What Makes the Trifluoride Anion F3- So Special? A Breathing-Orbital Valence Bond ab Initio Study

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Abstract
The ground states of the F3- and H3- hypercoordinated anions are investigated and analyzed in terms of valence bond structures by means of the breathing-orbital valence bond method. While H3- is described reasonably well as the interplay of two major Lewis structures, H2 + H- and its mirror image, the description of F3- requires a further structure, of the type F•F-F•, which strongly stabilizes the trimer relative to the dissociation products, and endows the F3- ground state with a predominant three-electron bond character. It follows that the simple picture that is closest to the true nature of F3- is a resonating combination of F2- + F• and its mirror image. This peculiarity of the F3- electronic structure is at the origin of its preferred dissociation channel leading to F2- + F• rather than to the most stable product F2 + F-, at high collision energies. The three-electron bond character of F3- is also the root cause for the failure of the Hartree−Fock and density functional methods for this species, and for its strong tendency to artifactual symmetry-breaking. As an alternative to the Rundle−Pimentel model, the origins of the stability of F3-, as opposed to the instability of H3-, CH5-, and other SN2 transition states, are analyzed in the framework of valence bond state correlation diagrams [Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586]. It is found that a fundamental factor of stability for X3- is the presence of lone pairs on the X fragment. The explanation carries over to other trihalide anions, and to isoelectronic 22-valence electron hypercoordinated anions.