` A complex ion is a charged species consisting of a metal ionsurrounded by ligands.` A ligand is a molecule or an ion having a lone pair of electrons thatcan be donated to the metal ion to form a covalent bond.` The number of ligands attached to a metal ion is called thecoordination number.` Metal ions add ligands one at a time in steps characterized byequilibrium constants called formation constants, or stabilityconstants.