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.