George B. Kauffman
Fresno State College Fresno, California
Simplified Models of Inorganic Stereoisomers
A
recent paper by Myers1 in THIS JOURhas shown the modification of inexpensive commercial models to illustrate metal coordination compounds. With a few simplifications, such models are readily adapted to the depiction of stereoisomers of inorganic complexes. Scale models such as the Fisher-Hirschfelder-Taylor Metal-Coordination Atom Model Kit2 are ideal for research, since they accurately picture interatomic angles and distances, molecular sizes, steric hindrances, etc. They are also suitable for small classrooms. However, their cost and small size make them far less suitable for use with large undergraduate classes. Also, the compactness of the structure and the intricacy of the ligands complicate the issue by distracting the beginning student from the basic problem, via., the arrangement of the Viands around the central atom. The present author also uses the Sargent Organic Structure Model Set3 suggested by Myers, but for central metal atoms (square planar, octahedral, tetragonal pyramidal) he uses the rubber balls included in the Fisher Master Crystal Model Set%ith which hole-drilling is unnecessary. To show unusual hypothetical configurations such as hexagonal planar or trigonal prismatic, additional holes can he easily made by using a No. 2 cork borer. The ligands are represented simply by differently colored balls. No attempt is made to show the structure of these li~andsas was done in the models described NAL
recently by Wendlandts and Myers.' The purpose of the proposed models is to demonstrate stereoisomerism only. Ligands are therefore depicted as simply as possible, consistent with their effects on the stereochemistry of the system. To make clear to the student the degree of simplification, one or two FisherHirschfelder-Taylor models may be shown next to their simpler counterparts. Multidentate groups are formed by linking the ligand balls with the helical springs provided in the Sargent set. Unsymmetrical groups are shown by using differently colored ligand balls or rubber balls. In most inorganic chemistry courses, proof of configuration by number of isomers is an important topic. The method consists of assuming the most likely configurations in which the equivalent groups can be symmetrically arranged about a common center, predicting the number of possible arrangements for compounds of different types for each of these configurations, and comparing this number with the number of isomers actually prepared. This classic example of an indirect method of structure proving can be effectively presented with these models (Fig. 1).
' MYERS,R. T., J. CHEM.EUUC.,35, 15'2 (1958). *Fisher Scientific Company, Pittsburgh, Pa. Catalogue 50. 12.822~ --
E. H. Sargent and Company, Chicago, Ill. Catalogue KO. S-61815. "isher Scientific Company, Pittshnrgh, Pa. Catalogue No. 12816. (1957). 5 W ~ ~W. ~W., ~J. CHEM. h ~ EDUC., ~ ~34, 223 ,
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Journal of Chemical Educofion
Figure>. Poriib'le con7iguraiion~brcoordination number six. ogonol ploner, b = trigonal prismatic, c = octahedral.
a = her-
The two-dimensional representations of the configurations for coordination number six commonly found in textbooks are apt t o be misinterpreted by the student. They emphasize the "outer structure" and do not point out that the bonds are directed from the central atom toward the ligands (contrast with Figure 1). For example, there is a distinct difference between the hexagonal planar structure and the benzene ring. Also, the conventional representation of the octahedron may give students the impression that the upper and lower positions are different from the four positions at the corners of the square. Such misconceptions are less likely to arise when the proposed models are used. Optical isomerism as well as geometric (cis-trans) isomerism can be demonstrated. Unsymmetrical chelate groups can be illustrated as shown in Figure 2a, while a slight modification allows the depiction of an unsymmetrical chelate group which is itself optically active (Fig. 26). Chelate groups are not limited to hidentate ligands; as a case in point, a compound
containing a sexadentate group recently syntheshed by F. P. Dwyer is shown in Figure 2c. The models are not limited to depicting mononuclear complexes. Binuclear and polynuclear complexes can also be shown.
Figure 2. Mononuclear hexocovalent complexes. a = tr;r(phenylbiguanidine)cobalt(lll) ion, [ C ~ ( @ i g H ) ~f 3l (one antipode), b = cia-dinitro (ethylenediomine)(propylenediomine)c~b~lt(lll) ion, cis-[Co en pn(N03)2]t(one of eightforms),c = 3,6-dithia-i,8-bi~(ralicylideneamiio). ocfonecobdt(li1) ion. [Co(C20HazN20zSz)l + (one antipode).
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Volume 36, Number 2, February 1959
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