Pseudorotation

The International Union of Pure and Applied Chemistry (IUPAC) defines a pseudorotation as a "stereoisomerization resulting in a structure that appears to have been produced by rotation of the entire initial molecule", the result of which is a "product" that is "superposable on the initial one, unless different positions are distinguished by substitution, including isotopic substitution.

"[1] Well-known examples are the intramolecular isomerization of trigonal bipyramidal compounds by the Berry pseudorotation mechanism, and the out-of-plane motions of carbon atoms exhibited by cyclopentane, leading to the interconversions it experiences between its many possible conformers (envelope, twist).

[citation needed] In these and related examples, a small displacement of the atomic positions leads to a loss of symmetry until the symmetric product re-forms (see image example below), where these displacements are typically along low-energy pathways.

[citation needed] The Berry mechanism refers to the facile interconversion of axial and equatorial ligand in MX5 types of compounds, e.g. D3h-symmetric PF5 (shown).

[1][3] Finally, in a formal sense, the term pseudorotation is intended to refer exclusively to dynamics in symmetrical molecules, though mechanisms of the same type are invoked for lower symmetry molecules as well.

Pseudorotation of Iron-Pentacarbonyl-Complex (Berry mechanism)
Pseudorotation of Iron-Pentacarbonyl-Complex (Berry mechanism)