What is meant by the term 'coordination number' ?

Explanation

Coordination number: The coordination number (C.N.) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.
For example, in the complex ions, `[PtCl_6]^(2)` and `[Ni(NH_3)_4]^(2+)`, the coordination number of Pt and Ni are 6 and 4 respectively. Similarly, in the complex ions, `[Fe(C_2O_4)_3]^(3-)` and `[Co(en)_3]^(3+)`, the coordination number of both, Fe and Co, is 6 because `C_2O_4^(2-)` and en (ethane-1, 2-diamine) are bidentate ligands.