M. Dale Alexander New Mexico State University Las Cruces, 88W1
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Demonstrating the Nomenclature for Absolute Configurations in Octahedral Complexes
The proposed nomenclature of absolute configurations for octahedral complexes of the cis-his-bidentate and trisbidentate type has been presented recently.' Many authors already use the convention. The convention is based upon the chirality of helices generated by the chelate rings about the threefold rotational axis ( C d in the trisbidentate type complexes and the analogous pseudo threefold axis in the bis-bidentate complexes. The system works as follows: consider tris-(ethylenediamine)cohalt(III) for example. If an octahedron representing the complex is inscribed in a cylinder such that the C8 axis of the complex is coincidental with the axis of the cylinder then each pair of apices joined by a chelate ring will describe a helix on the surface of the cylinder.
For one of the possible configurations of the complex each of the three pairs of apices joined by the three chelate rings will generate helices of right-handed chirality and for the other configuration the pairs of apices will generate helices of left-handed chirality. This is illustrated in Figure 1. The former configuration is designated A and the latter A. When students are introduced to this convention many have difficulty visualizing the concept even when twodimensional visual aids, like those in Figure 1, are employed. However, easily constructed three-dimensional models are very helpful in overcoming this difficulty. Cardboard octahedra with colored tape along edges occupied by chelate rings can be used to represent complexes. Transparent cylinders to inscribe the octahedra can be constructed from blank transparences used for overhead projectorals. Colored tape on the surface of the cylinders represent the helices. Figure 2, shows a model representing a A-tris-bidentate complex and the appropriate cylinder with right-handed helices. A soda straw through the octahedron indicates the Ca axis. Figure 3 Inorg Chem., 9, I(1970).
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A
Figure 1. Octahedra representing tris-bidentate complexes inscribed in cylinders (bald lines represent edges of octahedra Spanned by chelate rings).
Figure 4. Pattern for model of octahedron
Figure 2. (left) Octahedron representing A trs-bidentate complex and cylinder with right-handed helices. Figure 3. (right) Octahedron inscribed in cylinder.
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Figure 5. Pattern for transparent cylinder.
Volume 50, Number 2, February 1973 / 125
shows the octahedron inscribed in the cylinder with the p a i n of apices joined by the chelate rings touching the helices. The relationship between the disposition of the chelate rings and the chirality of the helices is quite evident. The pattern shown in Figure 4 can he used to construct an octahedron of a convenient size. A piece of cardboard is cut in the shape indicated by the solid lines and then bent along the dotted lines. Glue is placed upon the tabs which are used to join together the edges of the faces. Edges to he joined are designated with the same letter.
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Chemical Education
To make a cylinder of the proper size colored tape is placed upon a flat transparency as shown in Figure 5, and the transparency is rolled into shape of a cylinder with the left end of the transparency overlapping the right end to the dotted line. Transparent tape can he used to hold the cylinder together. If the transparency is rolled such that the strips of colored tape are on the outside of the cylinder the strips of tape will describe right-handed helices, and if the strips of tape are on the inside of the cylinder the helices will be left-handed.
