Atomic orbitals can be combined, in a process called hybridization, to describe the bonding in polyatomic molecules.
Descriptions of the bonding in CH4 can be used to illustrate the valence bond procedure. We must arrive at four bonds projecting from the carbon atom in procedure. We must arrive at four bonds projecting from the carbon atom in tetrahedral directions.
Lithus Pauling pointed out that the 2s and 3p orbitals of the carbon atom could be used to form new orbitals better suited to the description of the bonds. This procedure of combining orbitals to form new ones is called hybridization, and the new sets are called hybrid orbitals. The most suitable set can be found, according to Pauling, by forming wave functions which project out farthest from the central atom. When the four orbitals that they are concentrated along tetrahedral directions. Thus the sp3 hybrid orbitals are tetrahedrally oriented and are suitable for describing the bonding in CH4.
Other combinations of s, p and d orbitals can be constructed to provide orbitals suitable for molecules of other shapes, hybrid orbitals that project in linear, trigonal, tetrahedral and octahedral directions are produced by the combinations. The trigonal and linear hybrids, which leave one p and two p orbitals of the atom unchanged, are the basis for descriptions of double and triple bonds. The p orbitals form bonds and supplement the σ bonds, to notice that σ and ∏bonds are similar to those constructed for homonuclear diatomic molecules.
Hybrid orbitals from symmetry: the hybrid orbitals constructed by Pauling led to the geometry, or symmetry, of the molecule for which they were constructed. If the geometry of the molecule is taken as known, the approximate hybrid orbitals can be deduced from symmetry consideration alone. Consider the four tetrahedrally arranged carbon atom bond orbitals needed in this approach to describe the bonding in methane. For these orbitals the characters for the various symmetry operations of the Td group can be seen by calculating the number of unchanged bond orbitals, or bond lines, for each operation. We obtain:
| Td |
E |
8C3 |
3C2 |
6σd |
6S4 |
| σorb |
4 |
1 |
0 |
2 |
0 |
Thus we need atomic orbitals that transform as A1 and T2 to provide the basis for the tetrahedrally directed hybrid orbitals. The totally symmetric s atomic orbital transforms according to A1. In a similar way, the hybrid combinations of table can be deduced from the symmetry of the bonding situation for which they are to be used.
Some Hybridization used in describing σ bonding:
| Number of orbitals |
Shape |
Atomic-Orbital Combinations
|
Example |
| 2 |
Linear |
sp |
CH≡CH |
| 3 |
Trigonal |
sp2 |
CH2  CH2, BF3 |
| 4 |
Tetrahedral |
sp3 or sd3 |
CH4, MnO4- |
| |
Square planner |
dsp2 |
PtCl24-, Ni(CN)24- |
| 5 |
Trigonal bipyramid |
dsp3 |
PCl5, Fe (CO)5 |
| 6 |
Octahedron |
d2sp3 |
PtF6, CoF26- |