Do early and late transition metal noble gas complexes react by different mechanisms? A room temperature time-resolved infrared study of (η⁵-C₅R₅)Rh(CO)₂ (R = H or Me) in supercritical noble gas solution at room temperature

Using fast time-resolved infrared spectroscopy, the late transition metal noble gas complexes CpRh(CO)(L) and Cp*Rh(CO)(L) (Cp = η⁵-C₅H₅; Cp* = η⁵- C₅Me₅; L = Xe and Kr) have been characterized at room temperature in supercritical noble gas solution. The krypton complexes are more reactive than the corresponding xenon compounds. There is a significant difference in reactivity with CO between CpRh(CO)(L) and Cp*Rh(CO)(L) with CpRh(CO)(Xe) (k_co = 6.7 × 10⁵ dm³ mol^-1 s^-1) being ca. 20 times less reactive than Cp*Rh(CO)(Xe) (k_co = 1.4 × 10⁷ dm³ mol^-1 s^-1). Similarly CpRh(CO)(Kr) (k_co = 1.3 × 10⁸ dm³ mol^-1 s^-1) is also less reactive than Cp*Rh(CO)(Kr) (k_co = 4.3 × 10 ⁸ dm³ mol^-1 s^-1). This should be contrasted to the early transition metal noble gas complexes, where for a given metal-noble gas complex, CpM(CO)₂(L) and Cp*M(CO) ₂(L) (M = Mn or Re) have similar reactivity towards CO. The activation parameters for the reaction of CpRh(CO)(L) and Cp*Rh(CO)(L) with CO have been determined above room temperature, and suggest that the reaction proceeds by an associative mechanism.