Why Pentaphosphole, P5H, Is Planar in Contrast to Phosphole, (CH)4PH

In contrast to phosphole, (CH) 4 PH, pentaphosphole, P 5 H, has a planar C 2 v minimum. At RMP2(fc)/6-31G* + ZPE(RMP2(fc)/6-31G*//RMP2(fc)/6-31G*) the aromatic stabilization energy (ASE) of P 5 H (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 N 5 H (29 kcal/mol). However, pentaphosphole is thermodynamically stable by 58.2 kcal/mol (54.7 kcal/mol, MP4SDTQ(fc)/6-31G*//MP2(fc)/6-31G* + ZPE(RHF/6-31G*)) toward dissociation into PPPH and P 2 . Other P 5 H 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 -PH 2 substituent reduces the inversion barrier of tricoordinate phosphorus, Δ E inv (P), from 35 kcal/mol for PH 3 to 20.3 kcal/mol for HP(PH 2 ) 2 and 16.1 for P(PH 2 ) 3 . In addition, π conjugation of −P PH substituents reduce Δ 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 CH 2 ) 2 and P(CH CH 2 ) 3 reduce the inversion barriers only to 31.4 and 27.8 kcal/mol, respectively. Consequently, in P 5 H 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).