Solvation tetracoordinated lithium

A more recent study of ethynyllithium included the effect of lithium solvation on dimerization. At the 6-3H-G level successive coordinations of water to form HCCLi(OH2), HCCLi(OH2)2, and HCCLi(OH2)3 have AE of —21, —10, and —5 kcal mol , respectively. As solvation of lithium increases, the dimerization to the corresponding solvated ethynyllithium dimer becomes less exothermic. An interesting further feature is that the dimer with two waters per lithium has D2, symmetry, with each tetracoordinated carbon associated with two terminal ethynyl carbons and two waters. With fewer waters of solvation the coordination number of the lithiums is increased by association with the r-bond of the acetylene. This feature... 

The monolithiated cyclopropene dimer was computed to have an unusual structure with two planar tetracoordinate carbon fragments the planar structure 6 is found to be about 11 kcal mol more stable than the perpendicular form 7 with tetrahedral tetracoordinate carbon. Calculations on model dimers indicated that the planar geometry is even stabilized further by heteroatom substituents at the vinylic Cfi, because of the formation of favorable chelate rings. The solid-state structure of a solvated cyciopropenyllithium dimer reveals the perpendicular geometry with tetrahedral tetracoordinate carbons, but lithium chelation by C/0 and C/N dianions results in dimeric cyclopropenyl structures with a high degree of planarization of the tetracoordinate carbon environments. ... 

The n -coordination of lithium in 13a is retained in the most stable structure when the lithium is further coordinated with one or two molecules of dimethyl ether. Solvation with a third ether now completes the tetracoordination of lithium with four oxygens and the n -interaction is no longer necessary the C-O-Li bond in this fully solvated structure is almost linear. ... 

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