Computational Insights on Ligand-Induced Spin Multiplicity in Nickel (II) Complexes Featuring Diamine Moiety

This research investigates the coordination chemistry and optical properties of homoleptic nickel (II) (Ni (II)) complexes coordinated with diamine-based ligands. Using density functional theory (DFT) and time-dependent DFT (TD-DFT), we simulate the optimized geometry, electronic structure, and optical spectra of these complexes in both tetrahedral and octahedral symmetries. Our calculations are validated by experimental ultraviolet-visible (UV-Vis) spectroscopy data and contribute to a deeper understanding of the structure-property relationships within these systems. We found that both tetrahedral and octahedral homoleptic Ni (II) complexes are stabilized more readily in the triplet ground state than in the singlet state. The tetrahedral symmetry of the complexes is notably perturbed at their triplet ground state, compared to the singlet state. In contrast, octahedral configurations maintain greater symmetry in their triplet state and are more thermodynamically stable than tetrahedral counterparts. In tetrahedral complexes, coordination of two bidentate ligands induces a broad spectrum of triplet-to-triplet transitions at the range of 230-450 nm, predominantly showing ligand-to-metal charge transfer (LMCT) characteristics. Conversely, octahedral complexes display sharper and more intense absorption peaks at 250-400 nm due to a larger mixing between π-π^* and metal-to-ligand charge transfer (MLCT) character of these transitions. Obtained atomistic insights will pave the way for further exploration of their potential in developing advanced materials for catalysis, electronic devices, and photonic technologies.