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

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journal contribution
posted on 08.08.1996, 00:00 by Mikhail N. Glukhovtsev, Alk Dransfeld, Paul von R. Schleyer
In contrast to phosphole, (CH)4PH, pentaphosphole, P5H, has a planar C2v 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)4PH (ASE = 7 kcal/mol) but smaller than of (CH)4NH (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)/6-31G*//MP2(fc)/6-31G* + ZPE(RHF/6-31G*)) toward dissociation into PPPH and P2. Other P5H isomers have relative energies of 9 (tricyclic, Cs), 17.4 (bicyclic, Cs), 25.6 (monocyclic, P3PPH, Cs), and 53.5 (open chain, PPPPPH, C1) kcal/mol. Due to its electropositive character, the -PH2 substituent reduces the inversion barrier of tricoordinate phosphorus, ΔEinv(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, π conjugation of −PPH substituents reduce ΔEinv(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 P5 ring eliminate the inversion barriers. Other bonding types favoring planar tricoordinate phosphorus are described (including a bicyclic P8 isomer with two P5 rings only 18.4 kcal/mol higher in energy than two P4 molecules).