Tricoordinate phosphorus can be planarized, and used as an excellent buildi
ng block of aromatic compounds. The pyramidality can be reduced by (i) sigm
a donor and pi acceptor groups; (ii) steric strain, which increases the bon
ding angle about phosphorus; (iii) bulky substituent groups; and (iv) deloc
alization or aromatic stabilization effects. The aromaticity of pi systems
with planar tricoordinate phosphorus is largest among those rings containin
g nitrogen or chalcogen heteroatoms. For the flat systems the aromatic stab
ilization is just compensated by the energy cost of the planarization at ph
osphorus. Due to this counterbalancing effect a very rich chemistry can be
expected for these compounds. (C) 1999 Elsevier Science Ltd. All rights res
erved.