Ligands are atoms or molecules with electron pairs available; they may be neutral or negatively charged. Ligands are usually thought of as electron donors attracted to the metal (the electron acceptor) at the center of the complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal–ligand bonding can range from covalent to ionic. Furthermore, the metal–ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligand.".
Ligands can be classified in many ways, including: charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons donated to the metal (denticity or hapticity). The size of a ligand is indicated by its cone angle.
Examples of ligands
Monodentate ligands have one atom that can bind to a central atom or ion. Water (H2O) and ammonia (NH3) are examples of neutral monodentate ligands.
An ambidentate ligand is a monodentate ligand that can bind in two possible places. For example, The thiocyanate ion, SCN-, can bind to the central metal at either the sulfur or the nitrogen.