The pharmacologically and industrially important palladium(II) ion is usually characterised as square-planar structure in aqueous solution, similar to the platinum(II) ion. Our investigations by means of the most modern experimental and theoretical methods give clear indications, however, that the hydrated palladium(II) ion is hexa-coordinated, with four ligands arranged in a plane at 2.0 angstrom plus two additional ligands in axial positions showing an elongated bond distance of 2.7-2.8 angstrom. The second shell consists in average of 8.0 ligands at a mean distance of 4.4 angstrom. This structure provides a new basis for the interpretation of the kinetic properties of palladium(II) complexes. (C) 2007 Elsevier B.V. All rights reserved.