Digonal twist

Fig. 9. The digonal twist motion interconverting square-planar (Dti,) and tetrahedral (Trf) structures by D d and showing orbital correlations.

The next point to consider is how the two isomers interconvert. From the possible mechanisms, the most obvious conceptually is the intramolecular motion shown in Fig. 9, often referred to as a digonal twist, which leads to a direct change of ligand positions. The problem here lies in the fact that such a motion is forbidden. 

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