Electron ligand transfer

Many complexes of metals with organic ligands absorb in the visible part of the spectrum and are important in quantitative analysis. The colours arise from (i) d- d transitions within the metal ion (these usually produce absorptions of low intensity) and (ii) n->n and n n transitions within the ligand. Another type of transition referred to as charge-transfer may also be operative in which an electron is transferred between an orbital in the ligand and an unfilled orbital of the metal or vice versa. These give rise to more intense absorption bands which are of analytical importance. 

However, it seems that these are best viewed as platinum(II) species too, so that two-electron metal-to-ligand transfer has been effected. The structures of Pt(PPh3)2Z (Z = r]2-02, t 2-C3H4, t]2-CS2) (Figure 3.16) all involve square planar coordination as expected for platinum(II) rather than the tetrahedral 4-coordination anticipated for platinum(O). 

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... 

Table 10-1. Yield of chlorobenzene in chloro-de-diazoniation of benzenediazonium sulfate as a function of electron transfer reagent and ligand transfer reagent. Experiments 1-5 from Galli, 1981a), experiment 6 from Daasbjerg and Lund, 1992b).

Exp. no. Electron transfer reagent Ligand transfer reagent Yield ArCl (Vo)... 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

In each case, it is the carbon atom that has the negative formal charge that makes it the "electron-rich" end of the structure. Moreover, there are antibonding orbitals in both CO and CN that can accept electron density transferred from the nonbonding d orbitals on the metal (as described in Section 16.10). Because of the nature of these ligands, it is not uncommon for them to function also as bridging groups in which they are bonded to two metal atoms or ions simultaneously. 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

In the fully reduced model, four electrons are transferred to dioxygen through sequential one-electron oxidations of heme as s iron ion, the Cub ion, the heme a iron ion, and one of the bimetallic center s Cua ions. The sequence of electron transferal differs in the mixed valence model, and a tyrosine radical (tyr) is generated. The proposed formation of a tyrosine radical during catalytic turnover arises from the known post-translational modification in most CcO s in which a covalent bond is formed between the his240 ligand of Cub... 

The radical >C(Ar)-C < is oxidized by ligand transfer as Jenkins and Kochi (1972) indicated. If the cation-radical [>C=C<]+ obtained as a result of the initial electron transfer is not fully consumed in the reaction, it is reduced by Cu(I) and returns in the form of its geometrical isomer. In the olefin cation-radical state, cis—>trans conversion has to take place, and it indeed takes place in the systems considered (Obushak et al. 2002). 

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