Ligands and Donor Atoms

Ligands are the atoms or groups of atoms bound to the central atom or ion, and the specific atom within a ligand that is directly bonded to the central metal is called the donor atom -- it is the donor atom's lone pair that is actually shared with the metal to form the coordinate bond, so a ligand and its donor atom are not the same thing and must be named separately. For example, in K₄[Fe(CN)₆] the ligand is the CN⁻ ion, but the donor atom is specifically carbon (not nitrogen), because CN⁻ bonds to iron through its carbon end; whereas in [Co(NH₃)₆]Cl₃ the ligand is the neutral NH₃ molecule and the donor atom is nitrogen, the atom that actually carries the lone pair donated to cobalt. This ligand/donor-atom distinction becomes especially important for ambidentate ligands such as the …