LIGAND-EXCHANGE PROCESSES OF OSHCL(CO)(L)(PR3)(2) (L = VACANT, H-2, R'CN, O-2, R = CY, I-PR)

The reactivity of complexes formed by the addition of O-2, H-2 and R'C N to OsHCl(CO)(PR3)(2) (1a:R = Cy; 1b:R = i-Pr) has been examined. Und er 24 bar H-2 and 65 degrees C, the dioxygen ligand of OsHCl(CO)(O-2)( PR3)(2) (2a,b) is displaced to yield the trans-hydridodihydrogen compl exes OsHCl(eta(2)-H-2)(CO)(PR3)(2) (3a,b). Measurements of the equilib rium constant, K-H2 = [3a]/[1a][H-2], for the direct addition of H-2 t o 1a yield Delta H degrees = -49.1 +/- 2.4 kJ/mol and Delta S degrees = -95.7 +/- 7.9 J/mol K. 1a,b react reversibly with aryl and alkyl nit riles to produce the isolable complexes, OsHCl(CO)(R'CN)(PR3)(2) (4a,b ). The phosphine ligands of la,b and 3a,b exchange with unbound, bulky alkyl phosphines at a rate that is slow relative to the NMR timescale . In the presence of excess PCy3, complex 3b yields the exchange produ cts OsHCl(eta(2)-H-2)(CO)(Pi-Pr-3)(PCy3) and 3a. While a tris-phosphin e complex cannot be detected, limited kinetic data characterize the ex change as associative process. (C) 1998 Elsevier Science B.V. All righ ts reserved.