Thermodynamics of formation of complexes of nickel(II) and copper(II) with L- and DL-serine and L- and DL-threonine

Abstract

Complexes formed between L- and DL-serine (Ser) and L- and DL-threonine (Thr) and H⁺, Ni²⁺, and Cu²⁺ at intermediate pH values have been studied potentiometrically at 25 °C and I= 0.10 mol dm^–3(K[NO₃]). The complexes of H⁺ and Cu²⁺ have also been studied calorimetrically. Stereoselectivity in the Gibbs free-energy changes is insignificant. However, the standard enthalpy of formation of [Cu(L-SerO)₂] is more favourable than that of [Cu(L-SerO)(D-SerO)](SerO = serininate) by a small but significant amount (1.7 kJ mol^–1). This stereoselectivity is explained by outer-sphere co-ordination between Cu²⁺ and un-ionized hydroxy-oxygen donor atoms. The presence of protonated complexes of both SerO and ThrO with Cu²⁺ at pH ⩽4.8 has been demonstrated.