Eluvathingal D. Jemmis, Indian Institute of Science, Bangalore, India, and a colleague from India found that P₃F₉²⁻ or  (PF₃)₃²⁻ is an all-pseudo π* 2π-aromatic system. Its aromaticity comes solely from the interaction of pseudo-π* MOs. The dianion forms the first example of a three membered ring with all the vertices constituted by pentacoordinate phosphorus.

Since 1950 the analogy between CO and PF₃ has been known. The similarity is due to the fact that CO has a low-lying π* MO that is localized more on C, and PF₃ has a low-lying pseudo-π* MO that is localized more on P. In 1997 the deltate anion, (C₃O₃)²⁻, was discovered, however, its PF₃ analogue remained unnoticed until now.

The researchers assume that (PF₃)₃²⁻ can be synthesized via a procedure similar to the synthesis of C₃O₃²⁻ from CO via dissolving metal reduction. The heavier analogues, (AsF₃)₃²⁻ and (SbF₃)₃²⁻, are more weakly bound and may be harder to realize, they say.

• P₃F₉²⁻: An All-Pseudo-π* 2π-Aromatic,
  Chakkingal P. Priyakumari, Eluvathingal D. Jemmis,
  J. Am. Chem. Soc. 2013.
  DOI: 10.1021/ja408308g