A coordination complex contains a central metal atom or cation which is surrounded by a suitable number of anions or neutral molecules (called ligands).
Ligands are bonded to the central metal atom or ion through coordinate bonds.
A complex may be cationic, anionic or neutral species.
A complex retains its identity in solution as well as in solid state. The number of electron pairs accepted by the metal atom in the complex is called the coordination number of the metal atom.
A complex may be represented by a general formula
Some examples of coordination complexes are[Cu(NH )]2+ [Ag(NH ) ]+ [Fe(CO)5] [CO(H2O )J2+ [Ni(CO)4] [Ni(CN)4)2- [Fe(CN)6]3- [COCI4]2-
OXIDATION NUMBER OR OXIDATION STATE OF THE CENTRAL METAL ION
The oxidation number is the residual charge that appears on the central atom when all other atoms/ions are removed from it. While calculating the oxidation state of an atom it must be noted that:
(a) the sum of charges of the constituents is equal to zero if the complex is neutral.
(b) for an anionic or cationic complex, the sum of charges of the constituents is equal to the charge on the complex.
The above points are illustrated by taking the following examples:
(i) In the complex ion [CO(NH3) 2Cl4] 2- the charge on the complex is + 2. The oxidation number of the central metal ion may be calculated as:
X + 2(0) + 4(- 1) = – 2
X = + 2
(ii) In the complex, [Fe(CN)J4–, the charge on the complex ion is – 4. The oxidation number of central metal may be calculated as: