What is ligand?

In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. An array of such ligands around a centre is termed a complex.

The central atom usually has a positive charge which is stabilised by donation of negative charge from the ligands.

Neutral or negatively charged centres are also known, usually stabilised by donating electron density back to the ligand in a process known as "back-bonding". 

The charge on the central atom constrains the number of ligands that may be bonded, since each type of ligand donates a characteristic number of electrons and there is a requirement for overall neutrality. If the directly-bonded ligands (the "inner-sphere" ligands) do not balance the charge, this may be done by purely ionic interaction with another set of ions (the "outer-sphere" ligands).

The inner-sphere ligands arrange themselves in a certain geometry, fixed for a given complex but in some cases mutable by reaction to another stable isomer. Ligands which bind to the central atom through more than one site of their own are termed polydentate; a ligand binding through two sites, for example, is bidentate.