A 3-center-2-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. When visualised in 3D, it looks like a banana, and hence a 3-center-2-electron bond is often called a banana bond.

This type of bond is seen in many boron compounds, for example diborane, B₂H₆. The monomer BH₃ is unstable since the boron atom is only surrounded by six valence electrons. To complete its octet, a boron atom 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. This bonding pattern is also seen in trimethylaluminium (Al(CH₃)₃), where the carbon atom of a methyl group is bridging.

Chemical bonding

رابطة 3-مركز-2-إلكترون