Metal-to-ligand CT

Substitution reactions of cis-Ru(bipy)2X2 (X = F, Cl, or Br) with methyl-substituted pyridines have been studied in various non-aqueous solvents and the results indicate a dissociative mechanism via [Ru(bipy)2X] as the intermediate. The outer-sphere electron transfer between [Ru(NH3)g] and [(NH3)5Co02Co(NH3)s] has been monitored. Flash photolysis at the intense metal-to-ligand CT band (508 nm) of the pyrazine complex [(NH3)5Ru"(pyr)Cu"] causes an intramolecular electron transfer to [(NH3)5Ru (pyr)Cu ]. This allowed the unimolecular reverse electron-transfer process to be monitored. 

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...

For the sake of simplicity, electronic transitions in metal complexes are usually classified on the basis of the predominant localization, on the metal or on the ligand(s), of the molecular orbitals involved in the transition (4). This assumption leads to the well-known classification of the electronic excited states of metal complexes into three types, namely, metal-centered (MC), ligand-centered (LC), and charge-transfer (CT). The CT excited states can be further classified as ligand-to-metal charge-transfer (LMCT) and metal-to-ligand charge-transfer (MLCT). 

The charge-transfer (CT) transitions can be of the metal-to-ligand or of the ligand-to-metal type (MLCT or LMCT, respectively). They have been discussed extensively in the literature (18,51). To this we would like to add the following. 

Pal et al. [89] have reported on substituted ferrocenyl compounds, where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, that were synthesized and analyzed for their second-order nonlinearity ((3). Their results indicate that the metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as Ij, p-chloranil (CA), 2,3-dichloro-5,6-dicyano-l,4-bcnzoquinonc (DDQ), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. Bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base... 

As just implied, there are two broad classes ligand-to-metal (L—>M) and metal-to-ligand (M->L). In general, the former are better understood. In most cases charge-transfer (CT) processes are of higher energy than d-d transitions thus they usually lie at the extreme blue end of the visible spec-... 

The extraordinary reducing ability of ( CT)Ru(dipy )3 which was first discussed by Gafney and Adamson (33), has also been very well documented by Bock et al. (32) and by Navon and Sutin (34). A very recent work (35) also demonstrated that as much as 97% of the spectroscopically estimated excitation energy of this molecule can be used in energy conversion by electron transfer. This is in agreement with the fact that the Ru(dipy)3 excited state is metal-to-ligand charge transfer... 

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