Spectroscopic and structural characterization of a phosphavinylidene carbenoid, Mes*-P=C(Cl){Li(DME)2}:: Stabilization of a carbenanionic center by a cisoid lone pair interaction

Characterization of the phosphavinylidene carbenoid DME-solvate (Z)-Mes*-P=C(CI){ Li(DME)(2)} (Z-2a) or the corresponding THF-solvate (Z-2b) by low-temperature X-ray diffractometry and multinuclear NMR studies reveals the presence of discrete monomeric molecules with donor-solvated metal atoms both in solution and the solid state. The structural parameters of solid 2a and the observed intermolecular exchange of Li+ between different carbanion fragments in solution suggest a low degree of covalency for the C-Li bond. This is in accord with a theoretical analysis of the bonding which demonstrates that the electronic structure of the model carbenoids (E/Z)-Me-P=C(Cl){Li(DME)(2)} (5) is very similar to that of the carbanions (E/Z)-[Me-P=C(Cl)](-) (6). In particular, the low degree of covalency in the C-Li bond in 5 is confirmed by natural population analyses and an analysis of the Laplacian of the electron density. On the basis of these results, the bonding situation in the phosphavinylidene carbanion system is discussed in terms of an isolobal analogy to N2F2.