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

Abstract

The X(3)(-) hypercoordinated anions (H, F, Cl, Br, I) 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(*), 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(*), 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.

Polyfluorides and Neat Fluorine as Host Material in Matrix‐Isolation Experiments

“…Polyfluoride anions are of synthetic and theoretical interest, and their chemical bonding, stability, and properties have repeatedlyb een studied by quantum-chemical methods. [76,[79][80][81][82][83][84][85][86][87][88] At the coupled-cluster level (CCSD(T)/def2-ZVPP)t he reaction of free F À and F 2 to yield the free [F 3 ] À and [F 5 ] À ions is exothermicb yÀ116.6 and À131.9 kJ mol À1 ,r espectively. [34] By contrast, due to the relatively strong coulombic bonding interaction in M + [F] À ,t he formationo ft he M + [F 3 ] À and M + [F 5 ] À ion pairs is by far less favored.…”

A Valence Bond Model for Electron‐Rich Hypervalent Species: Application to SFn (n=1, 2, 4), PF5, and ClF3

“…It is concluded that the bonding mode of these hypervalent species and isoelectronic ones complies with Coulson's version of the Rundle-Pimentel model, but assisted by charge-shift bonding. The conditions for hypervalence to occur are stated.H À exchange reaction, lying 11 kcal mol À1 above the reactants, [8] although the isoelectronic F 3 À is stable. To quote Kutzelnigg, [9] "Whereas simple MO theory has no difficulty in describ-Scheme 1.…”

“…H À exchange reaction, lying 11 kcal mol À1 above the reactants, [8] although the isoelectronic F 3 À is stable. To quote Kutzelnigg, [9] "Whereas simple MO theory has no difficulty in describ-Scheme 1.…”

Alkali-Metal Trihalides: M+X3– Ion Pair or MX–X2 Complex?

“…5 However, our distance agrees well with the result (2.945 Å) at the CCSD(T)/aug-cc-pVTZ-PP level (all orbitals are correlated) by Braïda and Hiberty. 23 The difference in bond lengths calculated with ostensibly the same methodology, not to speak of what would be obtained with different levels of calculation, serves in a way to set the theoretical equivalent of an error bar on a calculation. Figure 3) optimized using the CCSD(T)/AWCVTZ method.…”