Hypovalent molecules, hypovalence

Exceptions to the Octet Rule Hypervalent and Hypovalent Molecules 21... 

Soon after the quantum revolution of the mid 1920s, Linus Pauling and John C. Slater expanded Lewis s localized electronic-structural concepts with the introduction of directed covalency in which bond directionality was achieved by the hybridization of atomic orbitals.1 For normal and hypovalent molecules, Pauling and Slater proposed that sp" hybrid orbitals are involved in forming shared-electron-pair bonds. Time has proven this proposal to be remarkably robust, as has been demonstrated by many examples in Chapter 3. 

The energy of near-UV photons is on the order of bond energies in organic molecules. Photoreactions are thus frequently initiated by the homolytic cleavage of a covalent bond, which leads to a biradical, if the primary product is still covalently linked (Figure 5.13). Biradicals, also called diradicals, are usually defined as molecular species that possess two (possibly delocalized) radical centres, that is, one bond less than the number predicted by the standard rules of valence. When the two radical centres are located on the same atom (1,1-biradicals), such hypovalent species are referred to as carbenes (Section 5.4.1), nitrenes (Section 5.4.2), and so on. The two radical centres may be connected through saturated covalent bonds (localized biradicals) or they may be delocalized over the same Jt-system (conjugated biradicals). 

One problem with the reactions of C6H5N with organic carbonyl containing molecules was that the carbonyl group reactivity scale could not be extended past the simple esters HC02CH3 and CH3C02CH3 because their rates were already at the lower limit of determination in the FA. To get around this limitation, we examined other hypovalent species such as... 

The Lewis acid properties of the triel Z atoms (Z = B, Al, Ga, In, Tl) were analyzed recently in triel trifluorides, ZF3 [67]. The strong acidic properties of the triel atoms in those planar molecules are connected with their electron stmctures since the triel atom possesses six electrons in the outer shell thus it is electron deficient. This electron stmcture is connected with the existence of the vacant p-orbital situated perpendicularly to the plane of the molecule. Consequently it results in the strong electrophilic properties of the Z-center. The octet mle is not fulfilled here what is often named as the hypovalency in the literamre [68]. The similar electronic stmcture and consequently the electron deficiency is observed for... 

We depart this section with the following message If d orbital participation is assumed to be insignificant, "hypervalent" molecules are actually hypovalent molecules, e.g., SF has three rather than four core-ligand bonds. On the other hand, if d orbital participation is strong "hypervalent" molecules are worthy of their title, e.g., SF has four core-ligand bonds. 

The search for boron-centered nucleophiles has been a fundamental challenge owing to the six-valence electron, hypovalent nature of boron which renders boron-based molecules electrophilic. A major breakthrough in this area came in 2006 when Nozaki and Yamashita isolated and structurally characterized a boryllithium reagent that was isoelectronic to an NHC and reacted as a boron-centered nucleophile. Since, there have been additional reports of boron-centered nucleophiles where the nucleophilic boron atom was a ligand to a transition metal [i.e. borylene). In the past five years, however, carbenes have emerged as excellent ligands to stabilize sp nucleophilic boryl anions. 

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