Bromoselenates , mixed-valence

From the complex reaction mixture of the redox system SeBr4/SeBr2/Se/Br in aprotic solvents, di-, tri, and tetranuclear mixed-valence bromoselenates(IFIV) containing selenimn in both oxidation states -1-2 and -1-4 were prepared [Se2Brg]2, [SesBrio], and [Se4Bri2] (Figure 20). 

Figure 20 Mixed-valence bromoselenate(IFIV) anions (a) [Sc2 Brg] in [Et4N]2[Se2Brg] (b) [SesBrio] in [(n-Pr)4N]2[Se3Brio] (c) [Se4Bri2] in [Ph4P]2[Se4Bri2]-0112012...

The diamagnetic mixed-valence bromoselenates(ll/lV) are to be assigned to class 1 in the Robin-Day scheme. From the structnral behavior, a significant valence exchange is not to be expected. The coordination geometries are always clear indications of dilferent oxidation nnmbers. 

By further evaluation of the redox system SeBr4/SeBr2/Se/Br in aprotic solvents (Section III,A), the first examples of bromoselenates-(II,IV) containing selenium in both oxidation states +2 and +4 were prepared. Three dinuclear, trinuclear, and tetranuclear types of anions are reported in this novel class of mixed-valence compounds Se2Brg, SeaBrio , and Se4Bri2 . They are shown in Fig. 31. Similar to the bromoselenates(II), they were obtained by nucleophilic addition reactions of bromide ions within the complex reaction mixtures of selenium bromides. Evidently, the formation of Bra" is important for the stabilization of Se(II) in addition to Se(IV), shifting the equilibrium... 

Fig. 31. Mixed-valence bromoselenate(II,IV) anions, (a) [Se Se" Brs] in [ (C2H5)4N]2[Se2Brg], (b) [SenaSet Brio]"- in [ (nC3Hs)4N]2[Se3Brio], (c) [Se"3Se Bri2]" in [(C6H5)4P]2[Se4Bri2] CH2CI2, with important interatomic distances (A) (254).

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