| The Berry pseudorotation is a classical mechanism
for interchanging axial and equatorial ligands
in molecules with trigonal bipyramidal geometry.
Teaching this mechanism presents particular pedagogic
problems due to both its dynamic and three dimensional
character. Our approach illustrates these processes
using interactive animations embedded in a Web
page and overcomes many limitations of a printed
page. The animated illustrations were created
using the open-source Jmol applet in a scripted
Web page, and are based on density functional
(B3LYP/6-31G[3d] or B3LYP/TZVP) molecular orbital
calculations applied to six molecules; PF5,
IF5, SF4, SF4Cl2,
IF7 and Sn(amidinate)2.
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.
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