Theory of Hypervalency: Recoupled Pair Bonding in SF_n (n = 1−6)

Abstract

To gain new insight into the nature of hypervalency, we have characterized the bonding across the entire SF(n) sequence (n = 1-6) with high-level quantum chemical theory (multireference configuration interaction and coupled cluster calculations using correlation consistent basis sets). In contrast to most previous studies, this work examined both the stable equilibrium structures and the process of SF(n)-F bond formation. We conclude that two different types of bonding can occur in these species: normal polar covalent bonding and a new type that we call recoupled pair bonding. The two bonding processes can be seen in diatomic SF, where hypervalent behavior first occurs. In the covalently bonded (2)Pi ground state, the bond is formed by straightforward singlet coupling of electrons in the singly occupied S 3p and F 2p orbitals. But there is also a low-lying (4)Sigma(-) excited state where the S 3p(2) pair of electrons must first be decoupled so that one of the electrons can singlet couple with the electron in the F 2p orbital, hence the term recoupled pair bonding. Energy is required to decouple the electron pair, but the bond energy of SF((4)Sigma(-)) is still a substantial fraction (about 40%) of the bond energy of SF((2)Pi). Recoupled pair bonding is the basis for hypervalent behavior: for example, the three unpaired electrons of SF((4)Sigma(-)) are available for further bond formation, and their spatial orientations clearly anticipate the structure of SF(4). The new model of hypervalent bonding introduced in this work accounts for the observed trends in the structures of SF(n) molecules and the variations in the (SF(n)-F) bond energies. The model also predicts the existence of low-lying excited states in some SF(n) species and provides explanations for their energetic separations and orderings.

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)

“…This is due to the dominant 3p character in the 3p lobe orbitals and the repulsive forces between the two PF bonds, the PH bond and the doubly occupied distorted 3s-like orbital. The quasi-linear structure is consistent with our finding that p-recoupled pair bond dyads are structurally rigid and prefer nearly collinear arrangements [9,12,13,31,40].…”

“…Woon and Dunning, on the other hand, noted that the two axial bonds in the transition state for inversion in PF 3 closely resemble those in the PF 2 (A 2 P) state, which has a recoupled pair bond dyad, and concluded that the ability of the P atom to form a very stable recoupled pair bond dyad is the source of this anomaly [9]. In fact, the formation of recoupled pair bonds is the basis for the bonding in hypervalent molecules, as shown in studies on SF n [12], PF n [9], ClF n [13] and a number of related compounds. The T-shaped transition state for inversion of PF 3 should, in fact, be considered hypervalent, even though it is tricoordinated, because it possesses one of the hallmarks of hypervalent compounds-a recoupled pair bond dyad.…”

“…This is the reason for the unusual increase in the length of the PF axial bonds between the ground and transition state: p-recoupled pair bond dyads have bond distances that are significantly longer than the corresponding covalent bonds [9,12,13,31,40]. In contrast, the P orbital participating in the equatorial PH bond is the singly occupied 3p orbital (See 2b in Fig.…”

Fundamental aspects of recoupled pair bonds. I. Recoupled pair bonds in carbon and sulfur monofluoride

“…42,43 However, the SF(a 4 Σ − ) state is bound. 7,44 In other words, a recoupled pair bond involving the 3p lone pair of sulfur is conditional; it will not be formed unless the ligand has the right attributes (see below).…”

“…5 Previous studies of the halides of the second row, late p-block elements, by our group have shown that hypervalent or hypercoordinated molecules possess a previously unrecognized type of bond, a recoupled pair bond. [6][7][8][9][10][11][12][13][14] Recoupled pair bonds and recoupled pair bond dyads 14 the stability of SF 4 and SF 6 , 7 as well as other hypervalent species such as PF 5 , 9 ClF 3 , and ClF 5 , 8 providing a simple, straightforward explanation of the low-lying states, structures, and energetics of these species as well as those of the intermediate SF n , PF n , and ClF n radicals. In fact, it has been found that the ability of elements beyond the first row to form recoupled pair bonds accounts for a number of the differences between the structures, energetics, and spectra of compounds of the first and second row elements, including differences in reactivity 13 -the so-called first-row anomaly.…”