Aromatic Compounds with Planar Tricoordinate Phosphorus

Tricoordinate phosphorus can be planarized, and used as an excellent building block of aromatic compounds. The pyramidality can be reduced by (i) σ donor and π acceptor groups; (ii) steric strain, which increases the bonding angle about phosphorus; (iii) bulky substituent groups; and (iv) delocalization or aromatic stabilization effects. The aromaticity of π systems with planar tricoordinate phosphorus is largest among those rings containing nitrogen or chalcogen heteroatoms. For the flat systems the aromatic stabilization is just compensated by the energy cost of the planarization at phosphorus. Due to this counterbalancing effect a very rich chemistry can be expected for these compounds.