The Barrier to Rotation of the Dihydrogen Ligand in the Complexes [mh(η²-H₂)PP₃]BPh₄ (M = Fe, Ru): Evidence for Stronger Back-Donation in the Iron Complex

The barrier to rotation of the dihydrogen ligand in the complexes [MH(η²-H₂)PP₃]BPh₄ (M = Fe, Ru), was determined from inelastic neutron scattering studies of the dihydrogen rotational energy levels. Our results demonstrate unambiguously that this barrier is higher in the Fe complex than in the Ru analog. With the assumption of a simple 2-fold barrier our analysis yields heights of 1.82 and 1.36 kcal/mol, respectively, where the rotational constant for the molecular hydrogen ligand (B = 43 cm^-1) was calculated from a value for the H-H distance of 0.87 Å given by the NMR results of Morris and collaborators on related complexes. Since previous studies have shown that this barrier has its origin primarily in the back-donation from the metal to the antibonding orbital of the dihydrogen, one may conclude that the Fe center is a better back-donor than Ru, contrary to what general trends in the periodic table would lead one to expect. This result provides direct support for a hypothesis made by Morris and collaborators to explain their NMR results on a different series of dihydrogen complexes of the Fe group.