Computational predictions of planar tetracoordinate carbon R 1 R 2 C R Li 2 arrangements in the cyclopropenyllithium dimer have now been realized experimentally in derivatives with additional chelating ligands. The dimer structures of two substituted cyclopropenyllithiums bearing lithium alkoxide and lithium amide side arms, C R and O dilithiated 1-(di-tert-butylhydroxymethyl)-3,3-dimethylcyclopropene (8) as well as C R and N dilithiated 1-(tertbutylaminodimethylsilyl)-3,3-dimethylcyclopropene (9), were characterized by single-crystal X-ray diffraction. The complexes of 8 with tetramethylethylenediamine (TMEDA), (8‚TMEDA) 2 , and with THF, (8‚2THF) 2 , show laddertype dimeric aggregation. The tetracoordinate R 1 R 2 C R Li 2 carbon environments in both crystal structures have a high degree of planarization (with 35-39°twist angles), partly due to chelation of one of the lithiums by the Omoiety. Compound 9 crystallizes as a dimeric THF solvate, (9‚2THF) 2 , with a puckered ladder structure. Chelation of one of the lithiums by the Nmoiety in the individual (9‚2THF) 2 units results in R 1 R 2 C R Li 2 environments which aproach planarity even more closely (17 and 30°twist angles). Bridging of the vinylic C R -C β bonds by the chelated lithiums in (9‚2THF) 2 results in an unexpected feature, the nearly planar tetracoordinate environment of C β (the cyclopropene carbon next to the lithiated site)! Becke3LYP/6-31G* calculations on mixed-anion (MeLi‚LiOH) 2 model aggregates are consistent with the experimental findings that dimerization of both (8‚TMEDA) 2 and of (8‚-2THF) 2 involves the Li-O rather than the Li-C bond. In contrast (but also consistent with the model computations and with the steric environments), the (9‚2THF) 2 dimer is formed via Li-C rather than Li-N bond aggregation.