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 MCL(4)(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 greater than or equal to 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 (CuC42-)-C-II to (FeCl42-)-Cl-II, while that of (FeCl4-)-Cl-III is larger than that of (FeCl42-)-Cl-II. 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 (FeC4-)-C-III, and the variation observed is similar to that seen for the metal d-derived orbital covalency: Fe-III > Cu-II > Fe-II similar to Co-II similar to Ni-II. This study extends ligand K-edge XAS to the investigation of ligand-metal bonding in d(10-n) hole systems (n greater than or equal to 1) and forms the foundation for future ligand K-edge XAS studies of electronic structure in transition metal centers.