Geometries and Energies of Dilithioethylene Isomers and of Vinyl Lithium. An Ab Initio Study

Abstract

The structures (4‐31G optimization) and energies (6‐31G*) of vinyl lithium (5) and of the 1,1 (3 and 4), cis‐1,2 (6), trans‐1,2 (7) and doubly bridged (8) dilithioethylenes have been calculated using standard ab initio methods. Vinyl lithium (5), indicated to have a classical geometry, is about 11 kcal/mole more stable than ethyl lithium in an hydride exhange reaction (Eq. (2)). Singlet trans‐1,2‐dilithioethylene, the most stable C₂H₂Li₂ isomer (see also addendum), has a highly distorted, partially bridged structure (7a) with < CCLi = 87°. Symmetrically bridged 8 and the cis‐isomer 6 are considerably less stable, 54 and 19 kcal/mole, respectively. The partial Li‐H bonding in 7a contributes significantly to this behavior. 1,1‐Dilithioethylene is found to prefer a perpendicular (3) over the usual planar (4) geometry both in singlet and in triplet states. Triplet symmetrically bridged 8 and triplet cis dilithioethylene (6) are indicated to be local minima, but are less stable than singlet trans 7a. Cis‐dilithioethylene (6) is indicated to be thermodynamically unstable towards dissociation into Li₂ and acetylene. This factor, and the low lying triplet state, may contribute to the experimental difficulties in preparing 6. The nature of the carbon–lithium bonding in these molecules is discussed. We find that an electrostatic model cannot account for the structural details or for the relative energies of the C₂H₂Li₂ isomers. The importance of the multicenter covalent nature of bonding to lithium is emphasized.