Crystal and molecular structure of chromium(II) sulphate pentahydrate and single-crystal electronic spectra and bonding of CrSO4·5H2O, CuSO4·5H2O, and CuSO4·5D2O

Abstract

The crystal and molecular structure of CrSO₄·5H₂O is reported; this compound is isostructural with the analogous copper(II) salt. The two independent molecules in the unit cell have quite similar, tetragonally elongated ligand co-ordination geometries, the distortion from regular octahedral arrangements being marginally smaller than those in the copper(II) compound. The average Cr–O distance is ca. 0.07 Å longer than the average Cu–O bond length, this difference being more marked for the in-plane than the axial bonds. The low-temperature single-crystal electronic spectrum of CrSO₄·5H₂O shows bands in the region 10 000–15 000 cm^–1 assigned as d–d transitions coupled to metal–ligand vibrations and a weak peak at 18 450 cm^–1 thought to be due to coupling with water O–H stretching vibrations. The spectrum of CuSO₄·5H₂O is also reported and is similar to that of the chromium(II) compound except that the bands are ca. 2 000 cm^–1 lower in energy. For the copper(II) complex the assignment of the peak due to coupling with O–H stretches of the water molecules was confirmed by measuring the electronic spectrum of the corresponding deuteriated compound. An analysis of the transition energies using the angular overlap model suggests that the ligand interaction with the d orbitals is ca. 20% higher in the chromium(II) compound than in the copper(II) derivative. In both compounds the d_z² orbital is depressed in energy compared with the predictions of the simple bonding model, and better agreement with the observed transition energies is obtained if the π-bonding interaction involving the water molecules is approximately isotropic about the metal–ligand bond axes.