Carbon-lithium bond covalent character

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy 36,134 143) which has shown that the carbon-lithium bond of allylic-lithium species can possess considerable covalent character. Worsfold has also previously published 43 > concentrated solution viscosity results where the ratio of flow times, before and after termination, of a poly(isoprenyl)lithium solution was about 15. This finding is clearly incompatible with the conclusion that viscometry cannot detect the presence of aggregates greater than dimeric. 

There is considerable controversy regarding the degree of covalent character in a carbon-lithium bond ". An uncritical comparison of electronegativities does indicate a high degree of ionic character as do extended Hiickel molecular orbital... 

Although the carbon—lithium bond of an alkylhthium (RLi) has covalent character, it is polarized so as to make the carbon negative ... 

Table 12.1 shows some organometallic lithium compounds. It is seen from their formulas that these compounds are ionic. As discussed in Section 12.2, 1A metals have low electronegativities and form ionic compounds with hydrocarbon anions. Of these elements, lithium tends to form metal-carbon bonds with the most covalent character therefore, lithium compounds are more stable (though generally quite reactive) than other organometallic compounds of group 1A metals, most... 

As discussed in Section 12.2, group 1A metals form ionic metal-carbon bonds. Organometallic compounds of group 1A metals other than lithium have metal-carbon bonds with less of a covalent character than the corresponding bonds in lithium compounds and tend to be especially reactive. Compounds of rubidium and cesium are rarely encountered outside the laboratory, so their toxicological significance is relatively minor. Therefore, aside from lithium compounds, the toxicology of sodium and potassium compounds is of most concern. 

The organometallic compound chemistry of the 2A metals is similar to that of the 1A metals, and ionically bonded compounds predominate. As is the case with lithium in group 1 A, the first 2A element, beryllium, behaves atypically, with a greater covalent character in its metal-carbon bonds. 

Theoretical calculations of organolithium species have received considerable attention. The low atomic number of lithium is suitable for the most sophisticated molecular orbital methods. Although much debate exists over the degree of covalency within lithium carbon-bonding interactions, the presence of some covalent character in Li bonds of alkyllithinm componnds is widely accepted. 

The nature of the carbon-metal bond varies widely, ranging from bonds that are essentially ionic to those that are primarily covalent. Whereas the structure of the organic portion of the organometaiiic compound has some effect on the nature of the carbon-metal bond, the identity of the metal itself is of far greater importance. Carbon—sodium and carbon-potassium bonds are largely ionic in character carbon—lead, carbon—tin, carbon—thallium, and carbon—mercury bonds are essentially covalent. Carbon—lithium and carbon—magnesium bonds lie between these extremes. 

Polymerization then proceeds in the manner of the alkyl-initiated reaction described above. It may be noted here that lithium (both as the free metal and as alkyls) stands in marked contrast to the other alkali metals in giving rise to polymers with high proportions of 1,4-units. This is attributable to the covalent character of the lithium-carbon bond, as will be discussed later (Chapter 18). 

The polarity of the metal-carbon bond increases upon going down in the periodic table the lithium alkyls have some covalent character and form tetrameric clusters, whereas cesium alkyls are purely ionic. The degree of clusterification of lithium alkyls varies with the nature of the solvent between dimer (LiCHs TMEDA) and hexamer (Li-n-C4H9 cyclohexane), as can be checked by osmometry, NMR and EPR. Li NMR (I =3/2 abundance 92.6%) and Li NMR (I = 1/2 abundance 7.4%) allow to also show the dynamic fluxionahty phenomena around the Li4 tetrahedron, reversible dissociation of tetramers to dimers and ion pairs (contact ion... 

It is known that the anionic polymerization of dimethyl trimethylene carbonate (DTC) in toluene with lithium cation as a counterion proceeds slower than that with potassium one due to the more covalent character of the lithium-oxygen bond compared with the potassium-oxygen bond this leads to a lower nucleophilicity of the lithium alkoxide and also the low tendency for complexation with PEG favors lithium as... 

The intense nucleophilic reactivity of these compormds is ascribed to the presence of nearly a full unit of negative charge on one of the carbon atoms. n-Butyl-lithium, on the other hand, is a colourless liquid which readily dissolves in saturated hydrocarbons, as a hexamer (Bu°Li) , believed to have an electron-deficient covalent constitution though the lithium-carbon bonds are certainly highly polar. The more covalent character of lithium alkyls is likely to be due mainly to the smaller radius and higher polarizing power of lithium relative to sodium. 

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