Coherent and incoherent dihydrogen dynamics in a ruthenium trihydride complex with the tris(pyrollyl)phosphine ligand

The ruthenium IV trihydride complex Cp * RuH 3 (Ppy 3 ) ( 1 ) accommodating the π‐accepting tris(pyrollyl)phosphine ligand has been synthesized by standard methods. This complex exhibits average exchange couplings J of the coherent dihydrogen exchange and average rate constants k of the incoherent dihydrogen exchange which are significantly larger than the corresponding triphenylphosphine and tricyclohexylphosphine analogs. No kinetic HH/HD isotope effects on the incoherent exchange are observed within the margin of error. Using a simple kinetic model involving incoherent vibrational excitation it is shown that in principle both 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. Nevertheless, it is found that chemical modification which reduces the energy of activation of J also reduces the energy of activation of k which demonstrates the relation between both kinds of processes.