Carbon-lithium bond molecular orbitals

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... 

Several alternative transition structures for the [2.3]-Wittig rearrangements (Z)-(48) —> threo- 49), and ( )-(50) — erythro-(51), have been explored by ab initio molecular orbital calculations at the 6-31G level in an attempt to explain why the observed stereo selection is opposite to that for alkenes which do not bear a 1-carboxylic group.49 It has been concluded that coordination of lithium cation to two oxygen atoms and the C(4) carbon plays a significant role in reactions of (48) and (50), thereby making it easier to break the 0(2)—C(3) bond. 

Spherical-domain models of three-center bonds in localized-molecular-orbital models of a nonclassical carbonium ion, B4CI4, and TaeClfJ have been described 49,52) a drawing of a spherical-domain model of the methyl lithium tetramer, (LiCH, is shown in Fig. 31. Large, outer circles represent domains of electron-pairs of C—H bonds. Solid circles represent domains of Li+ ions. Shaded circles represent 4-center lithium-lithium-lithium-carbon bonds — i.e., electron-pair domains that touch, simultaneously, three lithium ions and the kernel of a carbon atom. The... 

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. 

Molecular orbital calculations, by Bach et al., at the HF/631 -I- G /HF/4-31 -l-G level revealed that oxidation of the monomeric lithium enolate of an acetaldehyde proceeds by SN2 attack of the /1-carbon on the enolate along the O—N bond of the parent oxaziridine (Figure 2) <92JOC6l3>. In this transition state, the lithium cation is coordinated not only to the enolate oxygen atom, but to the oxaziridine oxygen and nitrogen atoms as well. 

To investigate this interaction it is necessary to examine the symmetry of the molecular orbitals that contain substantial contributions from these atomic orbitals. The bond lengths in the carbanions in the structures examined are close to those expected from simple Hiickel bond orders for the isolated carbanion. As noted, the carbanion 7r-orbital contribution to the HOMO of the complex is probably very similar to the HOMO of the free carbanions. The symmetries of the HOMO s of the free benzyl and fluorenyl carbanions and the orientation of the N—Li—N groups with respect to the carbanions are shown in Figures 25 and 26. The N—Li—N group is positioned to permit the appropriate symmetry overlap of the lithium p orbital, which is parallel to the carbanion plane and the appropriate pz orbitals of carbon atoms in the plane. 

The INDO molecular orbital coefficients for the HOMO of the benzyl carbanion are given in Table X. As indicated, the symmetry of this orbital is appropriate for overlap between the CL> and C7 pz orbitals and a combination of lithium px and py orbitals. In addition, the contribution of the Ci pz orbital to the HOMO is relatively small. The HOMO of the complex Li(NH3)2C7H7 (Table X) is made up of the HOMO of the carbanion and the Li px and pv orbitals with a substantially smaller proportion of lithium pz. The values of the overlap integrals, Li (px>py)— C(p ) and, for comparison, the value of the C(pz)—C(pz) overlap integral for adjacent carbon atoms in the benzyl carbanion are also given. Therefore, both symmetry and INDO calculations are consistent with a substantial degree of three-center carbanion-metal bonding of the type described. 

Molecular orbital modeling of the reaction of organolithium compounds with carbonyl groups has examined the interaction of formaldehyde with the dimer of methyllithium. The reaction is predicted to proceed by initial complexation of the carbonyl group at lithium, followed by a rate-determining step involving formation of the new carbon-carbon bond. The cluster then reorganizes to incorporate the newly formed alkoxide ion. ... 

The effecfs of boron addition were also calculated based on a semiempiri-cal molecular-orbital model. Results show that the introduction of boron is favorable for lithium intercalation. When a layer of BQ is coated onto the surface of natural graphite, the performance improves considerably. In contradiction, another theoretical study based also on a semiempirical molecular orbital method concludes that the substitution of the carbon by boron is not effective for lithium storage. This illustrates the complexity of the carbon structure. These results suggest that the exact bonding states of boron may markedly influence the properties of the carbon materials. 

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