Electron-deficient compound
A three-center two-electron bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-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. Examples of 3c-2e bonds are the trihydrogen cation H+
3, and the dimeric structure of aluminium chloride.
Three-center two-electron bonds are seen in many boron compounds, such as diborane (B2H6). The monomer BH3 is unstable since the boron atom is only surrounded by six valence electrons. A B−H−B 3-center-2-electron bond is formed when a boron atom shares electrons with a B−H bond on another boron atom. In diborane, there are two such 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, each boron is surrounded by a stable octet.
The two electrons (corresponding to one bond) in a B−H−B bonding molecular orbital are spread out across three internuclear spaces. The reported bond order for each B−H interaction is 0.5, so that the bridging B−H bonds are weaker and longer than the terminal B−H bonds, as shown by the bond lengths in the structural diagram.
This bonding pattern is also seen in trimethylaluminium, which forms a dimer Al2(CH3)6 with the carbon atoms of two of the methyl groups in bridging positions. This type of bond also occurs in carbon compounds, where it is sometimes referred to as hyperconjugation; another name for asymmetrical three-center two-electron bonds.
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