NMR AND INS LINE SHAPES OF TRANSITION METAL HYDRIDES IN THE PRESENCE OF COHERENT AND INCOHERENT DIHYDROGEN EXCHANGE

In this paper a unified description of the effects of the coherent and incoherent dihydrogen exchange on the NMR and INS spectra of transition metal hydrides based on the quantum-mechanical density matrix formalism of Alexander-Binsch is proposed. The dynamic parameters of the line shape analyses are the exchange couplings or rotational tunnel splittings J of the coherent exchange and the rate constants k of the incoherent exchange. As experimental examples, we report the temperature dependent values J and k for 1 equivalent to Cp*RuH3(PCy3) (Cp* equivalent to C-5(CH3)(5) and Cy equivalent to cyclohexyl) including the kinetic HH/HD/DD isotope effects on the incoherent exchange, determined by NMR, and for the tungsten dihydrogen complex 2 equivalent to W(PCy3)(2) (CO)(3)-(eta-H-2), determined by INS. The temperature dependence of J and Ic is interpreted qualitatively in terms of a simple reaction scheme involving at each temperature a ground state and a dominant ro-vibrationally excited state. Using formal kinetics it is shown that a coherent exchange in the excited state contributes to J only if this exchange presents the rate Limiting reaction step, i.e., if vibrational deactivation is fast. This is the case for levels located substantially below the top of the barrier. A very fast coherent exchange of levels located close to the top of the barrier contributes only to k. This result reproduces in a simple way the quantum-mechanical results of Szymanski, S. J. Chem. Phys. 1996, 104, 8216 and Scheurer, C.; Wiedenbruch, R.; Meyer, R.; Ernst, R. R. J. Chern. Phys. 1997, 106, 1. The results concerning the coherent and incoherent exchange processes in 1 and 2 are discussed in terms of the simplified reaction model.