of coordination compounds
a. Coordinate entity, b. Central metal atom/ion,
c. Coordination number d. Coordination sphere
e. Coordination polyhedron f. Oxidation no. Of central atom
g. Ligand
2. Define homoleptic complexes. Give an example
3. Define heteroleptic complexes. Give an example
4. Define didentate ligand. Give an example
5. Define monodentate ligand. Give an example
6. What is ambidentate ligand? Give an example
7. What is meant by chalet ligand? Give an example
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8. What is meant by denticity of the ligand

9. Which is referred as Lewis acid and Lewis base in the coordination compounds.
10. Define facial isomer and meridional isomer in case of coordination compounds. Give an
example for each.
11. Define Linkage isomerism. Give an example
12. Define coordinate isomerism. Give an example
13. Define ionisation isomerism. Give an example
14. Define solvate isomerism. Give an example
15. What are spectrochemical series?
16. What is meant by crystal field splitting?
17. What is meant by degeneracy of d – orbitals
18. Write the chemical formula for Wilkinson catalyst
19. Write the postulates of Werner’s theory of coordination compounds

20. Write the limitations of Werner’s theory of coordination compounds

21. Write the postulates of VBT for coordination compounds
22. Write the limitations of VBT for coordination compounds
23. State the pustulates of CFT for coordination compounds
24. Write the limitations of CFT for coordination compounds
25. Explain the hybridization, magnetic property and shape of the following complex
ions
(a) [CoF6] 3 – (b) [Co(NH3)2]3+ (c) [NiCl4]2 – (d) [Ni(CN)4] 2–
26. Explain the crystal field splitting in octahedral coordination entities
27. Explain the crystal field splitting in tetrahedral coordination entities
28. Discuss the nature of bonding in metal carbonyls
29. State the factors which govern stability of complexes
30. How does the magnitude of Δ0 decide the actual configuration of d orbitals in a
coordination entity?
31. What are primary valance of central metal atom/ion. Which species will satisfies
the primary valance of the metal atom/ion.
32. What are secondary valance of central metal atom/ion. Which species will satisfies
the secondary valance of the metal atom/ion.
33. Write the structure for the following coordination compounds
a. [PtCl2(NH3)2], b. [Fe(CO)5] c. [Mn2(CO)10] d. [Co2(CO)8]
34. What the structures for the following metal carbonyl compounds
a. Ni(CO)4 b. Fe[(CO)5] c. Cr(CO)6
How many ions are formed when K4[Fe(CN)6] is dissolved in suitable solvent.