Carboxylates bind to single metals by one or both oxygen atoms, the respective notation being κ- and κ-. In terms of electron counting, κ-carboxylates are "X"-type ligands, i.e., a pseudohalide-like. κ-carboxylates are "L-X ligands", i.e. resembling the combination of a Lewis base (L) and a pseudohalide (X). Carboxylates are classified as hard ligands, in HSAB theory.
For simple carboxylates, the acetate complexes are illustrative. Most transition metal acetates are mixed ligand complexes. One common example is hydrated nickel acetate, Ni(OCCH)(HO), which features intramolecular hydrogen-bonding between the uncoordinated oxygens and the protons of aquo ligands. Stoichiometrically simple complexes are often multimetallic. One family are the basic metal acetates, of the stoichiometry [MO(OAc)(HO)].