This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We propose multi-edge x-ray absorption studies to dissect the fundamental molecular details of the non-innocent behavior of ligands in coordination compounds. The non-innocent nature of ligands is best characterized macroscopically by ligand-based reactivity, where the central metal becomes a spectator and most often a steric scaffold rather than a reactive site. Microscopically this translates into valence orbitals that are rich in ligand contribution (i.e. large ligand covalency). We have already collected preliminary data for formally Ni(II)/Ni(III) redox couple system supported with a pincer-type ligand denoted PNP (PNP = N[2-P(CHMe2)2-4-methylphenyl]2) at the Ni L-, Cl and P K-edges that clearly demonstrate the feasibility of our measurements. Our data unexpectedly show the non-innocent behavior of the pincer-type ligand PNP originally thought to be innocent ancillary in the context of organometallic chemistry and catalysis. Previously this behavior was mainly observed for sulfur and oxygen containing ligands, such as dithiolenes and catecholes. We first plan to study two pincer-type ligands with PCP and PNP compositions that are coordinated to a series of first row transition metals as central metals in their formally mono-, di-, and trivalent forms. Given the reducing ability of phosphorus, we will also investigate pincer ligands having only nitrogen donors, specifically the monoanionic NCN system C6H3[1,3-CH2NiPr2]2. These proposed investigations are part of a larger project aiming to understand the physical-chemical factors that can contribute to non-innocent behavior as a function of the 3d transition metal ion (when shifting from late to mid) and the nature of the heteroatom in the ligand. We wish to set up guidelines for synthetic chemists to be able a priori predict non-innocent behavior.
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