A double bond with weak σ- and strong π-interactions is still a double bond

Abstract

Synthesis of 1,3 singlet diradicals in cyclic organic molecules has been the main route toward the formation of single π bonds, i.e., π-bonded atoms without σ-bonding1–4 by forming a long-range π-bond between unpaired electrons of the radicals that occupy p-atomic orbitals. In a recent study, Kyushin et al. reported synthesis and characterization of 1,2,2,3,4,4-hexatert-butylbicyclo[1.1.0]tetrasilane (2), a four-membered singlet diradical silicon ring with a 1,3 silicon-silicon single π-bond and identified a single π-bond by analyses based on canonical molecular orbitals (CMOs) and natural bond orbitals5 (NBOs). Although MO-based analyses confirm that the two sp2 hybridized silicon atoms are connected via a π-MO, the HOMO of the molecule4, a deep analysis on the basis of topological approaches indicates a different story. Here, I study this system in more details and show that this bond is not a single π-bond, but rather a unique double bond in which the role of σand π bonds is reversed, i.e., a weak σ-bond and a strong π-bond. The first evidence in favor of σ-bonding can be found in CMOs, where HOMO −6 of the molecule represents a σinteraction between two sp2 hybridized silicon atoms, Fig. 1. This orbital with HOMO −1, HOMO −2, and HOMO −3 constitute the sigma framework of the silicon ring. Kyushin et al. in their contribution suggest that HOMO −1 is an antibonding orbital that quenches the bonding effect of HOMO −6. However, MObased analyses do not provide a quantitative picture of bonding6. MOs change shape by choosing different isosurface values or by employing various types of orbitals. Besides, in polyatomic molecules, the role of bonding and antibonding MOs is not comparable with simple diatomics, where a qualitative analysis can explain the bonding characters of a system. To obtain a quantitative picture of the bonding in 2, the molecule was analyzed within the context of Bader’s theory7. Topological analysis of the electron density identifies no bond critical point (BCP) between the two sp2 hybridized silicon atoms. Nevertheless, the presence or absence of BCPs is not a reliable measure of bonding; therefore, other parameters were studied8. Ideally a pure π-bond should have 2 BCPs above and below the nodal plane of the π-MO; however, this feature is visible only if the σ-electron density is removed9,10. In ordinary molecules with double bonds, the core and σ-electrons mask the fingerprint of π-electron density. To visually search for a potential single π-bond, the derivatives of the electron density, namely the Laplacian of the electron density, ∇2ρðrÞ, and energy density were probed. The plot of ∇2ρðrÞ shows regions of electron density concentration on Si atoms corresponding to their atomic p orbitals, Fig. 2a. This feature becomes more pronounced after removing electrons from the σ-skeleton but the region in between two Si atoms has a positive Laplacian that is a feature of noncovalent interactions or that of charge-shift bonds7,11. The Laplacian in the central region of the ring remains positive after removing electrons from the σ-framework of the Si4 ring, Fig. 1b, c. Although no BCP between two Si atoms is found, some of the properties of the ring critical point (RCP) in the middle of the Si4 ring are Fig. 1 Occupied molecular orbitals. Occupied molecular orbitals of 2; HOMO −6 corresponds to a σ-bond across the ring. https://doi.org/10.1038/s41467-021-24238-x OPEN