Classical ions, trivalent

In electrophilic reactions with n and n donors, the trivalent carbenium ions play the main role. Nevertheless, the role of non-classical ions cannot be neglected in view of the equilibrium... 

Lewis valence bond structures]. The methyl cation or methenium ion (CH3+) may be considered the parent of the trivalent carbocations. The trivalent carbocations are also stiU referred to as classical ions. 

The norbornyl ion controversy """ centered on the question of whether the ion had the static bridged nonclassical structure (126) containing pentacoordinate hypercarbons or whether its structure was better depicted as a rapidly equilibrating pair of trivalent classical ions (127a and 127b). 

Olah emphasizes that the division of cations into classical and nonclassical is frequently arbitrary, since in many cations there is an intenn liate range of delocalization ( partial carbonium-ion character ) as in the 2-methylnorbomyl ion. The author does not want to name classical ions carbonium because it is restricted to highest valeiK state carbocations this requirement is nwt by penta-and tetracoordinate carbocations but not trivalent ones. On the other hand, while in the formation of other onium ions the atom of the donor (nitrogen, oxygen etc.) increases its covalence by one unit upon addition of the acceptor (electrophile), in the formation of a classical ion the covalence of the carbon atom decreases from 4 to 3. As for the name carbenium ion, in the author s opinion it reflects the logical relationships between the carbene and the carbeiunm ion, between the alkene and the carbenium ion ... 

Trivalent-tricoordinate (Hypocoordinate) carbenium ions Classical ions CHg"... 

Trivalent-tricoordinate carbenium ions "Classical" ions CH3-... 

Cations are kinetic chain carriers in cationic polymerizations. Such cations may be, for example, carbocations or oxonium ions. All electrophilic carbon atoms may be described as carbocations. These can be classified as carbenium ions (trivalent carbocations) and as carbonium ions (carbocations with coordination numbers of four or five). Carbenium ions such as, for example, R3C are classical carbocations. Carbonium ions such as, for example, R5C or R5C2 are nonclassical ions. 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

The classical dc polarography of vitamins A, B, B2, B6, BI2, and C, nicotinamide, tocopherols, and naphthoquinones has been reviewed [55]. Other studies have examined in detail the cyclic voltammetry of vitamin B12 employing rapid-scan voltammetry at the DME [90] and the HMDE [91]. Vitamin B12 is complexed with trivalent cobalt ion at the heterocyclic nitrogen atoms. As a result of the complexation, a catalytic hydrogen wave is formed for the compound. In addition to the catalytic wave, a wave corresponding to the reduction of the trivalent cobalt to the monovalent state is observed. 

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 26 have been more loosely defined. In recent years, a lively controversy centered on the classical-nonclassical ion problem.27-37 The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown, 31 who carried, however, the criticism to question the existence of any o-delocalized (nonclassical) ion. For these ions, if they exist, he stated ... a new bonding concept not yet established in carbon structures is required. ... 

Crystal chemistry of spinels. A classic example showing that transition metal ions display distinct site preferences in oxides stems from studies of spinel crystal chemistry. The spinel structure contains tetrahedral and octahedral sites normal and inverse forms exist in which divalent and trivalent ions, respectively, fill the tetrahedral sites. The type of spinel formed by a cation is related to its octahedral site preference energy (OSPE), or difference between crystal field stabilization energies in octahedral and tetrahedral coordinations in an oxide structure. Trivalent and divalent cations with large site preference energies (e.g., Cr3 and Ni2+) tend to form normal and inverse spinels, respectively. The type of spinel adopted by cations with zero CFSE (e.g., Fe3+ and Mn2+) is controlled by the preferences of the second cation in the structure. 

Olah s definition of two main classes of carbocations is most appropriate. A carbenium ion is a classical entity which contains an electron-deficient trivalent carbon atom possessing sp hybridisation and six electrons in the valence shell, e.g., CH3, (CH3)3C, CgH7 (benzenium ion), etc. The three atoms bound to such as carbon atom tend to be... 

The last three, the NH2, NH , and N ions, are found in the salt-like amides, imides, and nitrides of the most electropositive metals, but with the exception of the amide ion the stereochemistry of nitrogen is based on N with no lone pairs and N with one lone pair. These two states of the nitrogen atom correspond to the classical trivalent and pentavalent states, now preferably regarded as oxidation rather than valence states. 

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