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 substituted 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 stretching modes. In case of the monoprotonated systems, an equilibrium between metal- and nitrogen-protonated species exists that is clearly detectable in the IR spectra. Making use of theoretical force fields, a quantum chemistry assisted normal coordinate analysis (QCA-NCA) is performed for all three tungsten systems showing very good agreement with experimental frequencies. The resulting force constants for the metal-N and N-N bonds offer quantitative 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 feature of this "asymmetric" protonation pathway is the strengthening of the metal-N bond going along with each protonation step whereas the N-N bond is weakened at the same time.