Figure 1. Screenshot illustrating the interchange of axial and equatorial atoms in a trigonal bipyramidal molecules from Mechanisms That Interchange Axial and Equatorial Atoms in Fluxional Processes.

Our visual/dynamic treatment yields some surprises. IF₅ shows fluxional behavior with characteristics of Berry plus two other modes: one a turnstile rotation, and the other a lever mechanism. In recognition of the mixed character of this exchange mechanism, we have christened this a “chimeric pseudorotation”. The pure forms of the three mechanisms are illustrated with PF₅ for the Berry pseudorotation, with SF₄, which has a known lever mechanism as one of the pathways for terminal atom exchange, and with SF₄Cl₂, which has been shown to undergo a cis/trans isomerization via a turnstile rotation. Apical/equatorial exchange in IF₇, first analyzed by Bartell, shows fluxional behavior of mixed character, similar to that observed for IF₅.

We conclude by animating the fluxional behavior of a simple model for a chiral metal catalyst Sn(amidinate)₂, in which axial/equatorial exchange within the amidinate rings occurs via a Berry pseudorotation resulting in interconversion of the enantiomers. We argue that for students learning about fluxional behavior such as that observed in intramolecular axial/equatorial atom exchange processes in systems like these, it is important that they be able to view the dynamic process, and to be able to move the vibrating molecule into alternative views which correspond to their own ideal perception of the process. This can yield new insight into how molecules undergo such dynamic exchange.