Berry mechanism

The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands (see the figure) for two of the equatorial ones.

It is the most widely accepted mechanism for pseudorotation and most commonly occurs in trigonal bipyramidal molecules such as PF5, though it can also occur in molecules with a square pyramidal geometry.

[3] This rapid exchange of axial and equatorial ligands renders complexes with this geometry unresolvable (unlike carbon atoms with four distinct substituents), except at low temperatures or when one or more of the ligands is bi- or poly-dentate.

Unlike with pseudorotation in bipyramidal molecules, the atoms and ligands which are not actively vibrating in the "scissor" motion are still participating in the process of pseudorotation; they make general adjustment based on the movement of the actively vibrating atoms and ligands.

However, this geometry requires a significant amount of energy to occur of about 26.7 kcal/mol.