Crystal structures and electronic absorption spectra of two modifications of Cr(SeO2OH)(Se2O5)

Single crystals of two modifications (I and II) of Cr(SeO2OH) (Se2O5) have been synthesized under low-hydrothermal conditions. Their structures have been investigated using single-crystal X-ray diffraction data up to sin theta/lambda = 0.76 Angstrom(-1) [I: monoclinic, space group P2(1)/n, a = 7.517(2) Angstrom, b = 12.516(3) Angstrom, c = 7.381(2) Angstrom, beta = 91.46(1) Angstrom, Z = 4, R1 = 0.030 for 1562 F-o > 4 sigma(F-o) and 114 variables; II: orthorhombic, space group Pnma, a = 8.782(2) Angstrom, b = 11.172(3) Angstrom, c = 7.545(2) Angstrom, Z = 4, R1 = 0.031 for 752 F-o > 4 sigma(F-o) and 65 variables]. In Cr(SeO2OH) (Se2O5)-I, sheets-built up from isolated CrO6 octahedra and Se2O5 groups-are linked via the SeO2OH groups and rather strong hydrogen bonds. This structure is isotypic with the corresponding compounds of Mn(III) and Fe(III). Cr(SeO2OH) (Se2O5)-II represents a new framework structure type, also composed of isolated CrO6 octahedra and Se2O5 and SeO2OH groups, but with very weak hydrogen bonds. The hydrogen-bonding schemes of both compounds were further investigated by FTIR microscope spectrometry in the range 7000-1000 cm(-1), yielding a typical AB-band spectrum for phase I. Furthermore, polarized electronic absorption spectra were measured in the range 33,000-12,500 cm(-1) using UV-VIS microscope spectrometric techniques. According to the weak distortions of the CrO6 octahedra in both compounds, the observed band splittings and polarization effects are rather weak. Hence, crystal field calculations were performed assuming O-h symmetry, resulting in the following crystal field parameter sets; I, Dq = 1558, B-35 = 653, B-55 = 760, C = 2920 cm(-1); II, Dq = 1641, B-35 = 647, B-55 = 730, C = 2815 cm(-1). These Dq values are slightly below average compared to octahedral Cr(III) in other oxygen-based structures, a tendency that was already observed for other 3d transition elements in crystal fields of selenite compounds. This might indicate a rather low position of the [SeO3](2-) anion within the range of oxygen-based ligands in the spectrochemical series. (C) 1998 Academic Press.