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Model of chemical bonding in which three atoms share four electrons
The 3 center 4 electron (3c–4e) bond is a model used to explain bonding in certain hypervalent molecules such as tetratomic and hexatomic interhalogen compounds, sulfur tetrafluoride, the xenon fluorides, and the bifluoride ion. It is also known as the Pimentel–Rundle three center model after the work published by George C. Pimentel in 1951, which built on concepts developed earlier by Robert E. Rundle for electron deficient bonding. An extended version of this model is used to describe the whole class of hypervalent molecules such as phosphorus pentafluoride and sulfur hexafluoride as well as multi center π bonding such as ozone and sulfur trioxide. There are also molecules such as diborane (B2H6) and dialane (Al2H6) which have three center two electron bond (3c 2e) bonds.

The σ molecular orbitals (MOs) of triiodide can be constructed by considering the in phase and out of phase combinations of the central atom's p orbital (collinear with the bond axis) with the p orbitals of the peripheral atoms. This exercise generates the diagram at right (Figure 1). 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. Using this model, one sidesteps the need to invoke hypervalent bonding considerations at the central atom, since the bonding orbital effectively consists of two 2 center 1 electron bonds (which together do not violate the octet rule), and the other two electrons occupy the non bonding orbital.

In the natural bond orbital viewpoint of 3c–4e bonding, the triiodide anion is constructed from the combination of the diiodine (I2) σ molecular orbitals and an iodide (I−) lone pair. The I− lone pair acts as a 2 electron donor, while the I2 σ* antibonding orbital acts as a 2 electron acceptor. that is, a situation in which the central atom is more electronegative than the peripheral atoms. Molecules of theoretical curiosity such as neon difluoride (NeF2) and beryllium dilithide (BeLi2) represent examples of inverted electronegativity. As a result of unusual bonding situation, the donor lone pair ends up with significant electron density on the central atom, while the acceptor is the "out of phase" combination of the p orbitals on the peripheral atoms. This bonding scheme is depicted in Figure 3 for the theoretical noble gas dihalide NeF2.

The valence bond description and accompanying resonance structures A—B···C− ↔ A−···B—C suggest that molecules exhibiting 3c–4e bonding can serve as models for studying the transition states of bimolecular nucleophilic substitution reactions.