The Reduction Pathway of End-on Coordinated Dinitrogen. II. Electronic Structure and Reactivity of Mo/W−N2, −NNH, and −NNH2Complexes

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Abstract
DFT calculations (B3LYP/LanL2DZ) of simplified models of [Mo(N2)2(dppe)2] and the two protonated derivatives [MoF(NNH)(dppe)2] and [MoF(NNH2)(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 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 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.