The hydration of Cu2+: Can the Jahn-Teller effect be detected in liquid solution?

Abstract

The long elusive structure of Cu͑II͒ hydrate in aqueous solutions, classically described as a Jahn-Teller distorted octahedron and recently proposed to be a fivefold coordination structure ͓Pasquarello et al., Science 291, 856 ͑2001͔͒, has been probed with x-ray-absorption spectroscopy by performing a combined theoretical and experimental analysis. Two absorption channels were needed to obtain a proper reproduction of the x-ray-absorption near-edge structure ͑XANES͒ region spectrum, as already observed in other Cu͑II͒ complexes ͓Chaboy et al., Phys. Rev. B 71, 134208 ͑2005͔͒. The extended x-ray-absorption fine-structure ͑EXAFS͒ spectrum was analyzed as well within this approach. Quite good reproductions of both XANES and EXAFS spectra were attained for several distorted and undistorted structures previously proposed. Nevertheless, there is not a clearly preferred structure among those including four-, five-, and sixfold coordinated Cu͑II͒ ions. Taking into account our results, as well as many more from several other authors using different techniques, the picture of a distorted octahedron for the Cu͑II͒ hexahydrate in aqueous solution, paradigm of the Jahn-Teller effect, is no longer supported. In solution a dynamical view where the different structures exchange among themselves is the picture that better suits the results presented here.

Hydration of Copper(II): New Insights from Density Functional Theory and the COSMO Solvation Model

“…[14][15][16][17][18][19][20][31][32][33][34][35][36][37] Our calculations favor Cu(II)-water complexes with first-shell coordination number of five for clustes with eight to ten water molecules. Note the results of solvation calculations are sensitive to the selection of the copper solvation radius when Cu(II) ions are in contact with the solvent dielectric boundary.…”

“…Conversely, Benfatto et al 18 and Frank et al 20 analyzed the XANES portions of the XAS spectra of Cu(II)-water solutions and concluded that a square-pyramidal model with one elongated axial water molecule provides a better fit of the data than a four-coordinate model or a Jahn-Teller axially elongated sixcoordinate model. More recently, Chaboy et al 19 used two absorption channels for simulating the EXAFS and XANES spectra of Cu(II)-water solutions. They concluded that the four-, five-, and six-coordinate structures are indistinguishable and are likely to dynamically coexist in solution.…”

“…This view, however, has not been supported by subsequent X-rayabsorption spectroscopy (XAS). 13,[18][19][20] Analysis of the extended X-ray absorption fine structure (EXAFS) spectra of aqueous solutions of Cu(II) by Persson et al, 13 Benfatto et al, 18 and Frank et al 20 indicated that the best fit to the experimental data can be achieved using either a distorted square pyramidal or a distorted octahedral coordination. Persson et al 13 selected the latter geometry based on a marginally better fit of wide-angle X-ray scattering (WAXS) data of aqueous solutions of Cu(II).…”

“…[1][2][3][4][5] Detailed information on ligands arrangement around the Cu(II) ion is only available in the solid state [6][7][8][9][10][11][12][13] whereas the structural information in the aqueous phase [13][14][15][16][17][18][19][20] is less definitive and available only for few Cu 2+ complexes. Although static [20][21][22][23][24][25][26][27][28][29][30] and dynamic [31][32][33][34][35][36][37] electronic structure calculations can, in principle, provide such information, realistic modeling of Cu(II) complexes in aqueous solution with highly flexible coordination environment still remains a challenge.…”

Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models

“…22,23 Surprisingly, the coordination environment even for the simplest hydrated Cu(II) ion has been the subject of extensive debate in recent years. 21,[24][25][26][27][28][29][30][31][32] The most recent X-ray absorption studies (EXAFS and XANES) of aqueous solution of Cu(II), 26 as well as theoretical calculations performed by us 30 and others, 31,32 showed that the [Cu(aq)] 2+ ion can be represented as a five-coordinate square-pyramidal structure with one elongated axial water molecule. Despite the flexibility of the coordination geometry, Cu 2+ is one of the most strongly coordinated divalent transition metal ions.…”