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.