The octahedron of six lithium atoms is shown. There are six methyl groups
occupying six of the eight ocatahederal faces. The two empty faces are
shown in red for the three possible isomers: trans with empty faces opposite
each other, apical where the empty edges touch at an apex, and edge where
they share an edge. Click on the isomer of interest above or make your
choice below. A table of energies is also available.
In the sections below the geometry is displayed in a static image and
an interactive image. The interactive images, on the right below, require
the Chime Plug-In and Netscape 3.01 or better. Microsoft Explorer 3.0 does
not support the Plug-In as of yet. The Chemscape Chime
Plug-In is available, without charge for academic or personal at-home
use. The software is related to the Rasmol
molecular viewer which may also be obtained. You may rotate the molecule
by dragging the mouse across the surface of the image. Initially, only
a wire frame image of the empty faces is presented to you. Many options,
including display choices, are available via a menu which is obtained by
clicking with the alternative button on the surface of the image.
In the trans isomer all six lithium atoms (red spheres) are equivalent
to each other. Similarly, the six methyl groups (grey spheres) are equivalent
to each other. The six methyl groups are located over six of the eight
faces of the distorted lithium octahedron leaving two empty faces. In this
isomer the two unoccuppied faces are opposite, trans, to each other. However,
the methyl groups are not located over the centers of the faces of the
distorted lithium octahedron but have moved towards that edge of the face
which is shared with one of the empty faces. This moves the negatively
charged methyl groups away from each other, thereby reducing methyl:methyl
electrostatic repulsion. Focusing on the six methyls groups it can be seen
that they have formed a distorted octahedorn around the lithium atoms which
themselves form a distorted octahederon.
In the apical isomer the the empty faces (red, pink, pink) meet at the
red apex and are indicated by the heavy edges. The methyl groups that nominally
over the (blue, pink, pink) faces sharing and edge with the empty faces
are actually located nearly over the common edge. We hypothesize that this
is to minimize repulsion between the negatively charged methyl groups.
The methyl groups take up a nearly octahederal array surrounding the octahederal
lithium array.
In the edge isomer the empty edges (red, red, pink) share a common (red,
red) edge and are shown in heavy outline. The four quivalent methyl groups
over the (red, pink, blue) faces make up a rectangle. The symmetry constraints
prevent the methyl groups relaxing into a lower energy geometry such as
is taken on in the trans or apical isomer. This results in the edge having
the highest energy of the three isomers.