Experimental and theoretical examinations of n→σ* negative hyperconjugation in pyridinium dichlorophosphinomethylide

An X-ray crystal structure investigation as well as theoretical DFT (hybrid B3LYP/6-31G** level) calculations of 2-ethylpyridinium dichlorophosphino-ethoxycarbonylmethylide confirm the existence of significant n→σ* negative (anionic) hyperconjugation between the ylidic charge and the σ * P – Cl bond in the most stable conformer, which has the phosphorus lone pair oriented orthogonally to the ylidic pz orbital, resulting in elongation of the P–Cl bond parallel to the pz orbital but shortening of the C–P bond.