United they are strong

J. Phys. Chem. Lett. 4, 3139–3143 (2013)

The most interesting chemical phenomena arise when common rules of thumb break down. One such rule is the relationship between bond length and bond strength. From infrared spectroscopy to organic synthesis, chemists usually assume an inverse correlation between the length of a bond and its dissociation energy and force constant. But as with every rule, there are exceptions: in various molecules with electronegative ligands such as fluorine, a longer bond can be a stronger bond.

The product of SF₂ dimerization, FSSF₃, is a particularly insightful representative of this class of compound, and has now been analysed theoretically by Beth Lindquist and Thom Dunning from the University of Illinois. FSSF₃ possesses four inequivalent fluorine atoms with four different sulfur–fluorine bond lengths (pictured). Removal of each fluorine atom yields a distinct structure that is a local minimum on the potential energy surface, facilitating the analysis of the bond length–bond dissociation energy relationship enormously. Surprisingly, the two longest S–F bonds, which are between the axial fluorine (F_cis and F_trans) and the hypervalent sulfur atoms, have the largest dissociation energies.

Credit: © 2013 ACS

Dunning and Lindquist explain this puzzle using generalized valence bond theory, with which they find substantial repulsive overlap between the two axial S–F bond orbitals (SF_cis and SF_trans). However, when one of these bonds is broken, the associated sulfur bonding orbital localizes on the hypervalent sulfur atom, which increases the repulsive overlap in the product. In other words, the axial S–F bonds are stabilized because two sulfur bonding orbitals get pulled away from each other, but this can only occur when both fluorine atoms are present. The authors rationalize this finding with the concept of a 'recoupled bond dyad'. This bonding model is closely related to the three-centre, four-electron bond, with the major difference being that the bonds in a recoupled bond dyad are highly ionic and localized. A number of other hypervalent compounds with electronegative ligands abide by the same united-they-are-strong rule, and we can expect more surprises from analysing their bonding situations in the future.