Secondary interaction

The heavier chalcogens are more prone towards secondary interactions than sulfur. In particular, the chemistry of tellurium has numerous examples of intramolecular coordination in derivatives such as diazenes, Schiff bases, pyridines, amines, and carbonylic compounds. The oxidation state of the chalcogen is also influential sulfur(IV) centres engender stronger interactions than sulfur(II). For example, the thiazocine derivative 15.9 displays a S N distance that is markedly longer than that in the corresponding sulfoxide 15.10 (2.97 A V5. 2.75-2.83 A, respectively). ... 

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

It will be noted that the central I-l distance is close to that in I5+ and that the terminal 1-Cl distance is very similar to that in )3-lCl (p. 826). There are also strong secondary interactions so as to form infinite zig-zag chains via trans-Cl atoms of the octahedral SbCle" anions (1- Cl 294.1 pm, angle Cl-1 1 177.6°). 

Method Characteristics Anode materials Current source Installation Possibilities of secondary interaction in foreign structures o VO... 

Some advice can be formulated for the choice of organic modifier, (i) Acetonitrile as an aprotic solvent cannot interact with residual silanols, whereas the protic methanol can. Thus, when measuring retention factors, methanol is the cosolvent of choice, as it reduces the secondary interactions between the solutes and the free silanol groups, (ii) For the study of the performance of new stationary phases one should use acetonitrile, as the effects of free silanol groups are fuUy expressed [35]. (iri) Acetonitrile with its better elution capacity can be considered as the best organic modifier for Hpophilicity measurements of highly Hpophihc compounds with adequate stationary phases [36]. 

Law, B. and Weir, S., Quantitative structure-retention relationships for secondary interactions in cation-exchange liquid chromatography, ]. Chromatogr. A, 657, 17, 1993. 

Figure 22 Some thiolate gold(I) complexes with several secondary interactions.

Despite the fact that the exo adduct is likely to be the more stable of the two thermodynamically, it is often (though not universally) found in Diels-Alder reactions that the endo adduct is the major, if not the sole, product. To explain this, it has been suggested that in endo addition stabilisation of the T.S. can occur (and the rate of reaction thereby speeds up) through secondary interaction of those lobes of the HOMO in, e.g. (32) and of the LUMO in (33) that are not themselves involved directly in bond-formation, provided these are of the same phase. Such interaction would not, of course, be possible in the T.S. for exo addition because the relevant sets of centres in (32) and (33) will now be too far apart from each other the endo adduct is thus the kinetically controlled product. It is significant in this connection that the relative proportion of exo... 

In Equation (15), R others encompasses all secondary interactions which are not included in the first two terms (for instance the interaction with an unpaired electron, the spin-rotation interaction,...). By contrast, the expression of the cross-relaxation rate is simply... 

In the solid state, the pyridine nitrogen atom of compound 10 forms secondary interactions with the mercury center. Intramolecular coordination of the nitrogen atom is, however, not observed in derivative 11 in which N —> Si 7t-bonding possibly monopolizes the nitrogen lone pair. 

Longer Hg-7r interactions are observed in the /> ra-/-butylcalix[4]arene mercury complex 162. The mercury atom forms primary bonds with the two sulfur atoms and engages in weaker secondary interactions with two arene rings of the calixarene whose centroids sit at 3.07-3.11 A from the metal center.201... 

From examination of Fig. 11, it is inferred that the zn-n state is less reactive, and a biradical mechanism should be the major reaction pathway. The degenerate stabilizing perturbation of the bonding levels is missing, and concerted pathways are not likely if stabilized only by much smaller secondary interactions. If the hi-n singlet state could be intercepted in some way, the all-suprafacial concerted mechanism would be favored [K(ji ) -0(jr )] relative to the suprafacial-antarafacial mechanism [O(tt) - -K( i )]. 

Cs3Se22 has recently been prepared under methanolothermal conditions138 and is isostructural to Cs3Te2219. In contrast to the [Te ] sheets (Figure 10b) of the polytelluride, the anionic sheets in Cs3Se22 are best described as [(Se3-)(Se3-)], in which disordered pairs of bent Se3 radical anions and Sef anions are linked through secondary interactions. 

The tendency of arsenic and to form additional secondary bonds has interesting consequences in the case of dialkyldithiophosphinate arsocane and stibocane derivatives, X(CH2CH2S)2MS2PR2, X = O, S R = Me, Et or Ph M = As,146 or Sb147,148 The conformational trends and intermolecular association in these compounds are influenced by the competition between endo-cyclic, transannular As X interactions and an exocyclic As- -S secondary interaction. 

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