Dynamic behavior in solution of the trans-hydridodihydrogen complex [OsHCl(η²-H<inf>2</inf>)(CO)(PiPr<inf>3</inf>)<inf>2</inf>]: Ab initio and NMR studies

Reaction of complex [OsHCl-(CO)(PiPr3)2] (1) with hydrogen gives the trans-hydridodihydrogen complex [OsHCl(η2- H2)(CO)(PiPr3)2] (2). The H-H distance in the dihydrogen ligand, determined by variable-temperature 1H T 1 measurements, is 0.8 Å. The fast-spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the η2-D2 ligand of [OsDCl(η2-D2)(CO)(PiPr3)2] ([D3]2). Measurements of the equilibrium constants for formation of 2 (K) give ΔH° = -14.1 ± 0.5 kcalmol-1 and ΔS° = -30 ± 1 e.u. An equilibrium isotope effect K d/K of 2.8 is found for this reaction. The activation parameters for the H2 loss from 2 are ΔH≠ = 14.6 ± 0.2 kcalmol-1 and ΔS≠ = 9.9 ± 0.5 e.u. Hydrogen exchange between the hydrido and η2-H2 ligands of 2 takes place at a slow rate (k2obs) at high temperatures. Activation parameters ΔH≠ = 17.4 ± 0.5 kcalmol-1 and ΔS≠ = 1.3 ± 1 e.u., and a kinetic isotope effect (k2obs/k2dobs of 4.6 at 333 K have been determined for this exchange. Ab initio calculations on the model system [OsHCl(H2)(CO)(PH3)2] confirm that the trans-hydridodihydrogen complex [OsHCl(η2-H 2)(CO)(PH3)2] (4) is the most stable species (rHH = 0.84 Å). In addition, a cis-hydridodihydrogen complex 5, with a relative energy of 13.8 kcalmol-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/η2-H2 exchange is discussed. © VCH Verlagsgesellschaft mbH, 1996.