Thermokinetic Study on the Complexation Reaction of the First-Row Transitional Metal Chlorides with Histidine

The enthalpy change of the complexation reactions of the first‐row transitional metal chlorides including CrCl₃, MnCl₂, FeCl₂, CoCl₂, NiCl₂ and CuCl₂ with L‐α‐histidine in water were determined by a microcalorimeter at 298.15‐323.15 K. The standard enthalpy of formation of Cr(His) (aq) and M(His) (aq) (M=Mn, Fe, Co, Ni and Cu) were calculated. Based on the thermodynamic and kinetic equations of the reactions, three thermodynamic parameters (the activation enthalpy, the activation entropy, the activation free energy), the rate constants, and three kinetic parameters (the apparent activation energy, the pre‐exponential constant and the reaction order) are obtained. The solid complexes of CrCl₃, MnCl₂, FeCl₂, CoCl₂, NiCl₂ and CuCl₂ with histidine were prepared and identified as Cr(His)₂Cl₃·H₂O. Mn(His)₂Cl₂·4H₂O. Fe(His)₂Cl₂·H₂O, Co(His)₂Cl₂·H₂O, Ni(His)₂Cl₂·H₂O and CU(HiS)₂Cl₂·H₂O by chemical and elemental analyses. The bonding characteristics of these complexes were characterized by R as well. The results showed that, with the atomic number increasing, three thermodynamic parameters, ΔG, ΔH and ΔS of the complexation reaction of these metal chlorides with L‐α‐histidine in water present an analogy regularity.