Non-rigid Molecular Skeletons-Berry Pseudorotation and Turnstile Rotation

Molecules rearrange by regular and irregular processes. The former occur with preservation of bonds and coordination numbers while the latter involve rupture and reformation of bonds via states possessing different coordination numbers. For molecules with skeletons that carry n ligands both processes can be represented by permutation operators belonging to the symmetrical group Sn. These operators are obtained from the descriptors of the particular molecules in a simple manner; the conceivable reaction mechanisms correspond to the skeletal symmetry subclasses of Sn. Regular rearrangements—e.g. of phosphorane derivatives with a monocentric skeleton, including their analogs, or of ethane derivatives with a dicentric skeleton—all submit themselves to classification based upon skeletal non‐rigidity. Flexible molecular skeletons are representable by a collection of rigid skeletal models which can be interconverted by deformations of bond angles and/or some form of internal rotation. To accomplish mechanistic understanding of these regular processes, it is useful to represent the symmetries of the rigid skeletal models and the existent ligand equivalencies by subgroups of Sn and consider the dependence of the skeletal flexibility upon the given ligand set. This procedure is based on the ideas of Polya and Longuet‐Higgins. The former introduced separation of permutational molecular symmetry into skeletal symmetry and ligand set symmetry, which, combined with the total molecular symmetry concept developed by Longuet‐Higgins for the treatment of non‐rigid symmetric molecules, yield the foundation for the following classification of regular pentacoordinate rearrangements and related processes. It will be shown that with the given quantum chemical properties of the pentacoordinate phosphorus bond system, including participation of the P3d orbital set, not only the BPR‘2’ mechanism but also single and multiple TR[2] processes must be considered in the interpretation of the observed phenomena. Although some experimental observations can be interpreted by either of the two mechanisms, and certain other regular rearrangements can be explained only by invoking the TR mechanism, no regular processes are known, as yet, that can be explained solely by the BPR mechanism.