MOLECULAR-ORBITAL BOND INDEX (MOBI) CALCULATIONS ON SELECTED ORGANO-LITHIUM AND LITHIUM ATE SPECIES - EVIDENCE FOR LI=H-C 3-CENTER INTERACTIONS

Molecular Orbital Bond Index (MOBI) calculations have been carried out on selected organolithium tetramers and hexamers, and on certain lithium 'ates, all of which had previously been shown to exhibit short Li ⋯ HC contacts in their solid-state structures. Species so examined comprised (MeLi)4, (EtLi)4 and their “dimerized” forms (to mimic the further association of these tetramers in the crystal), (Me2CHLi)6 and (H3SiCH2Li)6, [as models for (c-C6H11Li)6 and (Me3SiCH2Li)6, respectively], and (LiAlEt4)n and (LiBMe4)n, with n = 1,2. The calculational results indicate that the short Li ⋯ HC distances observed reflect genuine three-centre interactions between lithium centres and CH bonds: thus, summed Li ⋯ H bond indices (measures of the electron density between the nuclei concerned) contribute significantly (44 → 14%) to the total lithium valency, and concomitant weakening of the indices of involved CH bonds (0.80–0.90; cf. indices of distant CH bonds, ca 0.95) is observed.