Unusual Bonding

The interaction between two disulfide units in a 2,3,5,5-tetrathiabicyclo[2.2.2] oct-7-ene type radical cation with transannular delocalization between the four sulphur atoms was established by ESR. This radical cation was persistent enough to be crystallized and analyzed by X-ray crystallography. Remarkably, this species was formed after oxidation of 3,4,5,6-bis(bicyclo[2,2,2]octeno)-l,2- 

A triplet state dimer of a benzofused 1,2,3-trithiole dication was postulated. However the occurrence of this high-spin dimer is strongly dependent on the nature of the solvent and the oxidant129. The 2,5-dimethoxy-4-methylphenyl moiety in which the additional charge and the single electron are stabilized in a quinoidal structure after one-electron oxidation has been utilized as a probe for electron delocalization when two of these moieties were connected either by a trimethylene, a 1,4-disubstituted phenylene or a biphenyl bridge130. 

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The ability to perform molecular orbital (MO) calculations on metals is extremely useful because molecular mechanics methods are generally unable to treat metals. This is because metals have a wide range of valences, oxidation states, spin multiplicities, and have unusual bonding situations (e.g., dtt-pjt back bonding). In addition, the nondirectional nature of metallic bonding is less amenable to a ball and spring interpretation. 

TThe x-ray stmcture of the novel bis-dicarboUide sandwich compound i (9y y (9-3,3 -Si(3,l,2-SiC2B2H22)2 contains a siUcon atom iu a highly unusual bonding mode (245). TThis compound, which is quite stable thermally, is prepared iu good yield. 

Compare carbon-carbon bond distances in each diene. Which bond distances are unusually long or short Use propene as a model of a molecule with normal CC single and double bonds. Can resonance account for all of the unusual bond distances Is this the only explanation ... 

Boron forms perhaps the most extraordinary structures of all the elements. It has a high ionization energy and is a metalloid that forms covalent bonds, like its diagonal neighbor silicon. However, because it has only three electrons in its valence shell and has a small atomic radius, it tends to form compounds that have incomplete octets (Section 2.11) or are electron deficient (Section 3.8). These unusual bonding characteristics lead to the remarkable properties that have made boron an essential element of modern technology and, in particular, nan otechn ol ogy. 

Some yeasts are able to add other monosaccharides, like galactose (S. pombe), but with unusual bonds [70], and they also tend to phosphorylate their glycans everywhere to obtain an additional charge on proteins [71]. 

The most unusual bond in this system is the N-Cl bond. The nucleophilic substitution step must involve cleavage of this bond. No base is present, but S is an excellent nucleophile, even in its neutral form, so the first step probably entails formation of an S9-N2 bond. Now we have to make the C4-C10 bond and make the S9-N2 bond. Deprotonation of C4 gives an ylide, which as discussed in problem 4.15 is likely to undergo a [2,3] sigmatropic rearrangement. Tautomerization to rearomatize then gives the product. 

The mechanism in Fig. 14 applies equally well to both PVC and HPll. However, an unusual bond between the imidazole ring of His392 and the p-carbon of Tyr415 on the proximal side of the HPll heme has been identified (Fig. 13) (93), and subsequently its presence was correlated with heme oxidation. The apparent correlation between heme oxidation and His-Tyr bond formation suggested a mechanistic linkage between the two modifications and an alternate mechanism imique to HPll was proposed (Fig. 15). As with the first mechanism, the reaction assumes the formation of compound I that is available for reduction. Formation of the His-Tyr bond, involving a base catalyzed proton extraction from the... 

The agreement is typical but on reflection not surprising indeed if the observed and calculated parameters differed significantly one should suspect the structure determination (as it would imply unusual bond lengths). It is well to recall too that in the early days of X-ray diffraction, when it was difficult to determine the coordinates of light atoms, they were often obtained by just this procedure. 

Unfortunately, most of the structural information of IR spectra is contained in the often very crowded region of 500-1600 cm which was hardly exploited for diagnostic purposes except in the case of very small molecules with few vibrations, or for pattern matching of spectra of reactive intermediates obtained independently from different precursors. The reason for this was that the prediction of IR spectra was only possible on the basis of empirical valence force fields, and the unusual bonding situations that prevail in many reactive intermediates made it difficult to model the force fields of such species on the basis of force constants obtained from stable molecules. 

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