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

The ground states of the F(3)(-) and H(3)(-) hypercoordinated anions are investigated and analyzed in terms of valence bond structures by means of the breathing-orbital valence bond method. While H(3)(-) is described reasonably well as the interplay of two major Lewis structures, H(2) + H(-) and its mirror image, the description of F(3)(-) requires a further structure, of the type F(*)F(-)F(*), which strongly stabilizes the trimer relative to the dissociation products, and endows the F(3)(-) ground state with a predominant three-electron bond character. It follows that the simple picture that is closest to the true nature of F(3)(-) is a resonating combination of F(2)(-) + F(*) and its mirror image. This peculiarity of the F(3)(-) electronic structure is at the origin of its preferred dissociation channel leading to F(2)(-) + F(*) rather than to the most stable product F(2) + F(-), at high collision energies. The three-electron bond character of F(3)(-) 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 F(3)(-), as opposed to the instability of H(3)(-), CH(5)(-), and other S(N)2 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 X(3)(-) 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.