Structural, Electronic, and Spectral Properties of Metal Dimethylglyoximato [M(DMG); M = Ni, Cu] 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) [M = Ni, Cu] complexes. Ni(DMG) results are thoroughly compared with Cu(DMG) and also against available experimental data. Stronger H-bonding leads to greater stability of Ni(DMG) with respect to isolated ions (M and DMG) compared to Cu(DMG). In contrast, a relatively larger reaction enthalpy for Cu(DMG) formation from chemically relevant species is found than that of Ni(DMG) because of the greater binding enthalpy of [Ni(HO)] than that of [Cu(HO)]. In dimers, Ni(DMG) is found to be 6 kcal mol more stable than Cu(DMG) due to a greater extent of dispersive interactions. Interestingly, a modest ferromagnetic coupling (588 cm) is predicted between two spin-/ Cu ions present in the Cu(DMG) 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) compared to its Cu-analogue due to stronger H-bonding in the former complex. In fact, relatively weaker H-bonding in Cu(DMG) results in blue-shifted O-H stretching modes compared to that in Ni(DMG). On the other hand, qualitatively similar optical absorption spectra are obtained for both complexes with red-shifted peaks found for the Cu(DMG). Finally, computational models for axial mono- and diligand (aqua and ammonia) coordinated M(DMG) complexes are predicted to be energetically feasible and stable with relatively greater binding stability obtained for the ammonia-coordination.