MULTI-EDGE X-RAY ABSORPTION SPECTROSCOPIC STUDIES OF TRANSITION METAL DITHIOLENE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Dithiolene transition metal complexes are of longstanding interest in inorganic chemistry due to their fascinating physical properties and electronic structures. In biology, transition metal dithiolene ligands play an important role in molybdopterin active sites, which are involved in a wide variety of fundamental biochemical reactions. In material sciences, certain dithiolene complexes are candidate materials for nonlinear optical devices and laser applications due to their enormous absorption bands in the near-infrared region. However, the understanding of enzymatic reaction mechanisms or the design of new functional materials should be based on a thorough understanding of the electronic structures of such compounds. In this respect, a hotly debated question is, whether dithiolene ligands can exist as radicals or diradicals. In such 'non-innocent' complexes, the oxidation and spin states of the central metal is ambiguous and difficult to determine experimentally. Thus, x-ray absorption spectroscopy at the metal K-, and L-edges as well as the sulfur K-edges is ideally suited to provide fundamentally electronic structure insights into function in these systems. In particular, sulfur K-edge spectroscopy probes the ligand hole character in the electronic ground state and is therefore ideally suited to detect open shell character in the complexes. The metal K-edge spectra in the edge and pre-edge region will provide oxidation state information of the central metal ion while EXAFS can be used to get insight into metrical details. The metal L-edge is a particularly sensitive probe of the dn configuration and will be used to probe metal-ligand covalency through a quantitative analysis of the multiplet intensities. The experiments will be combined with quasi-relativistic density functional as well as multiplet calculations in order to obtain more insight into the experimental data. Our studies will focus on a series of complexes which systematically differ in their oxidation states and metal ion.