Athree-center two-electron (3c–2e) bond is anelectron-deficientchemical bond where threeatoms share twoelectrons. The combination of threeatomic orbitals form threemolecular orbitals: one bonding, onenon-bonding, and oneanti-bonding. The two electrons go into the bonding orbital, resulting in a net bonding effect and constituting a chemical bond among all three atoms. In many common bonds of this type, the bonding orbital is shifted towards two of the three atoms instead of being spread equally among all three. Example molecules with 3c–2e bonds are thetrihydrogen cation (H+
3) anddiborane (B
2H
6). In these two structures, the three atoms in each 3c–2e bond form an angular geometry, leading to abent bond.
An extended version of the 3c–2e bond model features heavily incluster compounds described by the polyhedral skeletal electron pair theory, such asboranes andcarboranes. These molecules derive their stability from having a completely filled set of bonding molecular orbitals as outlined byWade's rules.
The monomerBH3 is unstable since the boron atom has an empty p-orbital. AB−H−B 3-center-2-electron bond is formed when a boron atom shares electrons with a B−H bond on another boron atom. The two electrons (corresponding to one bond) in aB−H−B bonding molecular orbital are spread out across three internuclear spaces.[1]
Indiborane (B2H6), there are two such 3c–2e bonds: two H atoms bridge the two B atoms, leaving two additional H atoms in ordinary B−H bonds on each B. As a result, the molecule achieves stability since each B participates in a total of four bonds and all bonding molecular orbitals are filled, although two of the four bonds are 3-center B−H−B bonds. The reportedbond order for each B−H interaction in a bridge is 0.5,[2] so that the bridging B−H−B bonds are weaker and longer than the terminal B−H bonds, as shown by thebond lengths in the structural diagram.
2(HSiPh3).png)
Three-center, two-electron bonding is pervasive in organotransition metal chemistry. A celebrated family of compounds featuring such interactions is calledagostic complexes.
This bonding pattern is also seen intrimethylaluminium, which forms a dimer Al2(CH3)6 with the carbon atoms of two of themethyl groups in bridging positions. This type of bond also occurs incarbon compounds, where it is sometimes referred to ashyperconjugation- another name for asymmetrical three-center two-electron bonds.
The first stable subvalent Be complex ever observed contains a three-center two-electron π-bond that consists of donor-acceptor interactions over the C-Be-C core of a Be(0)-carbene adduct.[4]
Carbocationrearrangement reactions occur through three-center bond transition states. Because the three center bond structures have about the same energy as carbocations, there is generally virtually no activation energy for these rearrangements so they occur with extraordinarily high rates.
Carbonium ions such asethaniumC
2H+
7 have three-center two-electron bonds. Perhaps the best known and studied structure of this sort is the2-Norbornyl cation.
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