Influence of the crystal field stabilization energy of metal(II) ions on the structural distortion of matrix-isolated SO 42− guest ions in selenate matrices

Abstract

Infrared spectra of metal(II) selenate hydrates (MeSeO 4· nH 2O and Na 2Me(SeO 4) 2·2H 2O; n=6, 5, 4, 1; Me=Mg, Mn, Co, Ni, Cu, Zn, Cd) containing matrix-isolated SO 4 2− guest ions are reported and discussed with respect to the SO stretching modes ν 3 and ν 1. An adequate measure for the SO 4 2− guest ion distortion is the site group splitting Δ ν as (Δ ν ab and Δ ν ac in the case of a doublet and a triplet for ν 3, respectively; a, being the highest wavenumbered component of ν 3) and Δ ν max (the difference between the highest and the lowest wavenumbered SO stretching modes). It has been shown that the SO 4 2− guest ion distortion depends on both the number of the sulfate oxygen atoms involved in coordinative bonds with the metal(II) ions and the electronic configuration of the metal(II) ions, i.e. their crystal field stabilization energy (CFSE) additionally to the site symmetry and the local potential at the lattice site of the host lattice. The SO 4 2− guest ions matrix-isolated in MeSeO 4·H 2O (Me=Mn, Co, Zn) and in Na 2Me(SeO 4) 2·2H 2O (Me=Mn, Cu, Cd) exhibit three bands corresponding to the ν 3 modes as deduced from the site group analysis and Δ ν ab≅Δ ν bc. When SO 4 2− guest ions are incorporated in the triclinic Na 2Me(SeO 4) 2·2H 2O host lattices (Me=Co, Ni, Zn) the ν 3 stretching region resembles a higher local symmetry of the SO 4 2− guest ions (an approximate (A 1⊕E) splitting) than the crystallographic one (i.e. Δ ν ab>Δ ν bc instead of Δ ν ab≅Δ ν bc) and, hence, the ratio Δ ν ab/Δ ν bc has to be taken into account (the higher value of the ratio Δ ν ab/Δ ν bc, the weaker is the distortion of the SO 4 2− guest ions). The SO 4 2− guest ions incorporated in MeSeO 4· nH 2O ( n=6, 5, 4) exhibit a higher local symmetry of the guest ions than that deduced from the site group analysis (D 2d for the SO 4 2− guest ions in MeSeO 4·5H 2O, MeSeO 4·4H 2O and in the monoclinic MeSeO 4·6H 2O host lattices and close to T d in the tetragonal MeSeO 4·6H 2O host lattices). The analysis of the infrared spectra of selenate host lattices containing SO 4 2− guest ions reveals that the guest ions are stronger distorted when the adjacent metal(II) ions have CFSE≠0. These ions are more resistant to angular deformations of the MeO 6-octahedra (i.e. changes in the OMeO bond angles), thus facilitating the SO 4 2− guest ion distortion as compared to those having CFSE=0 which allow stronger angular deformations of the respective metal octahedra. Infrared spectra of kieserite-type compounds MeSeO 4·H 2O (Me=Mn, Co, Zn) containing matrix-isolated SO 4 2− guest ions and Me′ 2+ guest ions different from those of the host ions (i.e. Me′SO 4·H 2O in MeSeO 4·H 2O) are also presented and discussed (double matrix-spectroscopy).