Dynamic behavior in solution of the trans-hydridodihydrogen complex [OsHCl(eta(2)-H-2)(Co)(PiPr(3))(2)]: Ab initio and NMR studies

Reaction of complex [OsHCl-(CO)(PiPr(3))(2)] (1) with hydrogen gives the trans-hyrdridodihydrogen complex [OsH-Cl(eta(2)-H-2)(Co)(PiPr(3))(2)] (2). The H-H distance in the dihydrogen ligand, determined by variable-temperature H-1 T-1 measurements. is C.8 Angstrom. The fast-spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the eta(2)- ligand of [OsDiCl(eta(2)-D-2)(Co)(PiPr(3))(2)] ([D-3]2). Measurements of the equilibrium constants for formation of 2 (K) give Delta H degrees = -14.1 +/- 0.5 kcal mol(-1) and Delta S degrees = -30 +/- e.u. An equilibrium isotope effect K-d/K of 2.8 is found for this reaction. The activation parameters for the H-2 loss from 2 are Delta H-double dagger = 14.6 +/- 0.2 kcal mol(-1) and Delta S-double dagger = 9.9 +/- 0.5 e.u. Hydrogen exchange between the hydride and eta(2)-H-2 ligands of 2 takes place at a slow rate (k(2)(obs)) at high temperatures. Activation parameters Delta H-not equal = 17.4 +/- 0.5 kcal mol(-1) and Delta S-not equal = 1.3 +/- 1 e.u., and a kinetic isotope effect (k(2)(obs)/k(2d)(obs)) of 4.6 at 333 K have been determined for this exchange. Ab initio calculations on the model system [OsH-Cl(eta(2))(CO)(PH3)(2)] confirm that the trans-hydridodihydrogen complex [OsH-Cl(eta(2)-H-2)(CO)(PH3)(2)] (4) is the most stable species (r(HH) = 0.84 Angstrom). In addition, a cis-hydridodihydrogen complex 5, with a relative energy of 13.8 kcal mol(-1), occupies a local minimum in the potential hypersurface. The energy of other possible trihydrido isomers have also been evaluated. On the basis of the thermodynamic and kinetic data, and the results of the ab initio calculations, the possible mechanism for the H/eta(2)-H-2 exchange is discussed.