Berry mechanism

The Berry mechanism, or Berry pseudorotation mechanism, is a way trigonal bipyramidal molecules isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation.

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This happens when the two axial ligands close like a pair of scissors pushing their way in between two of the equatorial groups which scissor out to accommodate them. This forms a square based pyramid where the base is the four interchanging ligands and the tip is the pivot ligand, which has not moved. The two originally equatorial ligands then open out until they are 180 degrees apart, becoming axial groups perpendicular to where the axial groups were before the pseudorotation.