Stabilization of negative charge by the cobalticinium nucleus : II. Acidity of hydroxy-2,3,4,5-tetraaryl and hydroxy-2,3,4,5-tetraalkyl cobalticinium and rhodicinium salts

Abstract Hydroxy-2,3,4,5-tetra-substituted cobalticinium and rhodicinium salts exist in proteolytic equilibrium with stable cyclopentadienone complexes. The changes in IR, UV and NMR spectra upon dissociation are described. Acidity constants, K a , have been determined spectrophotometrically for the following hydroxymetallocinium salts, C 5 R 4 OH M + C 5 H 5 : R = CH 3 , M = Co, 4.30 ± 0.07; R = C 6 H 5 , M = Co, 2.42 ± 0.05; R = C 6 F 5 , M = Co, −0.60 ± 0.10; R = C 6 H 5 , M = Rh, 2.54 ± 0.03; R = C 6 F 5 , M = Rh, −0.41 ± 0.10. The acidity increases with increasing electronegativity of the R group and decreases slightly when Co III is replaced by Rh III . The lower acidity of the Rh compounds reflects a slightly lower electronegativity of Rh as compared to Co.