Stabilization of negative charge by the cobalticinium nucleus : III. The effect of substituents on the acidities of hydroxy-cobalticinium and -rhodicinium salts

Abstract

The acidity constant, K a, has been determined spectrophotometrically for 13 substituted hydroxycobalticinium and hydroxyrhodicinium salts. The acidity is increased with increasing electronegativity of the substituent but is decreased with rhodium(III) replacing cobalt(III) or with the addition of five methyl groups on the second cyclopentadienyl ring. An attempt has been made to relate acidities to Hammett's substituent constants. The p K a of the unsubstituted hydroxycobalticinium ion is estimated to be 3.38 ± 0.08. The p K as of 10 hydroxycobalticinium salts show a linear correlation with v(CO) of the deprotonated cyclopentadienone analogues. Values of p K a1 and p K a2 of cyclopentadienyl(tetraethyl- p-hydroquinone)cobalt have been found to be −2.42 ± 0.10 and 1.15 ± 0.06.