Coordination number (C.N.) of central metal ion

The coordination number (C.N.) of the central metal ion in a complex is the number of ligand DONOR ATOMS directly attached to it -- equivalently, the number of coordinate bonds (electron pairs) it is involved in. For a complex built only from monodentate ligands, C.N. simply equals the number of ligands present, since each contributes exactly one donor atom: in [Co(NH3)4Cl2]+, the four NH3 molecules and two Cl- ions (six ligands, all monodentate) give a coordination number of six for cobalt. The picture changes for polydentate ligands, since each one contributes MORE than one donor atom: in [Fe(C2O4)3]3- and [Co(en)3]3+, each complex has only THREE ligand units (oxalate or en, both bidentate), but since each ligand contributes two donor atoms, the total donor-atom count -- and hence the coordination number -- is still six for both Fe and Co. This is a common source of error: coordination number must always be counted in donor ATOMS, never simply in the number of ligand molecules or ions present. It is worth explicitly noting that 'coordination number' means something related b …