(complex compound). A compound formed by the union of a metal ion (usually a transition metal) with a nonmetallic ion or molecule called a ligand or complexing agent. The ligand may be either positively or negatively charged (such ions as Cl− or NH2NH3+), or it may be a molecule of water or ammonia. The most common metal ions are those of cobalt, platinum, iron, copper, and nickel, which form highly stable compounds. When ammonia is the ligand, the compounds are called ammines. The total number of bonds linking the metal to the ligand is called its coordination number. It is usually 2, 4, or 6 and often depends on the type of ligand involved. All ligands have electron pairs on the coordinating atom (e.g., nitrogen) that can be either donated to or shared with the metal ions. The metal ion acts as a Lewis acid (electron acceptor) and the ligand as a Lewis base (electron donor). The bonding is neither covalent nor electrostatic but may be considered intermediate between the two types. The charge on the complex ion is the sum of the charges on the metal ion and the ligands; for example, 4NH3 + 2Cl− + Co3+ forms the complex [Co(NH3)4Cl2]+. The brackets enclose the metal ion and the coordinated ligands.See Chelate; Sequestration; Metallocene.
