P3F92-: An All-Pseudo-pi* 2 pi-Aromatic

A qualitative MO analysis suggests (PH3)(3)(2-) as a candidate for an all-pseudo-pi* 2 pi-aromatic; however computational studies rule out its existence. Fluorine substitution which increases the contribution of p orbitals on P in the pseudo-pi* MO makes (PF3)(3)(2-) a minimum and an aromatic. The 2 pi aromaticity arising from the bonding combination of the three pseudo-pi* fragment MOs is comparable to that in C3O32- and is another example for the analogy between CO and PF3. The dianion (PF3)(3)(2-) forms the first example of a three-membered ring with all the vertices constituted by pentacoordinate phosphorus. The ability of PF3 to form the all-pseudo-pi* 2 pi-aromatic system is not shared by the heavier analogues, AsF3 and SbF3.