The mol. structures of askew and linear forms of H2C:CLiX (X = F, Cl) are investigated by ab initio MO theory. Complete geom. optimization reveals that the most stable structure for H2C:CLiF is the askew form, while H2C:CLiCl prefers the linear form. Calcns. indicate that the Fritsch-Buttenberg-Wichell rearrangement of H2C:CLiX is concerted, involving a simultaneous 1,2-hydrogen shift and elimination of LiX. The transition state structure is the p complex, H2CYCH甃iX, which eventually decomps. to free ethyne and LiX. No stereoselectivity is expected in various reactions if the unsatd. carbenoids take part in a linear form.

Key words: MOLECULAR STRUCTURE, CARBENE, LITHIUM COMPOUNDS, HALOHYDROCARBON, AB INITIO CALCULATION, MOLECULAR ORBITAL THEORY