In coordination chemistry, a binucleating ligand binds two metals.  Much attention has been directed toward such ligands that hold metals side-by-side, such that the pair of metals can bind substrates cooperatively.
A variety of metalloenzymes feature bimetallic active sites.  Examples include superoxide dismutase, urease, nickel-iron hydrogenase.  Many Non-heme iron proteins have diiron active sites, e.g. ribonucleotide reductase and hemerythrin.