Dinitrogen complexes are coordination compounds in which molecular nitrogen, N₂, acts as a ligand and coordinates to a metal through donation of its lone pair and, in many cases, back-donation from filled metal d-orbitals into the antibonding π* orbitals of N₂. The ability of a metal to form such complexes depends mainly on its oxidation state, electron configuration, availability of vacant coordination sites, and capacity for π back-bonding. Transition metals, particularly those of the middle and later parts of the transition series, form a large number of dinitrogen complexes. Metals such as titanium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium and platinum are known to form complexes containing coordinated N₂. Among these, molybdenum, tungsten, iron and ruthenium are especially important because of their strong metal–dinitrogen interactions. Dinitrogen can coordinate in several ways, most commonly in an end-on fashion through one nitrogen atom, represented as M–N≡N. It may also act as a bridging ligand between two metal centres, giving M–N≡N–M complexes. Coordination weakens the N≡N bond because of metal-to-ligand π back-donation into the antibonding orbitals of nitrogen.