In contrast to phosphole, (CH)(4)PH, pentaphosphole, P5H, has a planar C-2 upsilon minimum, At RMP2(fc)/6-31G* + ZPE(RMP2(fc)/6-31G*//RMP2(fc)/6-31G*) the aromatic stabilization energy (ASE) of P5H (17 kcal/mol) is larger than of (CH)(4)PH (ASE = 7 kcal/mol) but smaller than of (CH)(4)NH (25.5 kcal/mol) and N5H (29 kcal/mol), However, pentaphosphole is thermodynamically stable by 58.2 kcal/mol (54.7 kcal/mol, MP4SDTQ(fc)/6-3 1G*//MP2(fc)/6-31G* + ZPE(RHF/6-31G*)) toward dissociation into PPPH and P-2. Other P5H isomers have relative energies of 9 (tricyclic, C-s), 17.4 (bicyclic, C-s), 25.6 (monocyclic, P-3-P=PH, C-s), and 53.5 (open chain, PPPPPH, C-1) kcal/mol. Due to its electropositive character, the -PH2 substituent reduces the inversion barrier of tricoordinate phosphorus, Delta E(inv)(P), from 35 kcal/mol for PH3 to 20.3 kcal/mol for HP(PH2)(2) and 16.1 for P(PH2)(3). In addition, pi conjugation of -P=PH substituents reduce Delta E(inv)(P) further to 13.5 in HP(P=PH)(2) and 6.1 kcal/mol in P(P=PH)(3). In contrast, the vinyl groups in HP(CH=CH2)(2) and P(CH=CH2)(3) reduce the inversion barriers only to 31.4 and 27.8 kcal/mol, respectively. Consequently, in P5H and its derivatives endocyclic electronegativity effects together with aromaticity of the P-5 ring eliminate the inversion barriers, Other bonding types favoring planar tricoordinate phosphorus are described (including a bicyclic P-8 isomer with two P-5 rings only 18.4 kcal/mol higher in energy than two P-4 molecules).