Direct Contact versus Solvent-Shared Ion Pairs in NiCl2 Electrolytes Monitored by Multiplet Effects at Ni(II) L Edge X-ray Absorption

We investigate the local electronic structure in aqueous NiCl2 electrolytes by Ni L edge X-ray absorption spectroscopy. The experimental findings are interpreted in conjunction with multiplet calculations of the electronic structure and the resulting spectral shape. In contrast to the situation in the solid, the electronic structure in the electrolyte reflects the absence of direct contact Ni-Cl ion pairs. We observe a systematic change of the intensity ratio of singlet- and triplet-related spectral features as a function of electrolyte concentration. These changes can be described theoretically by a change in the weight of transition matrix contributions with different symmetries. We interpret these findings as being due to progressive distortions of the local symmetry induced by solvent-shared ion pairs.