Chromium species photochemical reactions

HCrO ] has been shown to be the photo-active species in the photochemical reaction of dilute solutions of chromium(vi) oxyacids with alcoholic reducing agents. The association of chromate with Np , Th, and Fe" in perchlorate solution has been studied spectrophotometrically and the respective formation constants, 63.6,4.70, and 1.93, have been determined. The greater stability of the Np complex has been interpreted in terms of the limited donation of 5/-electron density from Np to the d-orbitals of chromium in the chromate ion. ... 

In aspect of its toxicity, any pathway leading to abatement of chromate(VI) pollution arouse a vivid interest. One of such pathways seems to be created by cooperations between the iron and chromium photocatalytic cycles, which were reported as effectively converting chromate(Vl) into Cr(III) species. Photochemical coupling reactions between polycarboxylate Fe(III) complexes and chromate(Vl) were studied and strong collaboration between both photocatalysts was demonstrated, which was significantly affected by the oxygen concentration (16,17,95,261). On the other hand, chromium(Vl) reduction pho-toinduced by iron(lll) nitrilotriacetate accompanied by nta degradation was found to be independent of the O2 concentration, whereas the oxidation state of the chromium product depended on the pH (257). 

Although it is believed that the major photochemical reaction is loss of a carbonyl ligand trans to the alkene, an alternative photoprocess could involve conversion of the complex into a monoene chromium tetracarbonyl species/ ... 

Several examples are known of the effectiveness of organic compounds sensitizing photochemical reactions of cobalt(m) and of chromium(m) complexes, and biacetyl is known to photosensitize the aquation of [PtCU] ". Now the first example of photosensitization of substitution at platinum(n) by an inorganic species has been reported [Ru(bipy)3] + sensitizes photoaquation of the [PtCl4] anion. ... 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

One of the advantages of chromium complexes over those of, say, cobalt or rhodium in mechanistic studies is the much greater photoinertness of the chromium family. The superoxochromium CraqOO, for example, is almost unaffected by the > 320 nm light, but the superoxides of cobalt and rhodium photohomolyze readily even when exposed to visible light, X < 500 nm. Photochemical generation of radicals and other short-lived species is thus more easily accomplished in the presence of CraqOO. This feature facilitates kinetic studies, as illustrated by the reactions of CragOO " " with NO (Section III.C.2) and NO2 (218), described below. 

Another example of the photoassisted substrate conversion due to a short-lived intermediate in the ground state is shown in figure 6. Chromic acid esters form chromium(V)/alkoxy radical pairs within the photochemical primary reaction. In the presence of such iron(III), cobalt(III), or copper(II) complexes which are able to interact coordinatively or by second sphere interactions with chromium(V) within the radical pair cage reoxidation to chromate(VI) occurs under simultaneous reduction of the metal complexes to corresponding iron(II), cobalt(II), and copper(I) species, respectively. Unfortunately, the efficiency of this photoassisted reaction is limited by... 

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