Photochemical generation of reaction intermediates

An increasingly important application of photochemistry is in the production of proposed intermediates in thermal reactions. Flash photolysis and pulsed-laser techniques at low temperatures can produce these species and allow for their spectroscopic characterization and studies of their reactivity. Examples of this have been seen in the areas of C—activation and the addition of alkenes to metal centers. Some aspects of this area were reviewed by Harris and co-workers. In order to be relevant to the thermal mechanism, the species must be in its electronic and thermal ground state. 

These results are in general agreement with the values or limits determined from conventional kinetic studies by Halpem and Wong and Tolman et al. It was found that the dimer and the dihydrogen adduct react with CO rather slowly (Jl 2 s ) and by a unimolecular process. A remarkable feature of these results is the large rate constant for displacement of ethylene by CO, as shown in the following reaction  

one has a quite complete picture of the reactivity of the species thought to be important in the operation of this catalytic system. 

For both M = Rh and Ir in hydrocarbon solvents, about 80% of the initial photoexcited state decays to the ground state with x 40 ps. The remaining 20% dissociates CO and forms the solvent complex on the 10-ps time scale. However the x h value is 2 ps for Ir but 500 ps for Rh in cyclohexane. This great difference in reactivity is consistent with earlier theoretical predictions.  

Balzani, V. Carassiti, V. Photochemistry of Coordination Compounds-, Academic Press New York, 1970. 

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