Structural, Electronic, and Spectral Properties of Metal Dimethylglyoximato [M(DMG)₂; M = Ni²⁺, Cu²⁺] Complexes: A Comparative Theoretical Study

Dimethylglyoxime (DMG) usually forms thermodynamically stable chelating complexes with selective divalent transition-metal ions. Electronic and spectral properties of metal-DMG complexes are highly dependent on the nature of metal ions. Using range-separated hybrid functional augmented with dispersion corrections within density functional theory (DFT) and time-dependent DFT, we present a detailed and comprehensive study on structural, electronic, and spectral (both IR and UV–vis) properties of M­(DMG)2 [M = Ni2+, Cu2+] complexes. Ni­(DMG)2 results are thoroughly compared with Cu­(DMG)2 and also against available experimental data. Stronger H-bonding leads to greater stability of Ni­(DMG)2 with respect to isolated ions (M2+ and DMG–) compared to Cu­(DMG)2. In contrast, a relatively larger reaction enthalpy for Cu­(DMG)2 formation from chemically relevant species is found than that of Ni­(DMG)2 because of the greater binding enthalpy of [Ni­(H2O)6]2+ than that of [Cu­(H2O)6]2+. In dimers, Ni­(DMG)2 is found to be 6 kcal mol–1 more stable than Cu­(DMG)2 due to a greater extent of dispersive interactions. Interestingly, a modest ferromagnetic coupling (588 cm–1) is predicted between two spin-1/2 Cu2+ ions present in the Cu­(DMG)2 dimer. Additionally, the potential energy curves calculated along the O–H bond coordinate for both complexes suggest asymmetry and symmetry in the H-bonding interactions between the H-bond donor and acceptor O centers in the solid-state and in solution, respectively, well corroborating with early experimental findings. Interestingly, a lower proton transfer barrier is obtained for the Ni­(DMG)2 compared to its Cu-analogue due to stronger H-bonding in the former complex. In fact, relatively weaker H-bonding in Cu­(DMG)2 results in blue-shifted O–H stretching modes compared to that in Ni­(DMG)2. On the other hand, qualitatively similar optical absorption spectra are obtained for both complexes with red-shifted peaks found for the Cu­(DMG)2. Finally, computational models for axial mono- and diligand (aqua and ammonia) coordinated M­(DMG)2 complexes are predicted to be energetically feasible and stable with relatively greater binding stability obtained for the ammonia-coordination