Terminology

The important terms used in the study of coordination compounds are mentioned below.

Central Metal Ion

It is an acceptor atom containing vacant orbitals to which a fixed number of ligands are attached via co-ordinate bonds in definite geometrical arrangement.

Co-ordination Sphere:

Complex ion enclosed in square bracket, it behaves as a single unit.

Part of compound present outside co-ordination sphere, e.g

Co-ordination Number:

It is the total number of ligands attached to the central metal atom through co-ordinate bonds or the number of atoms of a ligand attached to the same central atom, e.g. hexadentate ligand should be counted as forming six co-ordinate bonds. 

Oxidation number:

It is the charge which the central atom appears to have if all the ligands are removed along with the electron pairs that are shared with the central atom.

                [Cr(H₂O)₄Cl₂]⁺

                x + (4 × 0) + (-1 × 2) = +1    [because the ligand H₂O is neutral and 2Cl^– carries - 2 charge]

                x + 0 - 2 = +1

                x = 3