Stability, onium carbanion

Schier and Schmidbaur93 performed a clever experiment that addressed part of this question does the orientation of the carbanion relative to the phosphorus atom play any role Scheme 2 shows two syntheses of ylides involving cyclopropyl substituents. In the first reaction, since the pKa of cyclopropane is considerably below that of propane, the expected product is the cyclopropylide. However, the isopropylide is the only recovered product. The second reaction also demonstrates the avoidance of the cyclopropylide product. The cyclopropylide possesses a very pyramidal carbanion that is directed away from phosphorus, allowing for minimal orbital overlap. The isopropylide is much less pyramidal and phosphorus can better assist in stabilizing the carbanion. While this stabilization does not require explicit orbital overlap (the electrostatic interaction of the carbanion with the onium is expected to be smaller in the cyclopropylide since it is directed away from P), it does suggest that some orbital interactions are involved. Hence, although the ylene contribution is small, it is unlikely that the ylene contribution is nil. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Ylides are 1,3-dipolar compounds, RjE—CRjRj, where E = N, P, As is a group-V element and RjRj is H, alkyl or aryl. The onium-stabilized carbanion is planar, or ca. sp -hybridization about the carbon atom. ... 

Extremely efficient stabilization can be observed for dipolar ions if the atoms or groups of atoms bearing the opposite charges are in the close vicinity. Such is the case with ylides, dipolar ions in which the carbanionic center is stabilized by the positive charge located on a neighboring multiple-electron atom such as phosphorus, sulfur, arsenic, etc. (onium centers). Typical ylides are represented in the structures 37 and 38 ... 

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