In chemistry, sigma bonds (σ bonds) are a type of covalent chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, pz+pz, and s+pz, and dz2+dz2 (where z is defined as the bond axis). Quantum theory also indicates that molecular orbitals (MO) of identical symmetry mix. As a practical consequence of mixing in diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of mixing (or blending) depends on the relative energies of the like-symmetry MO's.
For homodiatomics, bonding σ orbitals have no nodal planes between the bonded atoms. The corresponding antibonding, or σ* orbital, is defined by the presence of a nodal plane between these two bonded atoms.
The symbol σ is the Greek letter for s. When viewed down the bond axis, a σ MO resembles an s atomic orbital.
Sigma bonds in polyatomic compounds
They are obtained by head on overlap of atomic orbitals. The concept of sigma bonding is extended, albeit loosely, to describe bonding interactions involving overlap a single lobe of one orbital with a single lobe of another. For example, propane is described as consisting of ten sigma bonds, one each for the two C-C bonds and one each for the eight C-H bonds. The σ bonding in such a polyatomic molecule is highly delocalized, which conflicts with the two-orbital, one-bond concept. Despite this complication, the concept of σ bonding is extremely powerful and hence pervasive.
Pi bonds do not necessarily connect a pair of atoms that are also sigma-bonded.
In certain metal complexes, pi interactions between a metal atom and alkyne and alkene pi antibonding orbitals form pi-bonds.
In some cases of multiple bonds between two atoms, there is no sigma bond at all, only pi bonds. Examples include diiron hexacarbonyl (Fe2(CO)6), dicarbon (C2) and the borane B2H2. In these compounds the central bond consists only of pi bonding, and in order to achieve maximum orbital overlap the bond distances are much shorter than expected.
Sigma bonds in multiply bonded species
Compounds that feature multiple bonds, such as ethylene and chromium(II) acetate have sigma bonds between the multiply bonded atoms. These sigma bonds are supplemented by π-bonds, e.g. in the case of ethylene, and even δ-bonds, e.g. in the case of chromium(II) acetate.