9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Triple bonds and sp hybridisation

In a relatively small number of compounds, carbon is connected to another atom by a triple bond involving three electron pairs. This type of bond can be built from sp hybrid orbitals involving one s and one p orbital on each carbon atom, as shown at the bottom of the page. Two sp hybrid atomic orbitals extend out from an atom 180 apart, and the two remaining unhybridized p orbitals are at right angles to these and to one another. In acetylene, H-C=C-H, each of the two carbon atoms uses one sp hybrid orbital in a C-H bond and the other in the bond between carbons. Three electron pairs are employed in holding this a-bonded framework together.
The remaining two thirds of the triple bond involves the p orbitals. If the C-C bond direction is chosen as the z axis, then the two p. orbitals on carbon combine into one p MO, and the two p, orbitals combine into another. This means that the carbon atoms are held together by three electron pairs, one in a s bond and two in p bonds. The and MO's taken together from a symmetrical barrel of electron density around the carbon-carbon bond.

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