The specification of the production of three different complexes via thermodynamic investigations between vanadyl (IV) bis(acetylacetonate) and D-penicillamine at three various pHs

d-H2Pen-binding behavior to [VO(acac)2] at pH=7.00, 7.50 and 10.0 has been studied in thermodynamic viewpoint using UV/VIS spectroscopy. The optical absorption spectra of [VO(acac)2] were analyzed in order to obtain binding constants and stoichiometries using SQUAD software. The estimation of binding constant at various temperatures enabled us to calculate all of the thermodynamic parameters of binding using the van't Hoff equation. Studies of described reactions at pH=7.00, 7.50 and 10.0 show exothermic, endothermic and exothermic modality, respectively. d-penicillamine is a potentially tridentate ligand, but the pKa for the ‒ COOH, NH3 and S‒ H groups are 1.99–2.00, 8.0 and 10.6, respectively, as a result S‒H can not release proton at pH=7.00 and pH=7.50, so the coordination of S is not possible except at strong alkali mediums. At pH=7.00, d-H2Pen converts [VO(acac)2] to a vanadyl Schiff base complex (coordination mode is N2O2). At pH=7.50, d-H2Pen is converted to HPen− and the exchange of acac− with HPen− produces VO(HPen)2. It is clear that the coordination is formed via amine nitrogen and carboxylate oxygen. At pH=10.0, the main product is a vanadyl complex with the (S2O2) coordination mode. Studies in different ionic strengths of KCl confirm these products according to the number of ions in each medium. The formation constants of the products of VO(acac)2 with d-penicillamine at pH=7.00 and 7.50 are independent of ionic strength, but a Debye–Huckel-type equation was established for the dependence of the formation constant on ionic strength at pH=10.0.