Nature of N-N bonding within high-pressure noble-metal pernitrides and the prediction of lanthanum pernitride.

The nature of nitrogen-nitrogen bonding and the metal oxidation states within late-noble-metal pernitrides have been determined by a series of density-functional electronic-structure calculations. In contrast to alkaline-earth pernitrides such as BaN(2) which contain quasi-molecular double-bonded N(2)(2-) units, compounds such as PtN(2) incorporate a tetravalent metal and a N(2)(4-) species with a N-N single bond due to four surplus electrons within the antibonding 1pi(g)* molecular orbital. This fact is the source of the huge bulk moduli of PtN(2) and related materials such as OsN(2) and IrN(2). The crystal structure of lanthanum pernitride, LaN(2) <==> La(3+) + N(2)(2-) + e(-), yet to be made, has been predicted, and its electronic structure is compared with a likewise hypothetical LaN(2) which consists of both N(2)(2-) and N(2)(4-) pernitride units together with a trivalent lanthanum cation. Finite-temperature DFT calculations predict a very moderate reaction pressure toward LaN(2) starting from LaN and elemental nitrogen of less than 2 GPa at 300 K.