Inverted vs. classic ligand field: evolution of bonding features in gold halide square-planar complexes and ligands' electronegativity influence

Electron distribution and bonding in square planar gold complexes, traditionally classified as gold(III), have been investigated through DFT calculations. Molecular orbital (MO) and band structure analyses, used as diagnostic tools, indicate that the metal center, upon coordination with halide, retains d fully occupied orbitals, with a two-electron vacancy localized on ligand-based orbital combinations, consistent with the inverted ligand field (ILF) theory. The only exception is found in tetrafluoroaurate complex containing four fluoride ligands in the coordination sphere, for which a covalent bonding model appears to be more appropriate. Linear correlations were observed between halide electronegativity and the percentage contribution of gold to the LUMO or the bottom of the conduction band in the solid-state calculations. This trend is also reflected in mixed square planar halide‑gold complexes. Electronegatvity of the ligands can be used as a predicting tool not only of the metal contribution to the LUMO but also to classify Au-L bonding nature. The results provide a valuable basis for a critical and constructive reassessment of the bonding and electronic description of square planar gold halide compounds with meaningful consequences in the representation of both spectroscopic and reactivity features in gold chemistry