Stability, lability, spectral parameters and structure of complexes and stereoselective effects in the nickel(II) – l/d/dl-histidine – l/d/dl-methionine systems

The equilibrium constants, spectral characteristics of complexes, formation thermodynamic parameters and constants of the chemical exchange reactions in the nickel(II) – l / d / dl -histidine (HisH) – l / d / dl -methionine (MetH) systems (25 °C, 1.0 mol dm−3 KNO3) over wide ranges of pH and ligands to metal ratio were calculated from the pH-potentiometry, multi-wavelength electronic spectroscopy, calorimetry and NMR relaxation data. Based on the DFT computation the structures of some complexes have been proposed. Significant enantioselective effects are exhibited in the formation constants and in some electronic spectra and NMR relaxation parameters of a number of complexes with the meso-forms domination. For the Ni(His)2 complex the process of alkaline catalysis of proton exchange with the short-time N-H proton removal from the coordinated imidazole fragment was characterized. It was shown that in the ligand exchange reaction between Ni(His)2 and His− the Ni(His)3− complex participates as an intermediate. It was found that the Met− thiomethyl group is not bounded in the Ni(Met)+ and Ni(Met)2 complexes but coordinated in the Ni(His)(Met) form. Main factors determining stereoselectivity in the complex formation processes and ligand exchange reactions in the systems studied were identified as trans effect, solvation effect and a new type of interligand thiomethyl group–imidazole ring weak interaction.