Electron transfer, paramagnetic transition metal complexes

Some electron exchanges of cobalt complexes involve changes in spin configuration, from low-spin to high-spin such that S S V2, and this can lead to another source of non-adiabaticity. The spin forbidden factor, Xs> is entirely empirical within the present formalism, as well as within the TM formalism. For the complexes Co(phen)3 + + experimental evidence, also supplemented by quantum mechanical calculations, suggest that Xs 10. and as a rule of thumb we will employ this value for spin-forbidden electron transfers in transition-metal complexes. Where no heavy metals or paramagnetic species are involved, we expect to have Xs 10 , as shown in Chapter 15 for aromatic hydrocarbons. 

A second way to overcome this spin conservation obstacle is via reaction of 302 with a paramagnetic (transition) metal ion, affording a superoxometal complex (Fig. 4.1, Reaction (3)). Subsequent inter- or intramolecular electron-transfer processes can lead to the formation of a variety of metal-oxygen species (Fig. 4.2) which may play a role in the oxidation of organic substrates. 

These reactions turned out to be preparatively very useful since they allow the performance of radical cyclization reactions but produce an organozinc halide as a final product (Scheme 9-33) [65-70]. The treatment of an unsaturated alkyl halide 36 (X = Br, I) with a palladium(O) or nickel(O) complex produces, via a one-electron transfer [72], a paramagnetic nickel(I) or palladium(I) salt MLj,(X) (M = Ni, Pd) and a radical 37 which undergoes a smooth cyclization reaction and produces, after recombination with the transition-metal moiety, the nickel(ll) or palladium(II) species 38. After transmetallation with a zinc(II) salt, a stable organozinc cyclopentylmethyl derivative of type 39 is produced. 

In paramagnetic complexes, such as some octahedral complexes of transition metal ions, the metal -ligand interactions are responsible for transfer of electron spin from the metal to the ligands. The existence of net electron spin population in the hydrogen Is orbitals of the ligands causes very large chemical shifts, either for high or low frequency. Three of the several mechanisms involve tt interactions as follows. 

Not classified as free radicals are atoms or molecules in ground or excited electronic states wifli multiplicities larger than two (e.g. O, P O2, X N, " 8 molecules in excited triplet states), transition metal ions and their complexes deriving their paramagnetism exclusively or mainly from d- and f-eleclrons and charge transfer complexes. However, a number of polyatomic molecular species which do not fidfill die above definition are included because their properties closely resemble those of stmcturally closely related free radicals. These are... 

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