Jahn–Teller Distortion
The geometric distortion of a complex that removes an electronic degeneracy and lowers its overall energy.
Unit
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In depth
Any non-linear system with an unevenly occupied degenerate orbital set will distort to break that degeneracy. The effect is most pronounced for unevenly filled e_g orbitals, producing octahedra elongated along one axis, and it explains many anomalous structures and spectra.
Examples in the real world
[Cu(H₂O)₆]²⁺ having four short and two long Cu–O bonds because of its d⁹ configuration.