Phosphonium ylides bond angles

Dixon and Smart71 examined a series of fluorine-substituted phosphonium ylides. Fluorine stabilizes carbanions via an inductive effect, favouring pyramidal carbanions. Dixon and Smart argued that the first substitution of F on the ylidic carbon (H3P=CHF) causes the carbon to become very pyramidal (the sum of the angles at C is 338.5°), which reduces the potential overlap of the anionic orbital with any P orbitals. The leads to the long P—C distance of 1.723 A. The second F substitution (H3P=CF2) actually breaks the P—C bond and the system is best described as a weak interaction of phosphine with CF2. Trifluoromethyl groups act to stabilize the anions via hyperconjugation. This leads to a planar ylidic carbon in H3P=C(CF3)2. 

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