Stereoselectivity in the formation of mononuclear complexes of histidine and some bivalent metal ions

Abstract

The formation constants of complexes of Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, and [VO]²⁺ with D- and L-histidine have been measured at 25.0 °C and I= 0.10 mol dm^–3(K[NO₃]). The importance of protonated complexes in all the systems studied has been demonstrated and the structures of the various species are discussed. Thermodynamic stereoselectivity has been confirmed in the bis(histidinato)-complexes of Co²⁺, Ni²⁺, and Zn²⁺, and has been also found in the monoprotonated bis(histidine) complexes of Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺. Stereoselectivity as exhibited in the enthalpy change (ΔH) accompanying formation of bis(histidinato)cobalt(II) has been measured calorimetrically; ΔH for formation of the meso-bis complex is more favourable by 2.4 ± 0.2 kJ mol^–1.