Structural analysis and self-absorption correction of 1.5 mM and 15 mM Ni (II) complexes: Probing the limit of dilute systems with identical coordination number, and conditions for subtle hypothesis testing

Abstract

We apply a novel correction for the self-absorption distortion in fluorescence X-ray Absorption Spectroscopy (XAS) data collected from both 15 mM and 1.5 mM Ni (II) complex solutions. Self-absorption is an inevitable systematic effect that distorts fluorescence XAS data and limits the available information content. This work builds on an earlier project (Trevorah et al., 2019) and demonstrates that this method is valid for dilute samples. Structural analysis of the corrected data yields significant new insights with tight constraints on the determined molecular structure and dynamic bond lengths. The samples considered are nickel (II) complexes, bis(N-n-propyl-salicylaldiminato) nickel(II), ‘n-pr’, and bis(N-i-propyl-salicylaldiminato) nickel(II), ‘i-pr’. These complexes are notionally square-planar and tetrahedral, respectively, and have identical coordination numbers. In transmission detection XAFS, the geometry and distortion were well observed in 15 mM dilution for both high point density data sets and for high point accuracy but using more sparse data sets. In fluorescence the high point density data sets were just as insightful; however the sparse data sets provided insufficient data points in the region accessible with FEFF theory, and hence in this case were unable to distinguish between the distorted square planar and the distorted tetrahedral geometries, principally because of the increased difficulty in observing three-legged path contributions.