chemical bnding

Every element has its primary and secondary valency.

Primary valency corresponds to the oxidation number of the element. It is satisfied by anions.

Secondary valency corresponds to the co-ordination number of the element. It is satisfied by either anions or neutral molecules like water, ammonia, ethylene diamine etc.

These anions and neutral molecules satisfying the secondary valency are know as ligands.

These ligands donate electron pairs to the element.

In chemistry, a ligand is either an atom, ion, or molecule (see also: functional group) that binds to a central metal to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electrons. The metal-ligand bonding ranges from covalent to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known involving Lewis acidic "ligands."
Metal and metalloids are bound to ligands in virtually all circumstances, although gaseous "naked" metal ions can be generated in high vacuum. Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Ligand selection is a critical consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry.