Synthesis and Characterization of Coordinately Unsaturated Phosphine Complexes of Divalent

the H2 reductive-elimination process of importance to homogeneous and heterogeneous hydrogenation catalysis, the isotope effect observed supports the model of a late transition state where the H-H bond forms nearly completely either in the transition state or in a metastable q2-H2 intermediate. The relatively low (10-20 kcal/mole) enthalpies of activation for hydrogen reductive elimination for these late metal dihydrides emphasize the small energy difference between the dihydride, q2-dihydrogen, and free dihydrogen structures. The high stability of platinum(I1) dihydrides that contain bulky phosphine ligands must derive primarily from kinetic stability. Our results show that hydrogen reductive elimination is surprisingly solvent sensitive, with the dihydride complex being kinetically stabilized in polar-coordinating solvents. Invoking the principle of microscopic reversibility allows this solvent effect to be viewed in another context. Addition of molecular hydrogen to form a cis-dihydride complex must involve some charge separation, and polar solvents might aid in stabilizing dihydride formation. The similarity between the isotope effects for dihydrogen reductive elimination, and those observed r e ~ e n t l y ' ~ . ~ ~ for alkane 27, 1775-1781 1775