CHAPTER 9-COORDINATION COMPOUND

NOTES

OBJECTIVE OF THE LESSON-

o Coordination Compound
• Ligand and its types
• Nomenclature of coordination compounds
• Isomerism in coordination compounds and its types
• Valence bond theory (VBT)
• Crystal field splitting theory (CFT)
• Metal carbonyls
• Applications of 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+, NH+4 and SO2-4 ions.

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
central atom or ion. It is also referred as Lewis acid. e.g., in (NiCI2(H2O)4].

Ni is central metal atom. It is generally transition element or inner-

transition element.

BY-YOGESH sir
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.

(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., NO-2, SCN–, etc.

(v) Chelating ligands Di or polydentate ligands cause cyclisation around

the metal atom which are known as chelate IS , Such ligands USes 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.

BY-YOGESH sir
π – acid ligands are those ligands which can form π – bond and n-bond by
accepting an appreciable amount of 1t electron density from metal atom to
empty π or π – orbitals.

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

In polydentate ligands.

Coordination number = number of ligands * 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.

BY-YOGESH sir
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(CN4)3-, 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 au alphabetical order before the

name of central metal atom or ion.

BY-YOGESH sir
4. For more then one similar ligands. 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(H2CH2CH2H2)3]2(SO4)3

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

(iii) K4 [Fe(CN)6]

potassium hexacyanoferrate (II)

BY-YOGESH sir
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.,

[Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl or pentaammine nitrito- N

Cobalt (III) chloride and pentaammine nitrito-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 arise due to exchange of

ionisable anion with anionic ligand. e.g..

(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

BY-YOGESH sir
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 isomensm 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

BY-YOGESH sir
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)+.

Octahedral complexes of formula [MX2A2], where X are unidentate ligands

and 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 (fae) 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 images 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.

BY-YOGESH sir
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.

BY-YOGESH sir
Inner orbital complexes or outer orbital complexes

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 sp3d2. 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 d2sp3. They are also octahedral in shape.

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

(i) Inner orbital complex [Co(NH3)6]3+

All electrons are paired, therefore complex will be Diamagnetic in nature.

BY-YOGESH sir
(ii) Outer orbital complex, [CoF6]3-

Complex has unpaired electrons, therefore, it will be Paramagnetic in

nature.

2. 4-ligands (unidentate) tetrahedral entity.

BY-YOGESH sir
(i) Inner orbital complex, [Ni(CN)4]2-

All electrons are paired so complex will be diamagnetic in nature.

(ii) Outer orbital complex, [CoCI4]–

Since, complex has unpaired

electrons. so it will be paramagnetic in nature.

BY-YOGESH sir
Limitations of VBT

.• It does not tell anything about the spectral properties of the complexes.

• It does not give quantitative interpretation of magnetic data.

• It does not distinguish between strong and weak ligands.

• It does not explain the colour exhibited by coordination compounds.
• It does not give a quantitative interpretation of the thermodynamic or
kinetic stabilities of coordination compounds.

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.

BY-YOGESH sir
(ii) The other two d-orbitals, i.e., d x2 – y2 and d z2 oriented along the x – y %
axes are called 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.

[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 is-

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 – y2 and d z2 orbitals.

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.

If Δo > P, it becomes more energetically favourable for the fourth electron to

occupy a t2g orbital with configuration t42g 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.

BY-YOGESH sir
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.

BY-YOGESH sir
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.

Ligand Field or Molecular Orbital Theory

This theory was put forward by Hund and Mulliken. According to this
theory, all the atomic orbitals of the atom participating in molecule
formation get mixed to give rise an equivalent number of new orbitals,
called the molecular orbitals. The electrons are now under the influence of
all the nuclei.

Stability of Coordination Compounds

The stability of complex in solution refers to the degree of association

between the two species involved in the state of equilibrium. It is expressed
as stability constant (K).

The factors on which stability of the complex depends :

(i) Charge on the central metal atom As the magnitude of charge on

metal atom increases, stability of the complex increases.
(ii) Nature of metal ion The stability order is 3d < 4d < 5d series.
(iii) Basic nature of ligands Strong field ligands form stable complex.

The instability constant or the dissociation constant of compounds is

defined as the reciprocal of the formation or stability Constant.

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Organometallic Compounds

They contain one or more metal-carbon bond in their molecules. They are
of the following types:

1. Sigma (σ) bonded compounds

Metal-carbon bond is sigma bond, e.g., (C2H5)4 Pb, Zn(C2H5)2 R – Mg – X, etc.

2. Pi(π) bonded compounds

In which molecules/ions containing π bonds act as a ligand. e.g., Ferrocene,

Dibenzene chromium and Zeise’s salt.

Zeise’s salts is K[PtCI3(η2 – C2H4)] In which ethylene acts as a ligand which

do not have a lone pair oi electron.

In ferrocene, Fe(η5 – C5H5)2 represents the number of carbon atoms with

which metal ion is directly attached.

3. σ and π bonded compounds

Metal carbonyls are their examples. Metal-carbon bond of metal carbonyls

have both σ and π – bond character. They have CO molecule as ligand, e.g.,

Wilkinson’s catalyst (Rh(PPh3)3CI] is used as homogeneous catalyst in the

hydrogenation of alkenes. Zeigler-Natta catalyst
[Ti CI4 + (C2H5>3Al] acts as heterogeneous catalyst in the polymerisation of
ethylene

Applications of coordination compounds

• EDTA is used in the estimation of Ca2+ and Mg2+ in hardwater. The Ca2+
and Mg2+ ions form stable complexes with EDTA.

BY-YOGESH sir
• Metals can be purified by the formation and subsequent decomposition
of their coordination compounds. For example, impure nickel is converted
to [Ni(CO)4], which is decomposed to yield pure nickel.
• In analytical chemistry [Ni(DMG)2]2+ complex is used in the detection of
Ni in chocolates.
• In medicine, cisplatin, a cis isomer of [Pt(Cl)2(NH3)2] is used in the
treatment of cancer.
• Solutions of the complexes like [Ag(CN)2]– and [Au(CN)2]– can be used
for the smooth and even electroplating of metals by gold or silver.
• Chlorophyll, a pigment responsible for photosynthesis, is a coordination
compound of magnesium. Also haemoglobin, the red pigment of blood
which acts as oxygen carrier is a coordination compound of iron.

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BY-YOGESH sir