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 t he trans-hyrdridodihydrogen complex [OsH-Cl(eta(2)-H-2)(Co)(PiPr(3))(2 )] (2). The H-H distance in the dihydrogen ligand, determined by varia ble-temperature H-1 T-1 measurements. is C.8 Angstrom. The fast-spinni ng 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 equ ilibrium 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 equilibriu m isotope effect K-d/K of 2.8 is found for this reaction. The activati on 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. Hyd rogen exchange between the hydride and eta(2)-H-2 ligands of 2 takes p lace at a slow rate (k(2)(obs)) at high temperatures. Activation param eters Delta H-not equal = 17.4 +/- 0.5 kcal mol(-1) and Delta S-not eq ual = 1.3 +/- 1 e.u., and a kinetic isotope effect (k(2)(obs)/k(2d)(ob s)) 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 additio n, a cis-hydridodihydrogen complex 5, with a relative energy of 13.8 k cal mol(-1), occupies a local minimum in the potential hypersurface. T he energy of other possible trihydrido isomers have also been evaluate d. 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.