A Simple Algorithmic Method for the Recognition of Theoretically Chiral Octahedral Complexes Students who are not acquainted with symmetry theory and are faced with the problem of recognizing potentially ehiral octahedral configurations are usually forced to compare the s t ~ c t u r drepresentation of a complex with its mirror-image in a test for no"-superimposihility. This procedure is time consuming and places a premium on the ability to visualize in three dimensions. A simple algorithmic procedure is available which allows the student to unambiguously determine whether a particular octahedral configuration is c h i d or aehiral without recourse to drawing mirrorimages or knowingly applying symmetry theory. The six ligand positions in an octahedral complex are divided into three pairs, each pair consisting of positions trans to each other. Two simple conditions must be satisfied to give the chiral case
1) No trans pair contains identical members 2) No two pairs are identical
If both these conditions are satisfied the confuguration is ehiral and such a complex should be capable of existing in enantiomorphie forms. If neither, or only one, condition is satisfied the configuration is achiral. When two or three identical bidentate ligands are present the above conditions can still be applied provided that corresponding donor centers in different ligands are regarded as non-identical. For example, in the classical case of the cis and trans isomers of the bis-ethylenediaminedichlorocobalt(III)cation in the trans isomer the "trans pairs'' are
C-NH,, -N&'I,[-NHz
-NH/I, [CI, CI1
(where -NH2 and -NHs' signify that the nitrogen donor centers are on different molecules of ethylenediamine). This violates both conditions and this isomer is acbiral-it bas of course three planes of symmetry. For the cis isomer the "trans pairs" are [-NH,',
-NHJ, [-Ma, -Cl],[-NH/,
-C1]
this satisfies both conditions and this isomer should he separable into enantiomorphic isomers. Another classical ease which the student not familiar with Symmetry Theory finds difficult is that of tris-ethylenediaminecobalt(II1). This gives the ''trans pain" [-NH,
-NH,'l,
[-NH,, -NH,'a
[-NHH -NHH,"]
and must therefore be c h i d in agreement with the absence of a rotation-reflection axis of symmetry. Educationally it is desirable that students have a deeper appreciation of the nature of c h i d octahedral complexes than the mechanical application of an algorithmic procedure such as that above but it has been successfully used by our students in examination conditions where time and certainty are at a premium. Robert T. Richardson Glasgow Callege of Technology Glasgow, Scotland
Volume 51. Numbers, May 1974
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