MECHANISTIC STUDY OF THE SUBSTITUTION BEHAVIOR OF COMPLEXES OF THE TYPE M(CO)5(THF) (M = CR, MO, W)

The kinetics and mechanism of the substitution of M(CO)5(THF) (M = Cr, Mo, W) by piperidine, triethyl phosphite, and triphenylphosphine in tetrahydrofuran (THF) were studied with use of conventional and high-pressure techniques. The reactions follow the reactivity order Mo > Cr > W and are characterized by DELTA-H values between 38 and 78 kJ mol-1, DELTA-S values between 0 and -120 J K-1 mol-1, and DELTA-V values between -2 and -15 cm3 mol-1. The latter values exhibit a specific trend toward more negative values along the series Cr, Mo, and W, which is interpreted as evidence for a gradual changeover in mechanism from a more dissociative to a more associative activation process along the series of complexes.