Effect of the phosphonate group on the reactivity of carbenes. Neighbouring phosphonate group participation

The electrophilicity of phosphonylcarbenes is dramatically reduced as one changes the neighbouring group from the phosphonate ester to the acid anion, and phosphonate carbenes are experimentally classified as typical nucleophiles; the results are nicely explained in terms of neighbouring phosphonate group participation in which the vacant p orbital strongly interacts with the phosphonate oxygen anion resulting in suppression of the availability of the vacant p orbital.