Tri-coordinated nitrogen and phosphorus in planar eight π electron systems: Intriguing conformational differences

The phosphorus analogues (phosphanyl ketones) of pyridones are shown by theory to exhibit qualitatively different structures. Energy minimum of meta-phosphanyl ketone is planar while the planar conformations of the ortho- and para-isomers are transition state. Analysis of the electron density using the electron localization function (ELF) rationalizes the structural differences. In these molecules, there are competitive effects on geometry due to the octet rule, electron delocalization, and pyramidality of the tri-coordinated nitrogen or phosphorus. A delicate balance determines the molecular conformations.