Physical Organic Chemistry of Transition Metal Carbene Complexes. 8.1 Kinetic and Thermodynamic Acidities of Alkoxyalkylcarbene Pentacarbonyl Complexes of Cr, Mo, and W in Aqueous Acetonitrile. Dependence on Metal, Alkyl Group, and Alkoxy Group

A study of the acidity of various Fischer-type (alkoxyalkylcarbene)pentacarbonylmetal complexes and of their rates of deprotonation by OH- and amines in 50% acetonitrile−50% water at 25 °C is reported. The pKa values of (CO)5MC(OMe)CH3 for M = W (12.36) and M = Mo (12.81) are quite comparable to that for M = Cr (12.50) determined previously, and the pKa of (CO)5WC(OMe)CH2Ph (10.18) is similar to that of the chromium analog (10.40) reported earlier. These results indicate that the stabilization of the negative charge of the anion by the (CO)5M moiety depends little on the metal. The pKa of (CO)5CrC(OEt)CH3 (12.98) is 0.48 units higher than that of the methoxy analog; this increase probably reflects an enhanced stabilization of the zwitterionic resonance structure of the carbene complex, (CO)5Cr-C(O+Et)CH3, by the stronger electron-donating effect of the ethyl group. Intrinsic rate constants for the deprotonation of the carbene complexes by OH- (koOH) were estimated based on the Marcus equation, and intrins...