The effect of inner orbital splitting on the thermodynamic properties of the transition metal complexes of serine and threonine

Abstract

According to the Ligand Field Theory, the incomplete inner shells in the transition metal ions modify the thermodynamic properties of their compounds. In co-ordination complexes of the 3d transition metal ions, with the exception of those with d⁰, d⁵ and d¹⁰ electronic configuration, the splitting of the 3d orbitals in the ligand field results in the stabilization of the inner electrons compared to the energy they would have had in the absence of the ligand field. George and McClure[1] have shown that the Ligand Field Stabilization Energy can be determined from the heats of complex formation. This paper deals with the results of our studies on the thermodynamics of interaction of the 3d⁵-3d¹⁰ transition metal ions with the aminoacids dl-serine and dl-threonine in aqueous solution. The thermodynamic quantities ΔG (free energy change), ΔH_c (enthalpy change) and ΔS (entropy change) refer to the following equilibrium: [M(H₂O)_x]²⁺ + 2(A^-) M(A)₂(H₂O)_x-y + yH₂O where [M(H₂O)_x]²⁺ represents the aquated divalent transition metal ion and A^- the aminoacid anion.