Stabilization of negative charge by the cobalticinium nucleus

Abstract The acidity of alkylcobalticinium salts is greatly enhanced and the basicity of aminocobalticinium salts is greatly reduced in comparison to alkyl- and amine-benzenes and -ferrocenes. The methyl hydrogens of 1,1′-dimethylcobalticinium ion exchange in 0.91 M NaOD in D 2 0 with a rate constant of 1.40 x 10 −2 min −1 at 70°C and an activation energy of 25.4 ± 1 kcal mol −1 . The p K a of benzhydrylcobalticinium ion is 15–16 at 25°C, approximately 10 11 times more acidic than triphenylmethane. The following basicities (P K b ) were observed: aminocobalticinium, 15.6 ± 0.1, 1,1′-diaminocobalticinium, 13.5 ± 0.1 and 18.1 ± 0.1, 1-amino-1′-carboxycobalticinium, 18.06 ± 0.05. These values are interpreted in terms of strong electron-withdrawing field, inductive and resonance effects exerted by the cobalticinium moiety.