Hypervalent atoms

Three-Membered Ring Molecules with a Hypervalent Atom... 

The Gillespie-Nyholm rules usually can also be applied to other polyanionic compounds with hypervalent atoms. As an example, some polytellurides are depicted in Fig. 13.6. The Te

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Lewis recognized that certain molecules such a PCI5 and SF6 are exceptions to the octet rule because their Lewis structures indicate that the central atom has more than eight electrons in its valence shell 10 for the P atom in PCI5 and the S atom in SF4, and 12 for the S atom in SFg (Figure 1.17). Such molecules are called hypervalent because the valence of the central atom is greater than its principal valence. To write a Lewis structure for such molecules, the Lewis symbol for the hypervalent atom must be modified to show the correct number of unpaired electrons. For the molecules in Figure 1.17 we would need to write the Lewis symbols as follows ... 

Triheterapentalenes have numerous structural variations of the generic structure 6. They can possess either S, Se, or Te as its central hypervalent atom Y. The number of permutations increases dramatically since each of the atoms X and Z can be replaced by S, Se, O, or NR. Also, one or more of the remaining ring carbons can also undergo aza replacements. 

TBP of RR E X2 (and/or R2EX2) will be in equilibrium with molecular complexes (MC), together with the components, in solutions (Scheme 2).20 22 Both X-E-X and E-X-X bonds are hypervalent 3c-4e.15e 20 According to the N-X-L notation, proposed by Martin,23 the hypervalent atoms in X-E-X (TBP) and E-X-X (MC) are represented by 10 t 4 and 10-X-2, respectively. The notation will be employed in this chapter. 

The structure and stability of hypervalent. J-difluorothiaziridine were theoretically investigated within a comprehensive study of the structures and stabilities of three-membered rings containing a hypervalent atom - Si, P, and S (Figure 5 i.e., 23) <2001PCA10711>. 

According to the 1983 lUPAC recommendations (1984PAC769), the position and the valency of a hypervalent atom in a molecule is indicated by using the Greek letter X. In the lambda nomenclature, the symbol X" is used to indicate any heteroatom in a nonstandard valence state (n) in a formally neutral compound. The lambda terminology is broadly used in the modem literature to indicate the general type of a hypervalent compound and to specify the number of primary bonds at the hypervalent atom. 

Allyl and benzyl cations are the prototype delocalized carbenium ions (look at Figures 1.21 B and 1.22). Conventional it delocalization does not convert a carbenium ion to a carbo-nium ion in this case. No hypervalent atoms are involved in any of the resonance structures for allyl or benzyl. The same orbitals that carry the negative charge in allyl and benzyl anion carry the positive charge in allyl and benzyl cation. 

The ability of an atom in a molecular entity to expand its valence shell beyond the limits of the Lewis octet rule. Hyper-valent compounds are common for the second and subsequent row elements in groups 15 -18 of the periodic table. Hyperva-lent bonding implies a transfer of the electrons from the central (hypervalent) atom to the nonbonding molecular orbitals which it forms with (usually more electronegative) ligands. A typical example of a hypervalent bond is a linear three-center, four-electron bond, e.g., that of the Fap-P-Fap fragment of PF5. 

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