Molecular Motion in the Interconverting σ-H, Di-, and Tri-hydride Regimes: .

The transition-metal complex is known to exist in three possible isomeric forms, including a nonclassical, σ-bound dihydrogen complex and two classical dihydride isomers. As such, it has served as a model complex for the energies of conversion between these limiting structural regimes. In the present study, ab initio molecular dynamics computer simulations, combined with enhanced sampling techniques, were utilized to directly assess the degree of motion and isomerization of the dihydrogen/dihydride moieties in this complex. Ligand rotations (for both the H unit and the phosphine units) were found to be dominant in the low-temperature (298 K) regime, and the classical thermodynamic distribution showed no probability of thermally accessing dihydride forms, although unrestrained molecular dynamics trajectories showed fleeting configurations outside of the σ-H configuration. Simulations at higher temperatures surprisingly revealed new hydride isomers that are energetically competitive with the σ-H and -/-dihydride isomers. Low-energy pathways to hydrogen/hydride transfer and phosphine dissociation were readily accessible, which considerably expands the known isomeric flexibility of this complex.