Berry Mechanism in Trigonal Bipyramidal Structure
The process of pseudorotation occurs 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. Both the axial and equatorial constituents move at the same rate of increasing the angle between the other axial or equatorial constituent. 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. This requires about 3.6 kcal/mol.
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.
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