The reduction pathway of end-on coordinated dinitrogen. I. Vibrational spectra of Mo/W-N-2, -NNH, and -NNH2 complexes and quantum chemistry assisted normal coordinate analysis
N. Lehnert et F. Tuczek, The reduction pathway of end-on coordinated dinitrogen. I. Vibrational spectra of Mo/W-N-2, -NNH, and -NNH2 complexes and quantum chemistry assisted normal coordinate analysis, INORG CHEM, 38(8), 1999, pp. 1659-1670
Infrared and Raman spectra of [M(N-2)(2)(dppe)(2)] (M = W, Mo) and the two
protonated derivatives [WF(NNH)(dppe)(2)] and [WF(NNH2)(dppe)(2)](+) (dppe
= 1,2-bis (diphenylphosphino)ethane) are presented. Using isotope substitut
ed compounds (N-15 and D) the vibrations of the Y-M-N2Hx (x = 0, 1, 2; Y =
N-2, F) central unit are identified, in particular the M-N and N-N stretchi
ng modes. In case of the monoprotonated systems, an equilibrium between met
al- and nitrogen-protonated species exists that is clearly detectable in th
e IR spectra. Making use of theoretical force fields, a quantum chemistry a
ssisted normal coordinate analysis (QCA-NCA) is performed for all three tun
gsten systems showing very good agreement with experimental frequencies. Th
e resulting force constants for the metal-N and N-N bonds offer quantitativ
e insight into the change of M-N and N-N bond strengths during protonation
of dinitrogen bound end-on terminally to transition metals. The salient fea
ture of this "asymmetric" protonation pathway is the strengthening of the m
etal-N bond going along with each protonation step whereas the N-N bond is
weakened at the same time.