Alfredo Pasquarello,^12* Ingrid Petri,³ Philip S. Salmon,³ Olivier Parisel,¹² Roberto Car,¹⁴ Éva Tóth,⁵ D. Hugh Powell,⁶ Henry E. Fischer,⁷ Lothar Helm,⁵ André E. Merbach⁵

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

¹ Institut Romand de Recherche Numérique en Physique des Matériaux (IRRMA), PPH-Ecublens, CH-1015 Lausanne, Switzerland.
² Ecole Polytechnique Fédérale de Lausanne (EPFL), PPH-Ecublens, CH-1015 Lausanne, Switzerland.
³ Department of Physics, University of Bath, Bath BA2 7AY, UK.
⁴ Department of Chemistry and Princeton Materials Institute, Frick Chemical Laboratory, Washington Road, Princeton University, Princeton, NJ 08544, USA.
⁵ Institut de Chimie Minérale et Analytique, Université de Lausanne, Bâtiment de chimie, CH-1015 Lausanne-Dorigny, Switzerland.
⁶ Department of Chemistry, Durham University, Durham DH1 3LE, UK.
⁷ Laboratoire pour l'Utilisation du Rayonnement Electromagnétique (LURE), Bâtiment 209d, Centre Universitaire Paris-Sud, Boîte Postale 34, F-91898 Orsay Cédex, France.
^*   To whom correspondence should be addressed. E-mail: Alfredo.Pasquarello{at}epfl.ch