Ligand K-Edge X-ray Absorption Spectroscopic Studies: Metal—Ligand Covalency in a Series of Transition Metal Tetrachlorides

X-ray absorption spectra (XAS) have been measured at the chlorine K-edge for a series of tetrahedral MCl₄ ^n–complexes (M = Cu^II, Ni^II, Co^II, Fe^II, and Fe^III) to investigate ligand–metal bonding. The intensity of the pre-edge feature in these spectra reflects excited-state multiplet effects, intermediate-strength ligand field excited-state mixing, and ligand–metal covalency in the partially occupied d-orbital-derived molecular orbitals of each complex. A methodology which relates covalency to pre-edge intensity for d^10–n hole systems (n ≥ 1) is developed. Application of this methodology to the experimental data provides quantitative information about the covalency of the ligand–metal bond. The energy of the pre-edge feature is related to both the charge on the ligand and the metal d-derived orbital energy. An analysis of the pre-edge and edge energies allows the relative energy of the metal d-manifold, as well as the charge on each chloride ligand, to be quantitated. Results show that the HOMO covalency decreases across the series from Cu^IICl₄ ^2–to Fe^IICl₄ ^2–, while that of Fe^IIICl₄ ^– is larger than that of Fe^IICl₄ ^2–. This is related to the experimentally determined d-manifold energies, which vary in the order Fe^III < Cu^II < Ni^II < Co^II < Fe^II. The metal centers with the deepest d-manifold energies (closest to the ligand 3p orbital energy) are involved in the strongest ligand–metal bonding interactions and exhibit the largest metal d-derived orbital covalency. The total charge donated by the chloride ligands to the metal is greatest in Fe^IIICl₄ ^–, and the variation observed is similar to that seen for the metal d-derived orbital covalency: Fe^III > Cu^II > Fe^II ~ Co^II ~ Ni^II. This study extends ligand K-edge XAS to the investigation of ligand–metal bonding in d^10–nhole systems (n ≥ 1) and forms the foundation for future ligand K-edge XAS studies of electronic structure in transition metal centers.