Physical organic chemistry of transition metal carbene complexes. 19. Kinetics of reversible alkoxide ion addition to substituted (methoxyphenylcarbene)pentacarbonylchromium(0) and (methoxyphenylcarbene)pentacarbonyltungsten(0) in methanol and aqueous acetonitrile

Rate and equilibrium constants for the nucleophilic addition of MeO- in met hanol and in 90% MeCN-10% MeOH, of HC=CCH2O- and CF3CH2O- in 50% MeCN-50% w ater, and of OH- in 50% McCN-504b water (rate constants only) to Fischer ca rbene complexes of the type (CO)(5)M=C(OMe)C6H4-4-Z (M = Cr and W) are repo rted. The reactions lead to the formation of tetrahedral adducts of the typ e (Co)(5)(M) over bar-C(oMe)(OR)C6H4-4-Z. The kinetics of general acid cata lyzed loss of MeO- from the MeO- adducts by phenol, LF-bromophenol, and 3,5 -dichlorophenol, to regenerate the carbene complex, were also investigated. Hammett rho and rho(n) values as well as approximate beta(nuc) (beta(nuc)( n)) values were determined. They suggest that the transition state for alko xide ion addition to the carbene complexes is imbalanced in the sense that charge delocalization into the CO ligands lags somewhat behind C-O bond for mation. This finding is consistent with the fact that: the intrinsic rate c onstants for these reactions are substantially lower than those for alkoxid e ion addition to carboxylic esters. Another type of imbalance is seen for the transition state of the general acid catalyzed MeO- departure from the MeO- adducts. Here proton transfer to the leaving group is ahead of C-O bon d cleavage.