Mechanisms of organometallic reactions. Part II. Kinetics of the reaction of acetonitrile with tricarbonyl-(cycloheptatriene) and -(arene) complexes of metals in Group VI

Abstract

Kinetic studies of the reaction of acetonitrile with [(C₇H₈)M(CO)₃](M = Cr, Mo, or W) complexes show the third-order rate law, Rate =k[Complex][MeCN]². This order is explained in terms of a mechanism involving pre-equilibrium association between the complex and acetonitrile, followed by rate-determining addition of a second molecule of MeCN. The ease of cycloheptatriene displacement varies with the nature of the metal in the order Mo > W > Cr. In contrast, similar studies of the reaction of acetonitrile with the complex [(C₆H₃Me₃)Mo(CO)₃] indicate the second-order rate law, Rate =k[Complex][MeCN]. The change in mechanism is rationalised in steric terms. Comparison with previous data for the corresponding reaction of acetonitrile with the cation [(C₇H₇)Mo(CO)₃]⁺ suggest the order C₇H₈ > C₆H₃Me₃ > C₇H₇ for ease of displacement of π-bonding organic ligands from [LMo(CO)₃]ⁿ⁺(n= 0 or 1) species.