The molecules or ions that are attached to the metal in a complex ion are called ligands. The interaction between a metal atom and the ligands can be thought of as Lewis acid-base reaction. A Lewis base is a substance capable of donating one or more electron pairs, every ligand has at least one unshared pair of valence electron.
The atom in the ligand that is bound directly to the metal atom is known as the donor atom. For example, nitrogen is the donor atom and Cu2+ is the acceptor atom in the [Cu(NH3)4]2+ complex ion.
Depending on the number of the donor atoms present, ligands are defined as monodentate, bidentate or polydentate. H2O and NH3 are monodentate ligands with only one donor atom in each. Ethylenediamine (en) is a bidentate ligand.
The coordination number in coordination compounds is defined as the number of ligand (donor) atoms/ions surrounding the central metal atom in a complex ion.
For example, the coordination number of cobalt in [Co(NH3)6]3+ is six. Similarly, the coordination number of Ag+ in [Ag(NH3)2]+ is 2, that of Cu2+ in [Cu(NH3)4]2+ is 4, and that of Fe3+ in [Fe(CN)6]3– is 6.
The central metal atom and the ligands which are directly attached to it are enclosed in a square bracket and are collectively termed as coordination sphere. The ligands and the metal atom inside the square brackets behave as single constituent unit.
The net charge on a complex ion is the sum of the charges on the central atom and its surrounding ligands. For example, in the [PtCl6]2- ion, each chloride ion has an oxidation number of –1, so the oxidation number of Pt must be +4. If the ligands do not bear net charges, the oxidation number of the metal is equal to the charge of the complex ion. Thus, in [Cu(NH3)4]2+ each NH3 is neutral, so the oxidation number of copper is +2.