Chromium complexes photosubstitution reactions

The very air-sensitive compounds [Mo(terpy)2] (purple) and [W(terpy)2] (green) have been described by a number of workers 44,174). The chromium compound may be prepared by the reaction of [Cr(CO)g] 44), [Cr(CN)g] 42,47), [Cr(CO)3(C6H6)] 47), or [Cr(bipy)3] 44) with two equivalents of terpy. The molybdenum 44,174) and tungsten 44) compounds have been prepared from [M(CO)g] in a similar manner. Electrochemical studies on [Mo(terpy)2] indicate three one-electron oxidations 8). Photosubstitution reactions of [M(CO)g] in the presence of limited amounts of terpy lead to the formation of [M(CO)4(terpy)] (M = Cr, Mo, or W) 205). It is unlikely that these compounds are seven-coordinate, and they may well provide examples of a bidentate terpy. [Mo(CO)3(terpy)] may be prepared by the reaction of terpy with [Mo(CO)3(mesitylene)], but attempts to prepare the other group VI complexes of this stoichiometry lead to the formation of [M(CO)4(terpy)] 205). 

Chromium(Ill) Complexes. The first quantitative studies of pressure effects on the photosubstitution reactions of transition metal complexes were reported by Kelm and co-workers [89 91] for a series of Cr111 complexes in... 

Thus the activation volume AV for the rate constant kp of an individual ES reaction pathway can be evaluated if the pressure dependencies of the photoreaction quantum yield, of intersystem crossing and of the ES lifetime can be separately determined. However, such parameterization becomes considerably more complex if several different excited states are involved or if a fraction of the photosubstitution products are formed from states that are not vibrationally relaxed with respect to the medium. Currently, parameterization of pressure effects on photosubstitutions has been attempted for a limited number of metal complexes. These include certain rhodium(III) and chromium(III) amine complexes and some Group VI metal carbonyls, which will be summarized here. 

Ligand field irradiation of chromium(III) complexes leads primarily to substitution reactions/ The most common reaction in aqueous solution is photosubstitution of a ligand by water. One of the earliest studies of photoaquation reactions was the light-induced exchange of water between Cr(H20) and the solvent. The reaction is followed by using isotopically labeled H2O as the solvent, where sequential photosubstitution of the H2O molecules leads to the formation of Cr(H20 )6 (Ref.3) ... 

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