A qualitative MO analysis suggests (PH3)3(2-) as a candidate for an all-pseudo-π* 2π-aromatic; however computational studies rule out its existence. Fluorine substitution which increases the contribution of p orbitals on P in the pseudo-π* MO makes (PF3)3(2-) a minimum and an aromatic. The 2π aromaticity arising from the bonding combination of the three pseudo-π* fragment MOs is comparable to that in C3O3(2-) 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-π* 2π-aromatic system is not shared by the heavier analogues, AsF3 and SbF3.

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We studied the effect of electronegativity perturbation on the isolobal behavior of tetra-coordinate hypervalent compounds of S (sulfuranes, SL 4 , L is any atom or group which can provide one electron for S-L bonding). Though formally the fragment SL 4 obtained from SL 6 is an isolobal equivalent of CH 2 , a qualitative molecular orbital study shows that only SF 2 H 2 with equatorial F atoms is a practical isolobal substitute for CH 2 and can form oligomers, (SF 2 H 2) 2 , (14), (SF 2 H 2) 3 , (15) and (SF 2 H 2) 4 , (16) analogous to ethylene, cyclopropane and cyclobutane, respectively. DFT computations at the B2PLYP/6-311++g(d,p), MP2/aug-cc-pVTZ and B3LYP/6-311++g(d,p) levels confirm these structures to be minima on the PES. The skeletal S-S bonds in these structures are formed solely by the bonding combination of anti-bonding fragment orbitals of SF 2 H 2. In contrast, per-fluorination, the usual way to stabilize hypervalent structures, is found to have an opposite effect here. Calculations at the same levels show (SF 4) 2 , (SF 4) 3 , and (SF 4) 4 not to be minima. The highly stable HOMO of SF 4 fragment and large HOMO-LUMO gap makes SF 4 a stable entity, preventing it from oligomerization. Out of the various isomers of SF n H 4−n , n = 0-4, only SF 2 H 2 with equatorial F atoms can form oligomeric sulfuranes. Substitution of F by heavier analogs of the group did not lead to any stable oligomers.