Pentavalent carbon

The stable 2-norbornyl cation has recently been shown to be a non-classical, unusually stabilized species. Olah et al. (1970) proved spectroscopically that this ion is a comer-protonated nortricyclene with a pentavalent carbon atom. The value for the carbonylation-decarbonyla-tion equilibrium constant K (= of the 2-norbomyl ion illustrates... 

All structures must be valid Lewis structures. An atom may become electron deficient, but, on the other hand, it must never be shown with more valence electrons than it can accommodate. For example, it is not possible to have pentavalent carbon. 

A carbocationic species in which there is at least one pentavalent carbon atom (e.g., CHs ). 2. Traditional name for chemical species that are now referred to as carbenium ions. Considerable confusion exists in the literature with this term for carbocations. The -onium suffix usually refers to a higher covalency when compared to the neutral atom thus, CH5+ would be a true carbonium ion (in terms of the first definition). Additional ambiguity results when the term ethyl carbonium ion is used to describe both CH3CH2 and to R—CH2CH2. For these reasons, the terms carbocation or carbenium ion are now preferred. 

One example of a reaction which seems to reoccur in the literature involving pentavalent carbon is Reaction 12. 

A concerted mechanism assuming the elecrophylic attack of one oxygen atom of the diiron ferryl to the C-H bond carbon with nucleophylic assistance of the second oxygen atom with the formation of a pentavalent carbon intermediate (Shteinman, 1996). 

Indeed, the majority of the higher-covalent silicon compounds studied which have an increased coordination number are formed by bi-, tri-, and tetradentate ligands similar effects are apparent with other Group IVA elements. It is likely that the structural factors are responsible for the existence of pentacoordinate pentavalence carbon species expanding the valence shell to 10 (Sect. 3.2). For the same reason that there are about twenty structures of the organotin complexes with seven- and eight-coordination... 

This particular transition state is intermediate between reactants and products not only in the time sequence but also in structure. Not all transition states arc intermediate in structure as shown on page 462, reactant and product in 8 2 reactions are tetrahedral, whereas the transition state contains pentavalent carbon.)... 

It is, of course, the carbonyl group that makes acyl compounds more reactive than alkyl compounds. Nucleophilic attack (Sn2) on a tetrahedral alkyl carbon involves a badly crowded transition state containing pentavalent carbon a bond must be partly broken to permit the attachment of the nucleophile ... 

In the first case, the carbon would now have eight electrons accommodated around it. Yet these eight electrons are required to bond five hydrogen atoms to the central carbon atom. This would make available less than two electrons for each bond. Even though this is possible, it does not occur under normal circumstances, and so for the present purposes we may assume that this molecule does not exist, because there are too few electrons to hold all the hydrogens in place. In the second case, the proposed molecule would have nine electrons, and so would violate the Octet rule. So, under normal circumstances pentavalent carbon atoms are not found, regardless of whether they are charged or neutral. 

Protonated cyclopropanes are an example of a now well-documented class of cationic species that involve pentavalent carbon, the simplest of which is The bonds shown as dotted lines represent three-center, two-electron bonds. 

The apparently pentavalent carbon atom in CHs has drawn much attention and in terms of simple intuitive chemical structures this species is best thought of as a CH3 ion solvated by a hydrogen (H2) molecule (Heck 1991). The tendency of a reagent ion BH like CHj to donate a proton to a gas molecule is conveniently expressed in terms of the proton affinity (PA) of a species B ... 

A carbocation is a structure with a positive charge that is associated primarily with a carbon center or, in the case of delocalized systems, a collection of carbons. Heteroatom substitution is allowed, and it will inevitably lead to some positive charge being on the heteroatoms. There are two types of carbocations—carbenium ions and carbonium ions. Carbenium ions are trivalent species with a formula of RaC. Carbonium ions encompass two related types of structures. First are pentavalent species of the general formula R5C. While not common in solution phase chemistry, such pentavalent carbons are common in the gas phase. Also referred to as carbonium ions are carbocations that have an important contribution from three center—two electron bonding. The nomenclature will become more clear as we present examples below. 

Never draw a pentavalent carbon because that would violate the octet rule. 

When drawing a structure, make sure to avoid drawing a pentavalent carbon atom, or even a hexavalent or heptavalent caibon atom ... 

However, one of these bonds cannot be made via a reductive amination, because the starting material caimot have a pentavalent carbon atom ... 

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