Three-electron bonds between different heteroatoms

5 Three-electron bonds between different heteroatoms 

A substantial number of investigations has been devoted to sulfur-halogen interactions, particularly neutral R2S X radicals.56,122,154,155 por X = iodine and bromine such species can be generated via oxidation of the sulfide and subsequent association of R2S + with the halide anion, or via ligand 

Optical data and equilibrium constants concerning Me2S. X radicals 

Examples for anionic species are rare and only some (RS Br) have been observed. 156 Radical cations have, however, been detected much more frequently. They include, for example, intramolecular sulfur-bromine, -iodine, -selenium, -nitrogen, and -phosphorus species of the general types 15, 16, and 17.140,151,157-161 

With regard to optical absorptions, influence of substituents and molecular structure all observed trends follow the same rules as outlined for the (S S)+ radical cations. The most stable (S . X)+ radical cations are those intramolecular species which attain five-membered ring structures, i.e., those with, for example, three linking carbons between the two heteroatoms. Particularly interesting are the S Br bonded systems since they represent a rare example of bromine-based cationic species in organic chemistry. 

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Let us now consider the formation of three-electron bonds between different atoms. Stabilization of an oxidized sulfur atom can, in principle, be achieved in cases of its interaction with other heteroatoms if they provide free (preferably p-) electron pairs. Nitrogen, oxygen, and halogens (except fluorine) can be mentioned as such heteroatoms (Anklam et al. 1988 Carmichael 1997). The stability of these bonds is generally not as high as that of a symmetric S.. S system. An important reference for the enhanced stability of symmetrical three-electron bonds is Clark s (1988) calculations. 

In view of (R2S.. OH2)+ being a distinct species it would not be unreasonable to also formulate the OH-adduct to a sulfide function, i.e., R2S (OH) as three-electron bonded radical R2S OH. However, being a neutral species, the latter cannot benefit anymore from any stabilization due to charge delocalization. The unpaired electron will, therefore, be driven towards the more electropositive sulfur by the full impact of electronegativity difference between the two heteroatoms and, therefore, the sulfuranyl notation, >S -OH, may thus... 

A general question arising in this context is whether these hydroxyl adducts, and possibly all (or at least some) of the sulfur-oxygen interaction species should be formulated as S.%0, that is, three-electron bonded species or perhaps better as sulfuranyl radicals, that is, >S -OH. The former is not unjustified because of many characteristics typical of a a weakened bond. On the other hand, the significant difference in electronegativity between these two heteroatoms drives the unpaired electron towards the more electropositive atom and, therefore, the >S -OH notation should perhaps be preferred since it describes the actual electronic situation more accurately. In this sense sulfur-oxygen (and in general X-Y) interactions approach the limits of the 2[Pg.184]

The presence of two heteroatoms with different electronic properties in one molecule (one atom possessing a lone electron pair and another one with a vacant orbital, donor and acceptor, base and acid) separated by some fragment results in an interaction between the heteroatoms. Three types of the phosphorus-boron interactions have been revealed through-bond, intramolecular trans-annular (dative P—B bond), and intermolecu-lar (dative P—B bond) (Fig. 3). 

The most widely employed heteroatom ligands are the phosphines. Although they are largely spectators and do not participate directly in bond formation (and when they do, the result is often highly undesirable), they are not innocent bystanders. The size and electronic nature of the three groups attached to phosphorus have a profound effect on the course of the reaction and may make the difference between success and failure. An example is with the Grubbs catalyst (Chapter 8). The bis(triphenylphosphine) complex is of little use. The bis(tricyclohexylphosphine) complex is Nobel-prize winning. 

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