Three-center four-electron bond

[1][2] It is also known as the Pimentel–Rundle three-center model after the work published by George C. Pimentel in 1951,[3] which built on concepts developed earlier by Robert E. Rundle for electron-deficient bonding.

[9] In a 1951 seminal paper,[3] Pimentel rationalized the bonding in hypervalent trihalide ions (X−3, X = F, Br, Cl, I) via a molecular orbital (MO) description, building on the concept of the "half-bond" introduced by Rundle in 1947.

More recent theoretical studies on hypervalent molecules support the Langmuir view, confirming that the octet rule serves as a good first approximation to describing bonding in the s- and p-block elements.

Three molecular orbitals result from the combination of the three relevant atomic orbitals, with the four electrons occupying the two MOs lowest in energy – a bonding MO delocalized across all three centers, and a non-bonding MO localized on the peripheral centers.

Molecules of theoretical curiosity such as neon difluoride (NeF2) and beryllium dilithide (BeLi2) represent examples of inverted electronegativity.

Sigma molecular orbitals of the triiodide anion, illustrating 3-center 4-electron bonding.
Figure 1: Diagram illustrating σ molecular orbitals of the triiodide anion.
A donor-acceptor interaction diagram illustrating formation of the triiodide anion sigma natural bond orbitals.
Figure 2: A donor-acceptor interaction diagram illustrating construction of the triiodide anion σ natural bond orbitals from I 2 and I − fragments.
Diagram depicting the natural bond orbital donor-acceptor interaction in neon difluoride. The central Ne atom acts as the donor, while the out-of-phase combination of the peripheral F atoms acts as the acceptor. The two orbitals have been overlaid on the same molecule framework.
Figure 3: Diagram depicting the natural bond orbital donor-acceptor interaction in neon difluoride. The central Ne atom acts as the donor, while the out-of-phase combination of the peripheral F atoms acts as the acceptor. The two orbitals have been overlaid on the same molecule framework.