handbook of carbon, graphite, diamond and fullerenes

36 Carbon, Graphite, Diamond, and Fullerenes
In an sp 2 structure such as graphite, the delocalized electrons can
move readily from one side of the plane layer to the other but cannot easily
move from one layer to another. As a result, graphite is anisotropic. The
sp 2 -hybridized structure of graphite will be reviewed in more detail in Ch. 3,
Sec. 1.2.
4.3 The Digonal-sp Orbital and the sp Bond
The sp orbital (known as a digonal orbital) is a merger of an s and a p
orbital which consists of two lobes, one large and one small, as illustrated
in Fig. 2.18. An sp bond consists of two sporbitals which, because of mutual
repulsion, form an angle of 180° and, consequently, the sp molecule is
linear. The bond, like all overlap bonds, is a sigma (a) bond and has high
strength. The sp orbitals account fortwo of the electrons of the carbon atom.
The othertwo valence electrons are free, delocalized pi (it) orbital electrons
which are available to form subsidiary pi (jt) bonds in a manner similar to the
sp 2 hybridization.
Examples of molecules having sp bonds are the gas acetylene,
HC-CH, and the carbynes, (C»C)n, which are cross-linked linear-chain
carbon polytypes, usually unstable.' 14 '
4. X —
2S Orbital 2P Orbital
SP (diagonal) orbitals showing
overlap (slgma) bond
Figure 2.18. Formation of the sp hybrid orbital and sp sigma bond.