Photoaquation reactions

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)  

The quantum yields at 261, 408, and 574 nm are rather low = 0.02) and wavelength independent. The quantum yields for this reaction decrease with decreasing temperature. These low quantum yields for the aquation reaction correspond with a mechanism involving labilization of a water ligand in the excited state. For other octahedral complexes such as Cr(NH3)6, the first step in the photoreaction in aqueous solution involves photoaquation of one of the ammonia ligands to give Cr(NH3)s(H20) +(Ref. 4)  

For the corresponding hexacoordinate tris chelated complex Cr(en)i, the initial photoreaction leads to substitution at one of the coordination positions to give Cr(en)2(enH)(H20). This photoreaction involves dissociation of one end of a coordinated ethylenediamine, followed by protonation of the nitrogen on the free hinged ethylenediamine arm of the chelate 

This hinging reaction is then followed by a second photoaquation step to give Cr(en)2(H20)2, a reaction that results in displacement of the monodentate enH ligand by water 

Photolysis of Cr(en)a under pulsed-laser conditions shows that Cr(en)2(enH)(H20) is formed within 20 ns of the laser pulse. The pseudo first-order rate constant (k) for this excited state substitution reaction is 10 s  

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A. Photosubstitution reactions can be aquation, anation or ligand exchange. (i) Photoaquation reactions of the type... 

In the following example a mixed ligand complex of Cr3+ can undergo two different photoaquation reactions ... 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

One of the more classical examples of inorganic photochemistry includes the photoaquation reactions of Cr(lll) ammines. The generic reaction is shown in Equation (17.57), but a variety of N-coordinating ligands can be substituted for NH3 in this expression and sometimes another ligand other than NH3 is dissociated. 

TABLE 17.17 Photoaquation reactions involving Cr(lll) ammine compounds and their quantum yields (4>). 

Waltz and Lillie have used pulsed-laser techniques to measure the phosphorescent lifetimes of CrfNHj) and conductivity to monitor the following photoaquation reaction ... 

The photoaquation reaction at the experimental conditions used (see the Experimental Section) has a half-life of 13 min at 25°C. This is five times faster than the thermal aquation at 140°C and 10 times faster than the thermal reaction is estimated to be at 25°C. 

In recent years there has been increasing evidence that for chromium(m) complexes photoaquation reactions following d-d irradiation involve stereochemical change. This is in marked contrast to the thermal aquations which nearly always proceed with retention of configuration. Further evidence in support of this generalization comes from a study of the photolysis of a mixture of cis- and trans-[Cr(NH3)4( NH3)Cl] ions. The ratio of the photoaquated ammonia... 

This photochemical process competes with a reversible photoaquation reaction leading to the formation of a pentacyanoaquo complex. 

Mixed-ligand complexes of chromium(III) also undergo photoaquation reactions. In general, two or more photoreactions occur simultaneously, and the predominant product from the photochemical reaction may be different from the product that is obtained under thermal conditions. An example of such difference in regioselectivity is observed with the halo complexes CrX(NH3)s. Under thermal conditions, aquation of the halide ion (X") is the preferred pathway ... 

The quantum yield for the photoaquation reaction of Co(CN)6 can be de-creased by the introduction of a supramolecular cation into the solution. When a... 

This actinometer, an aqueous solution of K[Cr(NH3)2(SCN>4] (Reinecke s salt), is based on the photoaquation reaction ... 

Mo(CN)8] solutions deal with the quantum yield of the primary photoaquation reaction.The photoaquation and photoredox processes in the photochemistry of [Mo(CN)8] have been succinctly contrasted in a paper whose main discussion is concerned with the solvated electrons formed during irradiation of aqueous solutions containing this complex ion at intemaJ-charge-transfer and charge-transfer-to-solvent frequencies. ... 

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