The Reduction Pathway of End-on Coordinated Dinitrogen. II. Electronic Structure and Reactivity of Mo/W-N(2), -NNH, and -NNH(2) 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(NNH(2))(dppe)(2)](+) (dppe = 1,2-bis(diphenylphosphino)ethane) provide quantitative insight into the reduction and protonation of dinitrogen bound end-on terminally to transition metals. This "asymmetric" reduction pathway is characterized 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 NNH(2) species which is neutral and hence corresponds to coordinated isodiazene. UV-vis data of Mo and corresponding W complexes support the calculated energy level schemes. Moreover, calculated vibrational frequencies and force constants show good 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 complexes with respect to the reduction and protonation of dinitrogen in small-molecule systems and nitrogenase are discussed.