Coordination geometry of Cr(VI) species: Structural and spectroscopic characteristics

In this review, the speciation of chromium(VI) entities, the key to understanding the further redox processes related to this ion, is presented and discussed. Two possible modes of Cr(VI) binding, i.e. as a centre of coordination and as a ligand, have been characterized. In contrast to the gas and solid-state phases, where coordination number four is generally preserved, the Cr(VI) coordination sphere may be more flexible and chromium(VI) forms can even be seven-coordinate in solution. In the solid state, the pseudotetrahedral stereochemistry of the [CrO4]2− species does not exclude versatility of its coordination modes, e.g. monodentate, polymeric bridging, bidentate, chelate, etc. The difficulties and various problems associated with structural aspects of Cr(VI) forms, mainly in relation to their spectroscopic properties, are shown.