The reduction pathway of end-on coordinated dinitrogen. II. Electronic structure and reactivity of Mo/W-N-2, -NNH, and -NNH2 complexes

DFT calculations (B3LYP/LanL2DZ) of simplified models of [Mo(N-2)(2)(dppe)( 2)] and the two protonated derivatives [MoF(NNH)(dppe)(2)] and [MoF(NNH2)(d ppe)(2)](+) (dppe = 1,2-bis(diphenylphosphino)ethane) provide quantitative insight into the reduction and protonation of dinitrogen bound end-on termi nally to transition metals. This "asymmetric" reduction pathway is characte rized by a stepwise increase of covalency and a concomitant charge donation from the metal center during each protonation reaction. The major part of metal-to-ligand charge transfer occurs after the first protonation leading to coordinated diazenido(-). In contrast, addition of the second proton is accompanied by a minor change of covalency leading to a NNH2 species which is neutral and hence corresponds to coordinated isodiazene. UV-vis data of Mo and corresponding W complexes support the calculated energy level scheme s. Moreover, calculated vibrational frequencies and force constants show go od agreement with experimental values determined in Part I of this series ( Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1659-1670). The implications of the electronic structure description obtained for the above model compl exes with respect to the reduction and protonation of dinitrogen in small-m olecule systems and nitrogenase are discussed.