Evidence of distorted fivefold coordination of the Cu2+ aqua ion from an x-ray absorption spectroscopy quantitative analysis.

A combined x-ray-absorption near-edge structure and extended x-ray-absorption fine-structure quantitative analysis, based on fitting procedures of both low- and high-energy ranges, has been carried out at the Cu K edge. This allowed an accurate determination of the structural parameters associated with the first hydration shell of the Cu2+ aqua complex. The existence of an average fivefold coordination has been evidenced, ruling out the recently proposed regular pyramidal configuration with five equal Cu-O distances. Our structural results are compared with recent neutron-diffraction data performed by the second-difference isotopic substitution method.