The Mechanism of Substitution of Dicobalt Octacarbonyl

The kinetics of the substitution of dicobalt octacarbonyl, Co2(CO)8, by triphenylarsine, AsPh3, tributylphosphine, PBu3, triphenylphosphine, PPh3, and l3C-enriched CO in hexane solution has been observed at temperatures in the range of +30 to -15 °C. Reaction with AsPh3 leads first to Co2(CO)7AsPh3 and then to Co2(CO)6[AsPh3]2. The rate is first order in Co2(CO)8 and zero order in AsPh3 and is reduced by added CO. The rate-determining step is presumably CO loss. At 25 °C, k1 = 2.0 X 10-2 s-1. From variation in k with temperature, ∆H* = 93 ± 4 kJ mol-1 and ∆S* = +42 ± 10 J K-1 mol-1. The exchange reaction of Co2(CO)8 with 13CO proceeds at 0 °C with pseudo-first-order rate constant at 0 °C of about 3 X 10-4 s-1 in good agreement with the extrapolated rate constant for reaction with AsPh3. Reaction with PBu3 is very fast; it leads to [Co(CO)3(PBu3)2]Co(CO)4 as the product. Reaction with PPh3 leads to an analogous product but the reaction is slower. The kinetic behavior is complex; the apparent order in Co2(CO)8 varies from values near 1.5 at the lowest temperatures studied to values of about 1.0 at the higher temperatures. The reactions are markedly inhibited by traces of oxygen. A radical chain mechanism fits the observations very well. Key steps in the radical chain involve an electron-transfer process of the form Co(CO)3L. + Co2(CO)8 - Co(CO)3L+ + Co2(CO)8-, rapid dissociation of the radical anion to form Co(CO)4- and Co(CO)4., and then rapid substitution of Co(CO)4 by L to form Co(CO)3L. Numerical integration of the rate equations, using reasonable values for the various rate constants, leads to computed behavior that closely approximates the observations.