Photoaquation of Cr

In addition to acid catalyzed reactions, this paper deals with charge and/or energy transfer reactions between adsorbed species and the lattice, or between co-adsorbed species. Reduction of lattice iron(III, as well as luminescence quenching of Ru(bpy), photoaquation of Cr(bpy)3 and photooxidation of water, are examples chosen for illustrating this aspect of the reactivity of clays. 

Waltz and co-workers [95] thoroughly reexamined pressure, temperature, and medium effects on the doublet emission and photoaquation of Cr(NH3)e+. Emission lifetimes vary with solvent and a strong correlation was observed between fcd(t ) and the solvent donor character in a manner attributed to solvent interactions with the N—H bonds of the coordinated... 

Photochemistry.—In the photoaquation of [Cr ena] +, irradiation being at d d or charge-transfer frequencies, only the first stage producing the... 

Proposed reaction coordinate diagram for the photoaquation of [Cr(NH3)5CI] explaining the observed stereochemistry using the edge displacement rule. 

Photoaquation of Cr(en)3 in acidic solution has a quantum yield of -0.4, independent of wavelength between 365 and 685 nm. The product distribution from A-Cr(en)3 also is wavelength independent and the isomers of (en)2Cr(enHXOH2) produced are 65% trans, 27% A-cis and 8% A-cis. The theory of Vanquickenbome and Cuelemans, which is often used to explain product distribution in this area, is not consistent with the dominant trans product and unspecified steric effects have been invoked as a rationalization of this failure. More recently, Irwin and Kirk have reported a flash photolysis study with conductivity monitoring. The rate constants for doublet decay and chemical change at pH <3 are very similar. For pH <3.9, the conductivity shows an exponential decrease due to the following reaction ... 

The rather contentious question of the relative importance of either doublet or quartet states in the photoaquation of [Cr(en)3] ion has been examined by three groups. " The evidence reported earlier in favor of a slow reaction from the doublet state has been reexamined by Kirk, who concludes that the involvement of a quartet state is more likely. Similarly, from a study of the photoaquation of A-[Cr(en)3] between 365 and 685 nm in aqueous acidic solutions, it is concluded that the lowest lying quartet state is photoreactive, with back intersystem crossing from the lowest doublet state to the quartet state responsible for the delayed reaction. " The wavelength-independent quantum yields of the three products, A-cis, A-cis-, and frans-[Cr(en)2(enH)(H02)] are 0.10, 0.03, and 0.24, respectively. Very recently, however, good evidence in favor of doublet state reactivity has been found. Irradiation at 669.2 nm populates the Eg state and enhances the reaction efficiency by 50%. At this wavelength there is little or no competitive absorption from the quartet state. ... 

The photoaquation of [Cr(en) (NH3)6 2nl ions n = 1, 2, or 3) has been investigated. When n = 1 or 2, the quantum yield for loss of en is larger than expected on statistical grounds. For example, when n = 2 the relative rate of loss of en and NH3 by photoaquation is 2.83, and when n = 1 the ratio is 1.64. The reason for the easier loss of en is ascribed to steric strain in the chelate rings rather than factors associated with different excited states. 

Previous studies of the photoaquation of [Cr(bipy)3] ion indicated that a seven-coordinate aquo complex is involved. In contrast, photosolvolysis in DMF solution proceeds by reduction to the + 2 oxidation state (L = bipy) ... 

Previous studies of photoaquation of [Cr(en)a] + and [Cr(ox)s] are linked by a study of photoaquation of the mixed complexes [Cr(en)aOx]+ and [Cr(en)(ox)2] . Both of these ions photoaquate to intermediates containing unidentate monoprotonated en. These intermediates aquate further thermally, with loss of the ethylenediamine. The rate of photo-racemization of [Cr(ox)3] in water-dimethyl sulphoxide decreases as the proportion of dimethyl sulphoxide increases, since solvating water plays a vital part in the racemization mechanism. ... 

Two mechanisms have been put forward to explain the photochemistry of chromium(in) complexes. Their photoreactivity has been ascribed to excitation either to the lowest spin-forbidden excited state, Eg, or to the lowest quartet excited states, and Mxg. Several years ago some quenching experiments on the photoaquation of the [Cr(NH3)2(NCS)4]- anion indicated that a quartet state was at least partially involved. Recently two papers concerning the photochemistry of the [Cr(CN)6] anion, chosen for the known large energy difference between the doublet and quartet states, have provided strong evidence for a quartet state as the photointermediale. Thus the observations that pyrazine and xanthone sensitize the photoaquation of [Cr(CN)e] , but that Michler s ketone and [Ru(bipy)3] + do not, rule out photoaquation via a doublet state. Likewise a comparison of phosphorescence and photolysis of solutions of [Cr(CN)6] in dimethylformamide showed that the photolysis, to [Cr(CN)5(DMF)] , could not proceed via the same excited state, Eg, as phosphorescence. Both of these investigations led to the implication of the... 

A detailed study of the photochemistry of Cr(en)33 + has been carried out.40 Both cis- and trans-isomers of Cr(en)2(H20)23 + were isolated as products of photoaquation of this complex.5... 

Irradiation of the Cr complexes [CrCCNlg] and [Cr(CN)6-x(OH),] (where x = 1, 3, or 6) in alkaline media has been described. Ligand photosubstitution by OH and oxidation to Cr both occur and in the case of [Cr(CN)s(H20)] and [Cr(CN)3(H20)3], photoaccelerated aquation is ob-served. " [CrL(CN)2(H20)]"- (where H4L = EDTA) also undergoes CN photoaquation, probably because labilization of other ligands is blocked or alternatively is not observable under the experimental conditions. This finding should be contrasted with the behaviour of cyano-ammine complexes which show primarily NH3 photoaquation.A complex photosubstitution occurs in the case of the nitrosyl complex [Cr(CN)5NO] and is followed by secondary thermal transformations to produce hydroxo-complexes of Cr . In the presence of oxygen, Cr04 ions are also formed. ... 

Further t-emperature dependence studies of the doublet lifetimes of coordination complexes of Cr(III) have sho%m that the RISC (reverse Intersystem crossing) Is the lifetime limiting process in compounds with a small energy separation.A small reduction of the lifetimes of excited fCr(NH3)g) and (Cr (cyclam) (MH3)2] (eye lam - 1,4,8,11-tetraazacyclotetradecane) with Increased solvent mobility has been observed but which Is absent in the deuterated analogues. This has been interpreted to indicate that increased anharmonicity in accepting modes enhances nonradiative rates. The photoaquation of... 

The first quantitative studies of pressure effects on the photosubstitution reactions of transition metal complexes were reported by Kelm et al. for a series of Cr(NH3)5X + ions (X" = SCN , Cl or Br ) in aqueous solution [50-52] (Eq. 6.20). These workers measured photoaquation quantum yields as a function of pressure and determined the apparent AV values from plots of In Oi vs. P. Notably, the AV values were substantially negative regardless of whether X or NH3... 

The quantum yield of photoracemization of [Cr(en)3] has been foimd to be independent of pH. Although some of the data obtained suggest that the twist mechanism of photoracemization might be preferred, it has not proved possible to discriminate definitively between this and the bond-rupture mechanism. In aqueous HCl solutions, irradiation of [Cr(en)3] or [Cr(NH2Me)6] is reported to lead to substitution of one ligand by water. Photoaquation also occurs in... 

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