Application of the Valence Bond Mixing Configuration Diagrams to Hypervalency in Trihalide Anions: A Challenge to the Rundle-Pimentel Model

The X-3(-) hypercoordinated anions (H, F, Cl, Br, 1) are studied by means of the breathing-orbital valence bond ab initio method. The valence bond wave functions describe the different X-3(-) complexes in terms of only six valence bond structures and yield energies relative to the two exit channels, X-2 + X- and X-2(-) + X-center dot, in very good agreement with reference CCSD(T) calculations. Although H-3(-) is unstable and dissociates to H-2 + H-, all the trihalogen anions are stable intermediates, Br-3(-) and I-3(-) being more stable than F-3(-) and Cl-3(-). As a challenge to the traditional Rundle-Pimentel model, the different energies of the hypercoordinated species relative to the normal-valent dissociation products X-2 + X- are interpreted in terms of valence bond configuration mixing diagrams and found to correlate with a single parameter of the X-2 molecule, its singlet-triplet energy gap. Examination of the six-structure wave functions show that H-3(-), Cl-3(-), Br-3(-), and I-3(-) share the same bonding picture and can be mainly described in terms of the interplay of two Lewis structures. On the other hand, F-3(-) is bonded in a different way and possesses a significant three-electron bonding character that is responsible for the dissociation of this complex to F-2(-) + F-center dot instead of the more stable products F-2 + F-. This counterintuitive preference for the thermodynamically disfavored exit channel is found to be an experimental manifestation of the large charge-shift resonance energy that generally characterizes fluorine-containing bonds.