Ligand centered electron transfer

The coordination chemistry of (phenoxyl)manganese complexes is rather more complicated because both metal- and ligand-centered electron-transfer processes are accessible in the normal potential range. The phenolato precursors are known to exist with manganesc(II), (III), and even (IV). In fact, three phenolato groups strongly stabilize the Mn(IV) oxidation state. 

A relatively simple case based on ligand-centered electron transfer involves the olefin complex (t] -TCNE)W(CO)3, TCNE = tetracyanoethylene (10) [46]. This compound undergoes a ii (C=C) -> h N), i.e. a 71 a rearrangement on ligand-based reduction to the anion and, in particular, to the dianion. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

The electrochemical reduction of permanganic acid [H0Mnvu(0)3], which is traditionally represented as a metal-centered electron transfer to change Mn7+ to Mn6+, is another example of a ligand-centered process ... 

This process is outer sphere because there is little electronic interaction between the metal centers through the ligand the electron transfer occurs through space as the metal centers move to the correct conformation by rotations about the C—C bond of the... 

In the case of an outer-sphere reaction the so-called bridging material is simply the material between the redox centers—solvent molecules and, in the case of metal complexes, ligands surrounding the metal centers. Electron transfer between donor and acceptor sites connected by a molecular bridge is now fairly well understood. The rates decrease with increasing separation of the donor and acceptor and can generally be interpreted in terms of a first-order rate constant kei-... 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

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