Nature of the Rh-H-2 bond in a dihydrogen complex stabilized only by nitrogen donors. Inelastic neutron scattering study of Tp(Me(2))RhH(2)(eta(2)-H-2) (Tp(Me(2)) equals hydrotris (3,5-dimethylpyrazolyl)borate)

The tunnel splitting of the librational ground state and the torsional frequencies of the dihydrogen ligand in Tp(Me2)RhH(2)(eta(2)-H-2) (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate) were measured using inelastic neutron The barrier for the rotation of the H-2 ligand and its H-H separation, calculated from these data, are 0.56(2) kcal/mol and 0.94 Angstrom, respectively. These values indicate that pi-back-donation from the Tp(Me2)RhH(2) fragment to H-2 is relatively weak and/or the interaction between the coordinated dihydrogen molecule and the two cis-hydride ligands significantly lowers the barrier for H-2 rotation.