Pentaphosphole: An aromatic ring with a planar sigma(3)-phosphorus

Correlated ab initio calculations show that pentaphosphole (P5H) is a planar five-membered heterocycle at all levels of theory investigated here. The HP out of plane bending potential, however, is rather shallow. All the PP bond lengths are nearly equal, indicating significant aromatic character. The Bird index is 77, a value that is among the largest for the five membered heterocycles. The average of the double bond characters is 59.6% that is smaller than the 67% for benzene. The stabilization as measured by an isodesmic reaction is comparable to the value for thiophene. In spite of the aromatic stabilization the inherent instability of the P=P-P=P linkage indicates that the ring is rather unstable. Several dimeric products (P10H2) were found, however, which were more stable than two P5H molecules by 28-35 kcal/mol. Since in case of P(5)R there is no possibility for kinetic stabilization by bulky substituents, it is questionable that the compound can be synthesized.