Electronic effects on the stability of heteroleptic nickel(II) complexes with aromatic and aliphatic ligands

Heteroleptic metal complexes are attractive systems because their properties can be tuned by combining ligands with distinct donor atoms and electronic characteristics. Xanthates are versatile sulfur-donor ligands that influence the stability and electronic structure of metal complexes. Nickel(II) complexes containing xanthate and nitrogen-donor ligands are therefore suitable models for investigating how sulfur- and nitrogen-based coordination environments affect thermochemical stability and excited-state properties. Here, six Ni(II) complexes containing n-butyl xanthate (L1) or 2-methoxyethyl xanthate (L2), combined with ethylenediamine (en), 2,2'-bipyridine (bpy), or 1,10-phenanthroline (phen), were investigated. The results indicate that the formation of these heteroleptic complexes is thermodynamically favorable and governed by steric and electronic ligand effects.

DFT calculations were performed using the M06L/def2-TZVP level of theory, chosen for its reliable description of transition metal systems, noncovalent interactions, and thermochemical properties at moderate computational cost. Geometry optimizations were carried out for six heteroleptic Ni(II) complexes ([Ni(en)(L1)₂], [Ni(en)(L2)₂], [Ni(bpy)(L1)₂], [Ni(bpy)(L2)₂], [Ni(phen)(L1)₂], and [Ni(phen)(L2)₂]). Vibrational frequency and thermochemical analyses, including BSSE corrections, were also performed. TD-DFT calculations were performed to simulate the UV-Vis spectra, considering up to 300 vertical excitations to cover the 200-800 nm spectral window. Solvent effects were considered using CPCM, with ethanol for thermochemistry/geometry and acetonitrile for TD-DFT UV-Vis simulations. Electronic transitions were analyzed using NBO/CMO analysis with Gaussian 09 and GaussSum.