Acidity of triorganosilane- and triorganogermane-carboxylic acids. The importance of pπ→dπ bonding

Abstract

The ionization constants of carboxylic acids of the type, Ph_nMe_{3 –n}MCO₂H (M = C, Si, and Ge) have been determined in 45 and 76%(w/w) ethanol–water at 25°. In all cases the triorganosilane- and triorganogermane-carboxylic acids are more acidic than their carbon analogues. The order of acidity observed for the triphenyl derivatives is Si Ge > C, which is in accord with the p_π→d_π bonding-order of these elements, rather than the inductive order. The relative acidities and basicities of the carbon, silicon, and germanium derivatives of R₃M(CO)X (X = Ph or OH) are considered in terms of inductive and p_π→d_π bonding effects. The p_π→d_π bonding appears to be an important factor in stabilizing the conjugate base forms of the triorganosilane- and triorganogermane-carboxylic acids.