Photochemical ligand exchange

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

Fulvene Complexes.—Several ( -fulvene)Cr(CO)3 complexes have been obtained by photochemical ligand-exchange reactions of benchrotrenes with 6,6-disubsti-tuted pentafulvenes. The crystal structure and C n.m.r. spectrum of the 6-(dimethylamino) complex have shown that it is best formulated as a... 

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

Quantum yield data for photochemically initiated ligand exchange reactions of a number of chromium(III) complexes having ligand fields of effective Oh symmetry are given in Table III. These data show that... 

In this review our focus will be largely on thermal ligand exchange and isomerizations. Other reactions, including photochemical ones, will be dealt with only briefly. Most examples will come from studies of cobalt(III) and chromium(III) compounds merely because the chemistry of these metals is more fully developed. 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

This is an electron transfer to the solvent, in which a hydrated electron is formed. In this complex there are well defined d-d transitions in the VIS and near UV (NUV), and CT bands in the UV regions. Irradiation in the d-d bands leads to ligand exchange, for instance to photoaquation in water. Irradiation in the CT bands results in electron transfer to the solvent. This provides a very good example of the dependence of the nature of photochemical reactions on irradiation wavelength in metal complexes. 

Similar chemistry occurs with R3SnCo(CO)4 if the reaction is performed photochemi-cally in hexane however, when conducted thermally in polar solvents, cobalt fluorides, R3SnF and cobalt cluster compounds are formed342. In the case of an attempted insertion into a Rh—Sn bond under photochemical conditions, ligand exchange chemistry occurred (equation 146)343. 

Monosubstituted complexes of the type Cp(OC)LMn(H)SiR3 [L = PR3, P(OR )3, CNR ] cannot be prepared from the dicarbonyl compounds by CO-ligand exchange, because the conditions necessary for this kind of reaction result in HSiR3 elimination, as discussed in more detail below. However, they are accessible by the photochemical route [Eq. (2)] from Cp(OC)2LMn, if the ligand L is not too bulky (13). 

Thus, a careful analysis of the rate constants for back electron transfer, hgand substitution, and dimerization leads to the conclusion that ligand exchange in the 17-electron radical (/cgub in Eq. 44) lowers the rate of back electron transfer from the acceptor radical (A ) (/c et in Eq. 43) to such an extent that dimerizations (and other possible follow-up reactions [118]) now become competitive and effect permanent photochemical transformations. The decrease of the back electron transfer rates is due to the attenuated reduction potentials of the phosphine-substituted radicals [176]. 

Numerous applications of [Cr(NH3)6]3+ and its derivatives to mechanistic studies of conventional or photo-assisted ligand-exchange reactions have been extensively reviewed.7,270-276 Among other common Cr111 complexes, [Cr(NH3)6]3+ has been used for the studies of reactivities of muonium and positronium atoms in aqueous solutions.313-315 Several computational methods, including DFT calculations,316 a combination of molecular mechanics and angular overlap model calculations, or vibrational analysis have been used for the prediction and interpretation of electronic spectra and photochemical properties of [Cr(NH3)6]3+. 

By means of the three general synthetic approaches outlined above, a fairly large number of trifluoromethylated transition metal complexes were synthesized during the period 1959-1980. Several studies had indicated that ancillary ligand exchanges, e.g., PPh3 for CO or Cl for Br, could be carried out either thermally or photochemically and that the removal of CF3 groups (which is commonly carried out by hydrolysis) could also be achieved as (27)... 

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