The structure of the hydrated Cu(II) complex was determined by both neutron diffraction and first-principles molecular dynamics. In contrast with the generally accepted picture, which assumes an octahedrally solvated Cu(II) ion, new experimental and theoretical results favor fivefold coordination. The simulation reveals that the solvated complex undergoes frequent transformations between square pyramidal and trigonal bipyramidal configurations. The preference of the Cu(II) ion for fivefold instead of six fold coordination, which occurs for other cations of comparable charge and size, results from Jahn-Teller destabilization of the octahedral complex
