Photocatalysis by chromium compounds

Both chromium forms are photochemically active, and the metal-centred (MC) photochemistry of the Crm complexes has been known for a long time [88]. Photo-substitution or photoisomerization is, however, of minor environmental relevance to facilitate or accelerate the mutual conversion of Cr111 and CrVI only photoredox behaviours are essential. 

The LMCT excitation of Cr111 complex with EDTA [91]  

Although chromate(VI) is photochemically inactive in all of its forms in neat aqueous solution, the photochemical oxidation of alcohols by chromate(VI) has been known for more than 80 years and interpreted in terms of photochemical reactivity of the chromate(VI) esters [94], Recent studies have shown, however, that LMCT excitation of CrVI species is quenched not only by inner-sphere but also by outer-sphere electron transfer [23, 87,92,94,95], Moreover, inner-sphere electron transfer in chromate(VI) esters was found to involve two electrons, yielding a CrIV species and appropriate aldehyde or ketone  

At higher pH, ie at more loose interaction between chromate(VI) and alcohol, the outer-sphere transfer of one electron generates a Crv species and the alcohol radical [94]  

The parallel outer-sphere PET mechanism was found in systems containing chromate(VI) plus an electron donor different from alcohol, such as oxalate or phenol and its halogen derivatives [92,95], 

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