Spectroscopic Investigation of Water-Ammonia Ligand Exchange in Ni(II) Complexes

Abstract

The substitution of water by an incoming ligand, such as NH₃, is a key mechanistic step in Ni(II) ligand exchange. However, the solution speciation and persistent formation of intermediate aqua-ammine complexes during stepwise substitution remain difficult to experimentally characterize. This study determined the stoichiometry of the Ni2+–H₂O–NH₃ system using UV-Vis spectroscopy. Beer’s law calibration of [Ni(H₂O)₆]²⁺ was combined with spectroscopic methods such as the mole ratio method and Job’s method of continuous variation to quantitatively determine stoichiometry. Mole ratio analysis at 394 nm showed a breakpoint corresponding to a Ni2+:NH₃ ratio of 1.1:4.6, suggesting partially substituted aqua-ammine complexes may be present during the substitution process. Job’s method showed a maximum corresponding to a Ni2+:NH₃ stoichiometric ratio of 1:6, which is consistent with formation of the hexamine complex [Ni(NH₃)₆]2+. These results demonstrate how spectroscopic stoichiometry methods can be used to characterize coordination complex speciation during ligand exchange and motivate future kinetic experiments to distinguish associative, dissociative, and interchange substitution pathways.