X-ray absorption methods for the determination of Ru–Cl bond covalency in olefin metathesis catalysts: On the normalization of chlorine K-edges in ruthenium complexes

A systematic approach to quantitative analysis of Cl K-edge XAS spectra in ruthenium complexes is outlined and applied to ruthenium carbene complexes involved in olefin metathesis catalysis. This method is generally well-suited for probing ligand donor contributions into empty transition metal d orbitals, but overlap of the Ru L2, 3 edge makes this very difficult in these complexes. A method for removing the Ru L2, 3 edges from the Cl K-edge spectrum is developed and tested. The normalization procedure makes use of the well-separated Ru L2 edge and an appropriate L3/L2 branching ratio to unambiguously remove contributions from these edges and allow for accurate normalization of the Cl K-edge spectrum. A generalized background subtraction that accounts for variability over the large energy range involved is also described. The methodology should be general for any similarly overlapping edges, as long as one component of the spin–orbit split metal (L- or M-) edge is unperturbed. The results of this study indicate a substantial difference in Ru–Cl bonding depending on the nature of the catalyst.