H D ISOTOPIC EXCHANGE ON THE SURFACE OF A TRIOSMIUM CARBONYL CLUSTER/

The reactions of both Os-3(CO)(10)(NCCH3)(2) and Os-3(CO)(11)(NCCH3) w ith HD Or with a mixture of H-2/D-2 in chloroform solution at room tem perature afford a mixture of H2Os3(CO)(10) (Ia), HDOs3(CO)(10) (Ib), a nd D2Os3(CO)(10) (Ic). The three isotopomers are also obtained by reac ting H2Os3(CO)(10) with D-2. By measurement of the rates of disappeara nce of Ia in the latter reaction at four different temperatures, the t hermodynamic parameters have been estimated (E(a) = 43.7 kJ/mol, Delta H-double dagger = 41.2 kJ/mol, Delta S-double dagger = -0.17 kJ/(mol K)). No H/D isotopic exchange has been observed after heating Ia and I c in toluene for several hours. A mechanism involving the reversible o xidative addition and reductive elimination of molecular hydrogen on H 2Os3(CO)(10) via the formation of a fluxional tetra(hydrido, deuterio) intermediate is proposed.