KINETICS OF (CO)-C-13 EXCHANGE WITH (CO)-C-12 IN [HM3(CO)11]- AND [DM3(CO)11]- (M = RU, OS) - STUDY OF THE EFFECTS OF ION-PAIRING AND DEUTERIUM LABELING ON THE EXCHANGE PROCESS AND HYDRIDE ACTIVATION

Rates of exchange of (CO)-C-13 gas (0.1-6.0 atm) with (CO)-C-12 in [HM3(CO-12)11]- and [DM3(CO-12)11]- (M = Ru, Os) were determined. The appearance of (CO)-C-12 in the gas phase above tetrahydrofuran solutions of the cluster salts was monitored by IR spectroscopy as a function of time. Overall (CO)-C-13-(CO)-C-12 exchange occurs by two parallel pathways. A dissociative pathway, which is first order in anion and zero order in CO, dominates the exchange process. As the concentration of (CO)-C-13 is increased, an associative step that is first order in cluster anion and first order in CO becomes increasingly significant. Exchange reactions involving [HRu3(CO)11]- are faster than those involving [HOs3(CO)11]-. With increased ion-pairing ability of the counterion of [HRu3(CO)11]-, the rate of exchange increases (PPN+ congruent-to PPh4+ < NEt4+ < K+ < Na+ < Li+), affecting, principally, the dissociative step. Similar results are observed for [HOs3(CO)11]- (PPh4+ < NEt4+ < K+ congruent-to Na+). Addition of BH3 increases the rate of exchange through the dissociative pathway. The associative pathway of exchange is related to the activation of hydride in these cluster anions when under water-gas shift reaction conditions. Examination of (CO)-C-13 exchange with (CO)-C-12 in the salts [X][HM3(CO)11] ([X] = Na+, [PPh4]+; M = Ru, Os) and [X][DM3(CO)11] ([X] = Na+, [PPh4]+; M = Ru, Os) reveals a kinetic isotope effect for the associative step, k2(H)/k2(D) = 1.4-1.8, which can be related to the displacement by CO of the bridging to a terminal site.