STUDY OF AROMATICITY IN PHOSPHORUS ANALOGS OF THE CYCLOPENTADIENYL ANION

Phosphorus analogues of the cyclopentadienyl anion are examined by complete structural optimization using the all-valence electron SCF MO method SINDO1. Optimized geometries obtained with inclusion of d-orbitals on phosphorus show that the ring bond lengths CC, CP, and PP are in the ranges 1.37-1.42 angstrom 1.74-1.77 angstrom and 2.17-2.20 angstrom, respectively. These values are between those of the bond lengths of the respective single and double bonds and are in close agreement with the observed values of the ligated rings in complexes. The bond-order definition of aromaticity, based on the ring-current concept, reveals that the phosphorus analogues of cyclopentadienyl anion retain greater-than-or-equal-to 86.0% aromaticity relative to the carbocyclic system C5H5-. A rather dramatic result is that the pentaphosphorus analogue P5- is as aromatic as the carbocyclic cyclopentadienyl anion.