COORDINATION COMPOUNDS

Introduction
A coordination compound consists of central metal atom or ion surrounded by atoms or
molecules.
e.g. a chemotherapy drug, cisplatin, Pt(NH3)2Cl2, is a coordination compound in which the
central platinum metal ion is surrounded by two ammoniamolecules and two chloride ions
Ligands: The species surrounding the central metal atom or ion are called ligands.
The ligands are linked directly to central metal ion through coordinate bonds.
A formation of coordinate bond occurs when the shared electron pair is contributed by ligands.
A coordinate bond is conveniently represented by an arrow →, where the arrow head points to
electron acceptor.

• The central metal atom or ion usually an electron deficient species, accepts an
electron pair
• while the ligands serve as electron donors. Coordination compounds having a
metal ion in the centre are common.
• In cisplatin two ammonia molecules and two chloride ligands utilize their lone
pairs of electrons to form bonds with the Pt(II).

• The donor nitrogen and chlorine atoms of the ligands are directly attached
to and form bonds with platinum.
• Formation of a coordination compound can be looked upon as the Lewis acid-
base interaction.
• The ligands being electron pair donors are Lewis bases.
• The central metal ion being electron pair acceptor serves as Lewis acid.
Types of ligands :
The ligands can be classified as monodentate and polydentate
Monodentate ligands :
A monodentate ligand is the one where a single donor atom shares an electron pair to
form a coordinate bond with the central metal ion.
 e.g.: The ligands Cl , OH or CN attached to metal have electron pair on Cl, O and N,
respectively which are donor atoms :

Polydentate ligands :
• A polydentate ligand has two or more donor atoms linked to the central metal ion.
e.g. ethylenediamine and oxalate ion.
Each of these ligands possesses two donor atoms.
Terms used in coordination chemistry:
Coordination sphere :
The central metal ion and ligands linked to it are enclosed in a square bracket. This is called a
coordination sphere
which is a discrete structural unit.
When the coordination sphere comprising central metal ion and the surrounding ligands
together carry a net charge, it is called the complex ion.
The ionisable groups shown outside the bracket are the counter ions. e.g. The compound
K4[Fe(CN)6] has [Fe(CN)6]4- coordination sphere with the ionisable K⊕ions representing counter
ions.

Charge number of complex ion and oxidation state of metal ion :

• The net charge residing on the complex ion is its charge number.
• It is algebraic sum of the charges carried by the metal ion and the ligands.
• The charge carried by the metal ion is its oxidation state (O.S.).
• The complex [Fe(CN)6]4 has charge number of -4. It can be utilised to calculate
O.S. of Fe.
Thus, charge number of complex = -4
= (O.S. of Fe + charge of ligands)
= (O.S. of Fe + 6 × charge of CN ion)
= (O.S. of Fe + 6 × (-1))
Therefore, O.S. of Fe = -4 + 6 = +2.
Coordination number (C.N.) of central metal ion :
e.g. [Co(NH3)4Cl2]⊕.
1) Here four ammonia molecules and two chloride ions,
that is, total six ligands are attached to the cobalt ion.
2) All these are monodentate since each has only one donor atom.
There are six donor atoms in the complex.
3) Therefore, the coordination number of Co3⊕ ion in the complex is six. Thus, the
coordination number of metal ion attached to monodentate ligands is equal to number of
ligands bound to it
• Consider the bidentate ligand C2O42 or ethylenediamine (en).
• The complexes, [Fe(C2O4)3]3 and [Co(en)3]3⊕, have three bidentate ligands each.
• The total donor atoms in three of ligands is six and the C.N. of Fe 3⊕and Co3⊕ in
these complexes is six.
• C.N. of metal ion in a complex is the number of ligand donor atoms directly
attached to it or the number of electron pairs involved in the coordinate bond.
Werner theory of coordination complexes :
The first attempt to explain nature of bonding in coordination compounds was put forth
by Werner.
The postulates of Werner theory are as follows.
(i) Unlike metal salts, the metal in a complex possesses two types of valencies : primary
(ionizable) valency and secondary (nonionizable) valency.
(ii) The ionizable sphere consists of entities which satisfy the primary valency of the
metal. Primary valencies are generally satisfied by anions.
(iii) The secondary coordination sphere consists of entities which satisfy the secondary
valencies and are non ionizable
The secondary valencies for a metal ion are fixed and satisfied by either anions or
neutral ligands. Number of secondary valencies is equal to the coordination number.
(iv) The secondary valencies have a fixed spatial arrangement around the metal ion.
Two spheres of attraction in the complex
[Co(NH3)6]Cl3 are shown.
A complex with coordination number six has octahedral structure. When four
coordinating groups are attached to the metal ion the complex would either be with
square planar or tetrahedral structure.
Classification of complexes:
The coordination complexes are classified according to types of ligands and sign of
charge on the complex ion.
Classification on the basis of types of ligands
i. Homoleptic complexes :
[Co(NH3)6]3⊕.
Only one type of ligands surrounds the Co3⊕ ion.
The complexes in which metal ion is bound to only one type of ligands are homoleptic.
ii. Heteroleptic complexes :
[Co(NH3)4Cl2]⊕.
Two types of ligands, NH3 and Cl attached to Co3⊕ion.
Such complexes in which metal ion is surrounded by more than one type of ligands are
heteroleptic.
Classification on the basis of charge on the complex
• i. Cationic complexes : A positively charged coordination sphere or a
coordination compound having a positively charged coordination sphere is called
cationic sphere complex.
e.g. [Zn(NH3)4]2⊕ and [Co(NH3)5Cl]SO4 are cationic complexes.
The latter has coordination sphere [Co(NH3)5Cl]2⊕;the anion SO42 makes it electrically neutral.
ii. Anionic sphere complexes : A negatively charged coordination sphere or a coordination
compound having negatively charged coordination sphere is called anionic sphere complex.
 • For example,
• [Ni(CN)4]2 and K3[Fe(CN)6] have anionic coordination sphere; [Fe(CN)6]3 and three K⊕
ions make the latter electrically neutral.
• iii. Neutral sphere complexes :
• A neutral coordination complex does not possess cationic or anionic sphere.
• [Pt(NH3)2Cl2] or [Ni(CO)4] have neither cation nor anion but are neutral sphere
complexes.

IUPAC nomenclature of coordination compounds :

Rules for naming coordination compounds recommended by IUPAC are as follows:
1. In naming the complex ion or neutral molecule, name the ligand first and then the
metal.
2. The names of anionic ligands are obtained by changing the ending -ide to -o and -ate
to -ato.
3. The name of a complex is one single word. There must not be any space between
different ligand names as well as between ligand name and the name of the metal.
4. After the name of the metal, write its oxidation state in Roman number which appears
in parentheses without any space between metal name and parentheses.
5. If complex has more than one ligand of the same type, the number is indicated with
prefixes, di-, tri-, tetra-, penta-, hexa- and so on.
6. For the complex having more than one type of ligands, they are written in an
alphabetical order. Suppose two ligands with prefixes are tetraaqua and dichloro. While naming
in alphabetical order, tetraaqua is first and then dichloro.
7. If the name of ligand itself contains numerical prefix then display number by prefixes
with bis for 2, tris for 3, tetrakis for 4 and so forth. Put the ligand name in parentheses. For
example, (ethylenediamine)3 or (en3) would appear as tris(ethylenediamine) or tris(ethane-1,
2-diamine).
8. The metal in cationic or neutral complex is specified by its usual name while in the
anionic complex the name of metal ends with 'ate'.
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)4Cl2]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 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
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., [CoCl2(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.