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-ed ge 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 in tensity of the pre-edge feature in these spectra reflects excited-stat e 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 relat es 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 cova lency 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 orbit al energy. An analysis of the pre-edge and edge energies allows the re lative energy of the metal d-manifold, as well as the charge on each c hloride ligand, to be quantitated. Results show that the HOMO covalenc y decreases across the series from (CuC42-)-C-II to (FeCl42-)-Cl-II, w hile that of (FeCl4-)-Cl-III is larger than that of (FeCl42-)-Cl-II. T his is related to the experimentally determined d-manifold energies, w hich vary in the order Fe-III < Cu-II < Ni-II < Co-II < Fe-II. The met al 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 covalenc y. The total charge donated by the chloride ligands to the metal is gr eatest 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-I I 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 syste ms (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.