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.