Reactivity of Charge Transfer States

In a LMCT state the metal is reduced and a ligand oxidized. This electron distribution determines the reactivity of LMCT states. Co(III) complexes are best suited to illustrate the photochemistry induced by LMCT excitation since these compounds have been studied extensively and a variety of different photoreactions have been identified [2,5,94]. 

The identity and reactivity of this primary product have been interpreted by various models. Only a few important features will be outlined at this point. The radical pair Co L / X- is assumed to be formed in a cage of solvent molecules. Geminate recombination leads back to the starting complex. 

Cage escape to the solvent-separated radical pair opens further reaction channels such as secondary recombination and other reactions. 

For the product formation a variety of possibilities exists. If the complex fragment or the ligand radical undergo a rapid irreversible reaction stable photoredox products are obtained. For example, the complex fragment Co(NH3 52+ in water decays to Co(H20)g2+ and ammonia with k 10 s l [95]. Accordingly, the photoredox decomposition of Co(ni) pentaammine complexes proceeds by equation (4) (X = halogen) 

In the case of Colil(CN)5X3- such a photoredox decomposition is not expected to take place since the primary complex fragment Co KCN)53- does not decay but undergoes a recombination with X- (or X2) [94]. However, if the ligand radical reacts rapidly and irreversibly Co(CN)5X3- will also yield stable photoredox products, e.g. (X = benzyl) [96]  

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