COHERENT AND INCOHERENT DIHYDROGEN DYNAMICS IN A RUTHENIUM TRIHYDRIDECOMPLEX WITH THE TRIS(PYROLLYL)PHOSPHINE LIGAND

The ruthenium IV trihydride complex CpRuH3(Ppy(3)) (1) accommodating the pi-accepting tris(pyrollyl)phosphine ligand has been synthesized b y standard methods. This complex exhibits average exchange couplings J of the coherent dihydrogen exchange and average rate constants k of t he incoherent dihydrogen exchange which are significantly larger than the corresponding triphenylphosphine and tricyclohexylphosphine analog s. No kinetic HH/HD isotope effects on the incoherent exchange are obs erved within the margin of error. Using a simple kinetic model involvi ng incoherent vibrational excitation it is shown that in principle bot h the coherent and the incoherent processes should be related. However , whereas the energy of activation of k corresponds to the barrier of rotation, the energy of activation of the coherent dihydrogen exchange J is much smaller i.e. determined by states well below the barrier. N evertheless, it is found that chemical modification which reduces the energy of activation of J also reduces the energy of activation of k w hich demonstrates the relation between both kinds of processes.