Factors that affect the kinetic and thermodynamic acidities of Fischer carbene complexes: New insights from the study of the cationic (methoxymethylcarbene)dicarbonylpentamethylcyclopentadienyl iron ([Cp*(CO)2Fedouble bond; length as m-dashC(OMe)Me]+) in

A kinetic study of the reversible deprotonation of [Cp*(CO)2Fedouble bond; length as m-dashC(OMe)Me]+ (16H+) by primary aliphatic amines, secondary alicyclic amines and carboxylate ions in 50% MeCN–50% water (v/v) at 25 °C is reported. A pK a value of 8.25 for 16H+ was determined kinetically. Intrinsic rate constants, k 0 for each buffer family were obtained by suitable extrapolation or interpolation of the corresponding Brønsted plots; they are View the MathML sourcelogk0RNH2=2.64,logk0R2NH=2.99 and View the MathML sourcelogk0RCOO=3.42, respectively. A major focus of this paper is a discussion of how structural variations of Fischer carbenes affect their acidities and intrinsic rate constants for proton transfer and how the results obtained for 16H+ add new insights into the interplay of these structural factors. These factors include the charge type of the carbene complex, the metal/ligand combination, the π-donor group attached to the carbene carbon, the push–pull resonance effect in conjunction with the metal moiety, the π-acceptor groups attached to the carbene carbon, and the potential for aromaticity/antiaromaticity of the conjugate base.