Inner-sphere charge transfer transitions

After an extensive review of MMCT transitions involving ions in solids, it seems wise to start this paragraph with some molecular species, because many of these have been investigated in much more detail than their counterparts in non-molecular solids. It is suitable to make a distinction between outer-sphere charge-transfer (OSCT) and inner-sphere charge-transfer (ISCT) transitions [1], In the former the metal ions do not have ligands in common, in the latter they are connected by a common ligand. Studies are usually performed on metal-ion pairs in solution. 

Excitation to inner-sphere charge transfer (ISCT) transitions induces radial charge shift within the coordination entity, which may result in redox reactions including the central atom and ligands. The charge redistribution increases the complex susceptibility towards protonation, isomerization, redox processes, or nucleophilic or electrophilic attack. 

Reactions that involve O2 as the reactant or the product may occur by an inner- or outer-sphere pathway when the redox partner is a transition metal. The same is true of reactions that consume or produce (V-. The inner-sphere reaction is defined by the presence of a bond to 02 in the transition state. Different types of inner-sphere reactions are possible including those that form covalent intermediates and those that do not. The outer-sphere reaction simply converts O2 to C>2, or vice versa, in the absence of covalent bonding. The free energy barrier in the reaction is expected to arise from the reorganization needed to accommodate electron transfer and the redistribution of charge. An additional contribution derives from lengthening or contracting the 0—0 bond. 

Figure 6.7 Inner-sphere redox processes induced by different CT (charge transfer) excited states in transition metal complexes. LMCT, ligand-to-metal charge transfer MLCT, metal-to-ligand charge transfer...

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