Chromium (III) Acetate, Chromium (III) Acetate Hydroxide, or µ3-Oxo-esakis-(µ2-acetato-O,O') - triaqua-trichromium (III) Acetate? Determining the Structure of a Complex Compound by Analytical and Spectroscopic Methods

An experiment illustrating the use of infrared spectroscopy and model compounds to elucidate the structure of chromium (III) acetate is reported. Two commercial samples with the following conventional formulations Cr3(CH3CO2)7(OH)2 and Cr(CH3CO2)3 were examined and compared to the model compound [Cr3(µ2-CH3COO)6(µ3-O)(H2O)3]DHB·2H2O, DHB = 2,6-dihydroxybenzoate(1-). The experiment may serve to illustrate the advantage of the joint application of analytical and spectroscopic data, in particular the use of symmetry concepts and IR-Raman selection rules, in the assignment of inorganic structures.All the samples exhibit the distinctive vibrational features predicted for a trimetallic carboxylate complex with the µ3-O-Cr3 cage. Therefore, it may be concluded that chromium (III) acetates are endowed with a trimetallic arrangement where six acetates bridge three metal ions connected by a triply-bridging oxygen so that a total of four carboxylate oxygens surround each chromium ion. Water molecules act as the terminal ligands and complete an almost regular octahedral coordination polyhedron at each metal center. Analytical data are in good agreement with the [Cr3(µ2-CH3COO)6(μ3-O)(CH3COO) composition.