We studied the effect of electronegativity perturbation on the isolobal behavior of tetra-coordinate hypervalent compounds of S (sulfuranes, SL₄, L is any atom or group which can provide one electron for S-L bonding). Though formally the fragment SL₄ obtained from SL6 is an isolobal equivalent of CH₂, a qualitative molecular orbital study shows that only SF₂H₂ with equatorial F atoms is a practical isolobal substitute for CH₂ and can form oligomers, (SF₂H₂)₂, (14), (SF₂H₂)₃, (15) and (SF₂H₂)₄, (16) analogous to ethylene, cyclopropane and cyclobutane, respectively. DFT computations at the B2PLYP/6-311++g(d,p), MP2/ aug-ccpVTZ 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 SF₂H₂. 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 (SF₄)₂, (SF₄)₃, and (SF₄)₄ not to be minima. The highly stable HOMO of SF₄ fragment and large HOMO-LUMO gap makes SF₄ a stable entity, preventing it from oligomerization. Out of the various isomers of SFnH₄−n, n = 0-4, only SF₂H₂ with equatorial F atoms can form oligomeric sulfuranes. Substitution of F by heavier analogs of the group did not lead to any stable oligomers.
Volume 134, 2022
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