Ligand replacement process, electron transfer

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

The monocationic intermediate Ar2pe+ subsequently looses its two aromatic ligands by replacing them with solvent (acetonitrile) molecules (Ari), and then reacts k2) with another 6 (arene)iron(II) acceptor in an electron-transfer chain (ETC) process which results in a catalytic deligation of Ar2Fe + [164, 165] (Eqs 39, 40). 

With this scheme the electron transfer step is rate limiting and k-2 — kikJk-2. An alternative scheme for Fe involving a rate-limiting ligand substitution process on [Fe(OH)] could not be ruled out. Replacement... 

Straightforward electron transfers to clusters can be often achieved by reaction with metal atoms. Transferred electrons replace either metal-metal or metal-ligand bonds in the cluster. One example illustrating a simple reduction process is the reaction of the anion [Rh 12(00)30] " with sodium in which the transfer of two electrons causes the rupture of one metal-metal bond. 

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