Ternary nickel(II) complexes with histidine and glycylglycine in solution: Thermodynamic approach

•The ionic composition of solutions in the nickel-histidine-glycylglycine system has been refined.•Mixed complexes of the types NiGGmHnHism (m = 1, 2; n = 0, 1,-1) have been revealed.•The thermodynamic parameters of glycylglycine addition to Ni(II) histidine complex are analyzed.•Coordination modes of amino acid and dipeptide residues have been proposed based on a thermodynamic approach.