Chromium, powder dimer

The crystal structure of these compounds is hexagonal. The detailed structure is known for the compounds Cs3Cr2X9 (X = Cl, Br), which belong to the space group P63/mmc with two dimers per unit cell. X-ray powder patterns indicate that the other compounds are isostructural to the chromium compounds. 

There is some controversy concerning the existence of the a isomers of these tris complexes, Israily reported a purple complex to be or-[Cr(gly)3] however, subsequent workers have shown that this substance most probably was the dihydroxy dimer [Cr2(gly)4(OH)2]. Careful chromatography, on potato starch, of solutions from chromium(III)/glycine reactions yielded red and purple fractions,the electronic spectra of which were consistent with jS and a isomers respectively. Solutions of the a complex were unstable even in the dark and cold. Hoggard has recently claimed the preparation of the a isomer of the glycine complex by a fractional crystallization. The complex was anhydrous, unlike its cobalt(III) analogue. X-Ray powder methods could hence not be used to confirm the identity of the complex the luminescence spectra were held to be consistent with meridional coordination. There have been a number of studies of the physical properties of /S-[Cr(gly)3], summarized in Table 99. 

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