Why edge inversion? Theoretical characterization of the bonding in the transition states for inversion in F n NH(3−n) and FnPH(3−n) (n = 0–3)

Abstract

As first noted by Dixon et al. (J Am Chem Soc 108:2461-2462, heavily fluorinated pyramidal phosphorus compounds, e.g., F n PH (3-n) with n [ 1, invert through a T-shaped transition state (edge inversion) rather than the D 3h -like transition states (vertex inversion) found in the corresponding nitrogen compounds and less fluorinated phosphorus compounds. Subsequent studies by Dixon and coworkers established that this is a general phenomenon and has important chemical consequences. But what is the reason for the change in the structure of the transition state? Recent theoretical investigations have resulted in the discovery of a new type of chemical bond, the recoupled pair bond. In particular, it was found that recoupled pair bond dyads account for the hypervalency of the elements beyond the first row. In this paper, we show that recoupled pair bond dyads also account for the existence of the edge inversion pathway in heavily fluorinated phosphorus compounds and likely account for the presence of the lower energy inversion pathways in pyramidal compounds of other elements beyond the first row.

Is the Inversion of Phosphorus Trihalides (PF3, PCl3, PBr3, and PI3) a Diradical Process?

“…Next consider the T-shaped C 2 v structure. Our results confirm the previous findings − that the transition structures for inversion are not necessarily of D 3 h symmetry but can be of C 2 v symmetry. In general, the C 2 v structures, like the singlet D 3 h ones, are planar, but one of the Cl–P–Cl bond angles (α 2 ) is around 180 deg and the other two (α 1 ) are around 90 deg.…”

“…Calculations by Creve and Nguyen showed that PH 3 and its methyl derivatives invert by the umbrella mode, but substitution of two or more hydrogens by F, Cl, or Br leads to edge inversion, again explained by a pseudo-Jahn–Teller effect. Dunning and co-workers , also investigated the reason for edge inversion in PF 3 and PHF 2 , and they explained it in terms of two singlet-coupled bonding pairs.…”

Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds

“…In all of these molecules there are polar covalent bonds, recoupled pair bonds, and recoupled pair bond dyads with the most stable structures and the bond energies being determined by the interplay of these three types of bonding. Dunning and co-workers have also shown that the formation of a recoupled pair π bond dyad is responsible for: (1) the HSO isomer being of lower energy than the SOH isomer, despite the fact that an OH bond is inherently stronger than an SH bond by tens of kcal/mol; (2) the greater strength and shorter length of the bonds in SO 2 as compared to the corresponding bonds in O 3 , and (3) the NSF isomer being of lower energy than the FNS isomer despite the fact that the reverse is true in the analogous oxygen compounds. , A recoupled pair bond dyad is also responsible for the unusual T-shaped transition states for inversion found for F 2 PH and PF 3 as compared to the D 3 h -like transition states in FPH 2 and PH 3 . , From these studies, it is clear that recoupled pair bonds and recoupled pair bond dyads involving n p lone pairs ( n > 2) are not a rarity in chemistry: they are ubiquitous in compounds of the second and third row elements and probably beyond.…”

“…159,160 A recoupled pair bond dyad is also responsible for the unusual T-shaped transition states for inversion found for F2PH and PF3 as compared to the D3h-like transition states in FPH2 and PH3. 161,162 From these studies, it is clear that recoupled pair bonds and recoupled pair bond dyads involving np lone pairs (n > 2) are not a rarity in chemistry: they are ubiquitous in compounds of the second and third row elements and probably beyond.…”

The inversion of tetrahedral p-block element compounds: general trends and the relation to the second-order Jahn–Teller effect

“…1D), which occupied generations of theoreticians and experimentalists. [41][42][43] Different approaches to explain the inversion of ER 3 , like perturbational molecular orbital theory, 1,[44][45][46] the second-order Jahn-Teller effect (SOJTE), [47][48][49][50] or recoupled pair bonding, 51 afforded a profound base and thereby challenged our understanding of the chemical bonding. 52,53 Likely, this diverging interest stems from the absent or potentially overlooked experimental observation of structural dynamics of the tetrahedron, which are classically considered congurationally stable.…”