Boron Compounds
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, and thus, to form a stable octet, shares electrons with a B−H bond on another boron atom, forming a B−H−B 3-center-2-electron bond. 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.
The two electrons (corresponding to one bond) in a B−H−B bonding molecular orbital spend half their time between one boron and the bridging hydrogen, and half their time between the other boron and the hydrogen. The bond order for each B−H interaction is therefore 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.
Read more about this topic: Three-center Two-electron Bond
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