A Straightforward Method for Assigning Stereochemical AlA Descriptors to Octahedral Coordination Compounds Santiago Herrero and Miguel Angel Uson Universidad de Zaragoza, 50009 Zaragoza, Spain Transferring a figure drawn on a two-dimensional surface (be it a textbook page or a blackboard) to a three-dimensional model is a n awkward task, not only for the heg i n n e r b u t often for t h e experienced chemist also. Therefore, i t is not surprising that many contributions to this Journal (1-21) concern simple methods to determine the configuration of a stereogenic center. All these contributions have been restricted to the RIS case (i.e., they apply only to tetrahedral centers, in mostly organic species). However. the stereochemistrv of octahedral coordination ~ and a busy field in compounds is illst, :in i m I ) o r t a subject inora;inic rt,ie,~rch.Indeed. the success of Werner in w a b lishi;ii: his coordination theory was h a s d on his ~hiliryto rcsolve the ennnriomers 2?, 2.71of
where X is Cl, Br; and en is ethylenediamine and [co(en),13+ For the cis-bis(chelate1 or tris(chelate1 octahedral complexes, the helicity symbols A or A are used (24,251 but, to the best of our knowledge, only two methods have been devised (26) for helping with the assignment of these stereochemical descriptors. One calls for the visualization of the structure in three dimensions, and the other requires that the projection be in a convenient orientation. As a result, hoth may prove too demanding for the students. We have developed a straightforward method (which does not depend on the orientation of the two-dimensional
drawing), in which the chirality is derived from the simple left-right concept without any need to compare the helicity of the structures. This is a three-dimensional concept. For a tridchelate) compound, the upper vertex of the octahedron is taken a s the viewing point, and a projection onto the equatorial plane is considered (see the figure) according to the following three steps. 1. One of the chelate ligands necessarily lies on this plane and will be represented as a curved line joining the came-
sponding vertices. 2: The projection of the chelate chain joining the upper vertex to the plane is represented as a straight line. 3. Finally, the figure soabtainedis rotated (movingthe sheet of paper is all that is needed!)so that the bent line is on top. If the straight line slopes down to the left, the compound is the A enantiamer (left-handed helix); if it slopes to the right, it is the A enantiomer (right-handedhelix).
This method can also be applied to cis complexes containing only two chelate ligands, taking into account that substitution of hoth monodentate ligands for a chelate one does not change the helicity. Thus, i t suffices to join the donor atoms of the monodentate ligands with a n imaginary linking chain and follow steps 1-3 for arriving a t the desired result. For the last few years this method has been proposed to our students, and i t has found great acceptance. In comparison to other methods, the degree of success in assigning the correct NA configuration of the cis-bis(chelat.4 or the tridchelate) complexes is superior, ranking to about 100%.
Diagram showing the application of the method to the four possible orientations in which a tris(chelate)octahedral coordination compound may be drawn: (a)A-isomer and (b) A-isomer. Volume 72 Number 12 December 1995
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