Do early and late transition metal noble gas complexes react by different mechanisms? A room temperature time-resolved infrared study of (5-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 =5-C5H5; Cp*= 5-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)(kCO= 6.7 × 105 dm3 mol–1 s–1) being ca. 20 times less reactive than Cp*Rh(CO)(Xe)(kCO= 1.4 × 107 dm3 mol–1 s–1). Similarly CpRh(CO)(Kr)(kCO= 1.3 × 108 dm3 mol–1 s–1) is also less reactive than Cp*Rh(CO)(Kr)(kCO= 4.3 × 108 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.