First solvation shell of the Cu(II) aqua ion: Evidence for fivefold coordination

We determined the structure of the hydrated Cu(II) complex by both neutron diffraction and first-principles molecular dynamics, In contrast with the g enerally accepted picture, which assumes an octahedrally solvated Cu(II) io n, our experimental and theoretical results favor fivefold coordination. Th e simulation reveals that the solvated complex undergoes frequent transform ations between square pyramidal and trigonal bipyramidal configurations. We argue that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods. The preference of the C u(II) ion for fivefold instead of sixfold coordination, which occurs for ot her cations of comparable charge and size, results from a Jahn-Teller desta bilization of the octahedral complex.