Editing Molecular orbital (section)

===Sigma and pi labels for MOs===<!-- this section-title is linked from some redirect pages...do not change it here without also updating them -->
The type of interaction between atomic orbitals can be further categorized by the molecular-orbital symmetry labels σ (sigma), π (pi), δ (delta), φ (phi), γ (gamma) etc. These are the Greek letters corresponding to the atomic orbitals s, p, d, f and g respectively. The number of nodal planes containing the internuclear axis between the atoms concerned is zero for σ MOs, one for π, two for δ, three for φ and four for γ.

====σ symmetry====
{{Further|Sigma bond}}
A MO with σ symmetry results from the interaction of either two atomic s-orbitals or two atomic p<sub>z</sub>-orbitals. An MO will have σ-symmetry if the orbital is symmetric with respect to the axis joining the two nuclear centers, the internuclear axis. This means that rotation of the MO about the internuclear axis does not result in a phase change. A σ* orbital, sigma antibonding orbital, also maintains the same phase when rotated about the internuclear axis. The σ* orbital has a nodal plane that is between the nuclei and perpendicular to the internuclear axis.<ref name = H&C>Catherine E. Housecroft, Alan G. Sharpe, ''Inorganic Chemistry'', Pearson Prentice Hall; 2nd Edition, 2005, p. 29-33.</ref>

====π symmetry====
{{Further|Pi bond}}
A MO with π symmetry results from the interaction of either two atomic p<sub>x</sub> orbitals or p<sub>y</sub> orbitals. An MO will have π symmetry if the orbital is asymmetric with respect to rotation about the internuclear axis. This means that rotation of the MO about the internuclear axis will result in a phase change. There is one nodal plane containing the internuclear axis, if [[Atomic orbital#Real orbitals|real orbitals]] are considered.

A π* orbital, pi antibonding orbital, will also produce a phase change when rotated about the internuclear axis. The π* orbital also has a second nodal plane between the nuclei.<ref name = H&C /><ref>Peter Atkins; Julio De Paula. ''Atkins’ Physical Chemistry''. Oxford University Press, 8th ed., 2006.</ref><ref>Yves Jean; François Volatron. ''An Introduction to Molecular Orbitals''. Oxford University Press, 1993.</ref><ref>Michael Munowitz, ''Principles of Chemistry'', Norton & Company, 2000, p. 229-233.</ref>

====δ symmetry====
{{Further|Delta bond}}
A MO with δ symmetry results from the interaction of two atomic d<sub>xy</sub> or d<sub>x<sup>2</sup>-y<sup>2</sup></sub> orbitals. Because these molecular orbitals involve low-energy d atomic orbitals, they are seen in [[transition metal|transition-metal]] complexes. A δ bonding orbital has two nodal planes containing the internuclear axis, and a δ* antibonding orbital also has a third nodal plane between the nuclei.

====φ symmetry====<!-- this section-title is linked from some redirect pages...do not change it here without also updating them -->
{{Further|Phi bond}}
{{multiple image
| footer    = Suitably aligned f atomic orbitals overlap to form phi molecular orbital (a phi bond)
| width     = 120
| image1    = Phi-bond-f-orbitals-2D.png
| image2    = Phi-bond-boundary-surface-diagram-2D.png
| caption   = Suitably aligned f atomic orbitals can overlap to form a phi molecular orbital (a phi bond)
}}
Theoretical chemists have conjectured that higher-order bonds, such as phi bonds corresponding to overlap of f atomic orbitals, are possible. There is no known example of a molecule purported to contain a phi bond.