Negative hyperconjugation in phosphorus stabilized carbanions.

The electronic Fukui function is used to give qualitative electronic proof on the existence of back-bonding from the carbon lone pair toward the sigma* P-Y and P-O orbitals in phosphorus stabilized carbanions. NBO analyses are used to investigate the energetic, electronic, and structural impacts of this negative hyperconjugation interaction. The observed energetic stabilization can indeed be attributed to the electronic delocalization of the lone pair toward the antibonding orbitals. This delocalization is furthermore responsible for the shorter P-C bonds, longer P-Y (P-O) bonds, and wider Y-P-Y angles observed for the anionic compounds compared to their neutral counterparts. From the electronic NBO analysis it becomes clear that phosphorus containing functional groups are best described as sigma donor/pi acceptors.