Crystal structure and infrared spectra of dicesium trans-tetraaquadichlorochromium(III) chloride

The crystal structure of dicesium trans-tetraaquadichlorochromium(III) chloride Cs2CrIIICl5·4H2O with trans-[MIIIX2(H2O)4]þ complex ions (space group C2/c, Z ¼ 4, a ¼ 1915.3(4) pm, b ¼ 614.1(1) pm, c ¼ 1392.0(3) pm, and b ¼ 118.24(3)8, final R1 ¼ 0.0246 for 2100 unique reflections) was redetermined by single-crystal X-ray diffraction studies. It was found to crystallize in a 2c super structure of the structure reported previously (Inorg. Chem. 20 (1981) 1566; Inorg. Chem. 36 (1997) 2248). The obtained structure data now agree with the results of infrared spectroscopic studies, which has been confirmed in this work, namely that there are two different hydrate H2O molecules in the structure. Phase transitions, static or dynamic disorder of the hydrate H2O molecules, and space group C2/m proposed in the literature were ruled out. The coordinates of the four hydrogen positions derived from the X-ray data have been improved via the O–H distances derived from the wave numbers of the OD stretching modes of matrix isolated HDO molecules (2426, 2323, and 2306 cm21, 263 K) by using the nOD versus rO–H correlation curve reported in the literature (J. Mol. Struct. 404 (1997) 63). The nOD versus rH· · ·Cl correlation curve reported by Mikenda (J. Mol. Struct. 147 (1986) 1) should be improved, especially for strong hydrogen bonds. The two hydrate H2O molecules of the title compound are strongly distorted with a weak and a relatively strong O–H· · ·Cl hydrogen bond each thus intramolecular coupling of the two OH stretching vibrations to coupled ones is largely reduced and, hence, the wavenumbers of the OH and OD stretching modes of the HDO molecules mainly resemble those of the H2O and D2O molecules. The strength of the hydrogen bonds is in accordance with the predictions of the competitive and synergetic effects. Chloro ligands are weaker hydrogen bond acceptor groups than chloride ions. q 2004 Elsevier B.V. All rights reserved.