PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES .8. 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)pen tacarbonylmetal complexes and of their rates of deprotonation by OH- a nd amines in 50% acetonitrile-50% water at 25 degrees C is reported. T he pK(a) values of(CO)(5)M=C(OMe)CH3 for M = W (12.36) and M = Mo (12. 81) are quite comparable to that for M = Cr (12.50) determined previou sly, and the pK(a) of (CO)(5)W=C(OMe)CH2Ph (10.18) is similar to that of the chromium analog (10.40) reported earlier. These results indicat e that the stabilization of the negative charge of the anion by the (C O)(5)M moiety depends little on the metal. The pK(a) of(CO)(5)Cr=C(OEt )CH3 (12.98) is 0.48 units higher than that of the methoxy analog; thi s increase probably reflects an enhanced stabilization of the zwitteri onic resonance structure of the carbene complex, (CO)(5)Cr-C(=O+Et)CH3 , by the stronger electron-donating effect of the ethyl group. Intrins ic rate constants for the deprotonation of the carbene complexes by OH - (k(0)(OH)) were estimated based on the Marcus equation, and intrinsi c rate constants for the deprotonation of some of the carbene complexe s by amines (k(0)(B)) were determined from Bronsted plots. The depende nce of k(0)(OH) and K-0(B) On the metal is rather small and hardly sig nificant, except possibly for the reaction of(CO)(5)W=C(OMe)CH3 with s econdary amines, for which k(0)B appears to be lower for the tungsten complex than for the chromium and molybdenum derivatives; k(0)(OH) for (CO)(5)Cr=C(OEt)CH3 is the same as for the methoxy analog.