Three-center two-electron bond

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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. The simplest example of a 3c-2e bond is in the trihydrogen cation, H+
3.

Boron compounds

Resonance structures of 3c-2e bond in diborane.

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 are spread out across three internuclear spaces.[1] The reported bond order for each B−H interaction is 0.5,[2] 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.

Diborane. The two central hydrogen atoms are simultaneously bonded to both boron atoms in 3c-2e bonds.

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.

Carbocations

Carbocation rearrangement 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 as ethanium C
2H+
7 have three-center two-electron bonds. Perhaps the best known and studied structure of this sort is the norbornyl cation.

See also

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ F. Albert Cotton, Geoffrey Wilkinson and Paul L. Gaus, Basic Inorganic Chemistry, 2nd ed. (Wiley 1987), p.113
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