Investigating pathways of molecular H-2 exchange in (mu-H)(2)Os-3(CO)(10)

The occurrence of two pathways responsible for the reaction of (mu -H)(2)Os -3(CO)(10) with molecular hydrogen-has been elucidated by comparing the res ults obtained from H-2/D-2 isotopic exchange experiments and para-H-2 effec ts observed in the cluster's H-1 NMR spectrum. The most efficient exchange process is based on an associative mechanism that leads to the formation of the elusive (eta-*H-2)(H)(mu -H)Os-3(CO)(10) species (*H-2 is either a D-2 or a para-H-2 molecule), which undergoes *H-H elimination. The other pathw ay (whose efficiency increases with temperature) is based on the dissociati on of H-2 from (mu -H)(2)Os-3(CO)(10) to form the highly unsaturated "OS3(C O)(10)" moiety, which promptly adds a *H-2 molecule, yielding the asymmetri cal *H(mu-*H)Os-3(CO)(10)S intermediate (where S is a stabilizing solvent m olecule), which releases the S molecule to re-form (mu-*H)(2)Os-3-(CO)(10).