The second-order variant of this phenomenon occurs to allow for a configuration interaction between a molecule's HOMO and LUMO and can be invoked to explain the stereoactive lone pair in xenon hexafluoride.
Ceder et al. recently used scanning tunneling electron microscopy to directly observe the cooperative version of this phenomenon in a sodium/manganese/O2 superstructure. The second-order variant of this phenomenon occurs to allow for a configuration interaction between a molecule's HOMO and LUMO and can be invoked to explain the stereoactive lone pair in xenon hexafluoride. A shoulder in the UV-Vis absorption band of aqueous titanium(III) is attributed to this phenomenon, which often occurs in a weaker (*) dynamic form at room temperature in complexes with T-sub-1g and T-sub-2g ground states. This phenomenon is weak in tetrahedral complexes, since none of the ligands directly overlap with any d-orbitals. The most common manifestation of this phenomenon is in the tetragonal elongation of six-coordinate transition metal complexes with four, seven, or nine d- electrons. For 10 points, name this effect in which a molecule distorts to relieve an orbital degeneracy.
Ceder et al. recently used scanning tunneling electron microscopy to directly observe the cooperative version of this phenomenon in a sodium/manganese/O2 superstructure.
Jahn-Teller effect [or  Jahn-Teller distortion]