“Hard and soft” behavior of Mn 2+, Cu 2+, and Zn 2+ with respect-to carboxylic acids and α-oxy- or α-thio-substituted carboxylic acids of biochemical significance

Abstract

An investigation has been made on the coordination of Mn 2+, Cu 2+, and Zn 2+, by several simple carboxylic acids and certain of α-oxy- and α-thiocarboxylic acids which have biochemical significance. The stability constants of the 1:1 complexes have been measured in water containing 50% dioxane ( I = 0.1; t = 25 °). With all three metal ions, the expected correlations between the basicity of the monodentate ligands and the stability of the complexes were found. The following potential bidentate α-oxyor α-thiocarboxylic acids were included in the measurements: Tetrahydrofuran 2-carboxylic acid (XI), hydroxyacetic acid (XII), tetrahydrothiophene 2-carboxylic acid (XIII), and S-carboxymethyl ethyl mercaptan (XIV). The complexes of the carboxylic acids with an oxygen in the α-position, (XI, XII), are more stable with all three metal ions than is expected on the basis of participation of the carboxyl function alone, i.e., chelates are formed. For the carboxylic acids with sulfur in the α-position, (XIII, XIV), such chelation is definite only for the Cu 2+ complexes. The values of the Zn 2+ and especially the Mn 2+ complexes are in the order expected from basicity of the carboxylate groups, i.e., there is no or only a weak coordination of the sulfur atom. NMR spectra of S-carboxymethyl ethyl mercaptan (XIV) with increasing amounts of the paramagnetic ions, Cu 2+ and Mn 2+, confirm these results: In the case of Cu 2+, the sulfur is strongly coordinated and a bidental chelate ligand results, while with Mn 2+ the spectra suggest that interaction with sulfur is weak and the simple monodental carboxylate complex probably dominates. These results are discussed with respect to Pearson's “hard-soft” rule and the selection of the coordinating groups in biological complexes, i.e., the question about the “selection rules” which determine the structure and reactivity in such complexes is considered.