Metal electron transfer

Internal ligand-to-metal electron transfer may be initiated by the action of an external oxidant on the ligand. This phenomenon of induced electron transfer has received rather scant attention. In the complex 3 shown in (5.82) the one-electron oxidizing center of the Co III) and the two-electron reducing ligand 4-pyridylcarbinol can coexist because of their redox incom-patability . The complex is therefore relatively stable. This situation is upset when a strong one-electron oxidant such as Ce(IV) or Co(III) is added to a solution of the Co(III) complex. The oxidant attacks the carbinol function to generate an intermediate or intermediates the intermediate in this case is oxidized internally by the Co(III) center for example. 

In the intramolecular photoreduction kinetics scheme, catalytic metallic nuclei are formed in the intramolecular ligand-to-metal electron-transfer process. For example. 

Figure 13 Scheme of the influence of the dose rate on the competition between the inter-metal electron transfer and the coalescence processes during the radiolytic reduction of mixed metal ion solutions. Sudden irradiation at high dose rates favor alloying, whereas low dose rates favor coreshell segregation of the metals because of metal displacement in the clusters. 

In the intramolecular photoreduction kinetic scheme, catalytic metallic nuclei are formed in the intramolecular ligand-to-metal electron-transfer process. For example, catalytic metallic Cu nuclei can be formed in the photochemical reaction (A < 3500 A) of cupric acetate (CuA) ... 

Hislop and Bolton (1999) elucidated the complex mechanism of this reduction process. Presumably, it includes a ligand to metal electron transfer in the iron-oxalate complex with formation of an oxalyl radical anion as shown in Eq. 5-13. 

Due to a kinetic competition between inter-metallic electron transfer and radiation-induced reduction of mixed metal ions, it was shown, thanks to radiolysis, that the core-shell or alloyed structure of a bimetallic nanoparticle depends not only on the redox potentials of the mixed systems but also on the metal ions reduction rate which is controlled by the dose rate. 

While 7-radiolysis at relatively low dose rates enabled the synthesis of a few alloyed clusters such as Ag-Pd nanoparticles (also chemically synthesized), radiolysis at very high dose rates (by electron beams) led to the synthesis, at room temperature, of a lot of new alloys such as Au-Pd bimetallic nanoparticles/ Like in the case of Ag-Au system, at low dose rate (7-irradiation), bilayered Au -Pd.i,ii nanoparticles were obtained. However, at high dose rates (electron beams), the reduction is faster than the possible inter-metal electron transfer, then alloyed clusters were prepared. Moreover, since the radio-induced reduction of metal ions is faster at high dose rates, the synthesized particles are, in these conditions, always smaller with a narrow distribution in size. 

In the synthetically attractive field of electron transfer catalysis (ETC) [2, 91], ESR data of radical intermediates can correlate with the variable efficiency of processes such as the ETC-catalyzed substitution (9) by substantiating the amount of ligand-to-metal electron transfer in the ground state [87]. 

The reaction of the 350 nm intermediate with O2 does not involve proton transfer, and is the rate-limiting step in TPQ biogenesis. This reaction has been suggested to proceed via intramolecular Tyr Cu electron transfer, yielding a copper(I) complex of a tyrosyl radical (step B of Figure 7). Consistent with this, an inhibition study has shown that coordination of the precursor tyrosine side-chain to copper is a prerequisite for TPQ formation. Once formed, the tyrosyl radical probably dissociates from the copper ion, to allow O2 access to the copper center (step C). A similar sequence of substrate-to-metal electron transfer prior to O2 activation... 

Although ligand to metal electron transfer transitions are theoretically possible the results of the calculations and band shifts through this series of complexes were taken to exclude aU but the iys - ni charge-transfer transitions. It was suggested that the nz - dyz transitions were forbidden. 

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