MOLECULAR MODELS in INORGANIC CHEMISTRY* RICHARD F. ROBEY The Ohio State University, Columbus, Ohio
The dificulties inherent in demonstrating clearly and effectively the many types of isomerism of inorganic coordimtion compounds on the blackboard, have been relieved by the adaptation of inexpensine organic molecular models and other simple materials. A brief discussion is given to show how such models may bs used i n the lecture.
T
HE USE of molecular models in the explanation of isomerism and homology to the students of elementary organic chemistry is quite common and indeed such models may be purchased on the market a t relatively small expense.t However, the use of similar models to explain the three-dimensional relationships existing in coordination compounds to students of inorganic chemgtry has been much less common despite their relative importance. In an elementary discussion of Werner's theory,f use is generally made of a tabulation of the platinic chloride ammonates and their properties as given in Table 1. I t is usually pointed out: (1) That observations on these ammines or ammonates disclose a general property of this type of c6mpound-namely, that while four atoms of chlorine have gone to make up each of these five compounds, only chloride ions to the number indicated for each compound react with such a simple reagent as silver nitrate. (2) That the equivalent conductances decrease with
* Presented before the Chemistry Division of the Ohio Academy of Science at Columbus, March 30, 1934; and the Division of Chemical Education of the American Chemical Society at Cleveland, September 11, 1934. t See, for example: N. M I N ~J.,&EM. EDUC.,6, 1984 (1929); A. L. POULEUR, ibid., 9, 301-lfi (1932); W. R. BRODEAND C. E. Boono, ibid., 9, 177442 (1932). $ For an excellent review of the electronic aspect of Werner's theory see C. A. BUEHLER, ibid., 10, 7 4 1 5 (1933).
decreasing ammonia content until the diammine of platinic chloride has no conductivity. (3) That in order to explain this and similar phenomena, Werner in 1893 proposed an entirely new theory of molecular structure, which was determined by the tendency of certain apparently saturated atoms or ions to attach or coordinate to themselves a definite number of other atoms or groups. This number he called the "coordmation number." Or, to be more specific, certain of the chloride ions of the platinic chloride ammonates are unavailable to silver nitrate, because the ammonia molecules and the unavailable chlorine atoms are occupying the coordination or primary sphere of the platinic ion, and in each of the five compounds cited, the number of atoms or groups occupying this sphere is constantly six. TABLE 1 TBB PLATINIC C B L O ~ DABU ~ N A Z B S
Ammanol~ Famuln PtCL-BNHa
of Cl-/me[.
A
4
523
PtCI..5NHa
3
404
2
1
229 97
0
0
Ptclv4NH1 PtCIa.3NHx PtClc2NHs
Gram-ions Wmncr's Pormuln lPt(NH41lCl~ [Pt(NHd~CllClr [P~(NHI)~CI~ICI, [Pt(NHdrCIsICI
[Pt(N&),CLI
(4) That experimentally the groups in the outer or secondary sphere are characterized by being ionized in water and by lending themselves to electrical conduction, while the constituents of the coordination group are not. (5) That to mark this distinction Werner introduced the square bracket in the formulas given to enclose those atoms which fonn the coordination group and are not ionized. (6) That Werner took a step even farther and assigned definite positions in space to the groups in the coordination sphere. He proposed, supported by isomeric proof, that Ule six coordinated units arrange themselves with the symmetry of the vertices of a sim-
ple octahedron about the centrally located coordinating ion. I t is not always immediately apparent to the student from blackboard sketches or from the frequently used planar diagram as shown in Figure 1, that this octahedral symmetry presumes all six positions to be equivalent, but with the aid of models adapted from the convenient and inexpensive organic molecular models, which may be oriented or modified before a class, this may be made quite obvious. Model H in Figure 2 represents, for example, the central coordinating ion surrounded by six units, five ammonia molecules and one chlorine atom, all a t right angles to one another, giving the octahedral arrangement, and may be thought of as the monochloropentammino-platinic ion. Since all six octahedral positions are equivalent, the ion shows no isomerism.
tain a rina structure. Such substances have been named "chilates." Figure 3 shows diagrammatically a typical chelate ring formed by ethylenediamine.*
With an ion of the type MA8B4"-that is, with two atoms of one kind and four of another in the coordination sphere, however, we have geometrical isomerism, as may he shown by the models of the cis (Model I) and trans (Model forms of the dichlorotetramminoplatinic ion, since the chlorine atom% may occupy either opposite or adjacent positions. The octahedral symmetry in this case is emphasized by outlining with wire. Another type of isomerism capable of existence in these compounds is the so-called "ionization isomerism." It may be demonstrated with Model H. It is easy to see that no indication is made as to the anions outside the coordination sphere of the cation. There is the possibility, and it has been borne out experimentally, that we may have a compound with a chloride ion inside the primary sphere and a bromide outside isomeric with a compound with a bromide inside and a chloride outside. In the cases just mentioned, one atom, molecule, or ion occupied each of the six positions of the coordination sphere. It is, however, equally possible for one molecule to occupy two of these six positions simultaneously, if two characteristic coordinating groups occur in the molecule. In that case the product must con-
The discovery of their nature is a masterpiece of Werner, whereby he established the octahedral theory of coordination symmetry of those'atoms with a coordination number of six. This consisted in the separation of the cis and trans isomers of dinitritodiethylenediammiuo-cobaltic chloride. Here aeometrical isomerism is again encountered because if the two possibilities: the groups may occupy adjacent or opposite positions as shown by models-B and E, respectively. If we let the paper spans or arcs represent the ethylenediammino chelates, and the spheres the nitrito groups, then these are the models of such isomers, keeping in mind that only the cation is represented because it is there that this type of isomerism exists. Since the cis form is asymmetric, Werner in turn has resolved it into the d and I forms of the non-superimposable optical isomers, i. e., mirror images (models A and B). This phenomenon may be demonstrated to a class very effectively by attempting to superimpose the models, or bv the actual use of a mirror. It mav also be ~ o i n t e d
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*Other ions or molecules which form symmetrical chelates include catechol, oxalate, m z h a t e , hydrazine, and a-a'-dipyridyl.
out here that in 1912, Werner clinched his theory by the resolution of the hi-ethylenediammino-cobalticchloride, the optical isomers of the cation of which are represented in models C and D. One observes from the structural formula (Figure 3) that the ethylenediammino chelate is symmetrical. Many compounds, however, f o m non-symmetrical chelates. The nou-symmetrical chelates have aided greatly in the proof of the symmetry of the coordination spheres of beryllium, boron, copper, and zinc; all of which have a coordination number of four, iustead of six. For example, the beryllium, copper, and zinc benzoylpyruvates, and borosalicylic acid have been resolved into their d and 1forms, which are usually represented in the planar form as shown in Figure 4. A more convincing illustration of this optical isomerism may be furnished by models similar to F and G. The uon-symmetrical chelate groups, attached to two points of the tetrahedron, may be represented by the unsymmetrical paper arcs, so that these models represent the d and I forms of the benzoylpyruvate compounds of beryllium, copper, and zinc; and also the anion in the case of borosalicylic acid. This isomerism proves the tetrahedral symmetry of the coijrdination spheres of these atoms because, of course, such isomerism could not exist if the groups were coplanar.'
~s-c==O H d
HOOC-C-----
\A/ 0
o==c-
COOH \CH
-C-
There are, nevertheless, two ions whose coordinated and coordinating groups are in the same plane, namely, the platinous ion and telluronium ion. The representation of their geometrical isomerism is so readily adaptable to the blackboard that models are of no aid in their cases. The tetrahedral symmetry of the coordination spheres of the atoms of nitrogen, silicon, phosphorus, arsenic,
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* Among other ions or molecules which f o m non-symmetrical chelates are salicylate, glycinate, cr-propylenediamine,picolinate, enol-acetylacetonate, a-nitroso-@-naphthol,8-hydroxyquinoline, and cupferron, the latter four of which are of importance because of their m e as mordant dves or as oreauic reaeents in the Dresence
and tin, all of which have a coordination number of four, has been proved by the resolution of asymmetric compounds with four different substitutions, much as the symmetry of the carbon valences has been proved, and, of course, the unaltered organic molecular models may be used to show this. The formula of another compound of special instructional interest is given in Figure 5A. Werner prepared and resolved i t in 1914 as a final proof that the optical activity of these coordinated compounds is not due to some conceivable rearrangement of the organic groups. The simple model K may be used to dispel the notion that optical isomerism is peculiar only to compounds of carbon.7
It represents the cation of one of the mirror-image f o m s of this compound. Here there are three cobalt coiirdination nuclei mutually acting as chelates through the sharing of their two hydroxyl groups with a single cobalt nucleus, arranged, as you'see, like the chelates of the triethyleuediammino model D. All coordiuation positions of the central nucleus are filled and ammonia molecules must be visualized on the remaining vertices of the other nuclei, much as we assume the hydrogen atoms when we draw the usual hexagonal structure for benzene on the blackboard. This model has been made up from sheet metal, which represents the polynnclear compounds somewhat better than the other type because of the simplified appearance. Many other polynuclear coiirdination compounds exist; for example, L is a model of the cation of a molecule of the formula given in Figure 5 B 3 which two nuclei are joined by an imino group, and is comparable to the diphenyl of the organic realm. And M is the model of a chromium compound (Figure 5C), in which there occur three chromium atoms, two of which are tetrahedrally coiirdinated, and the central atom octahedrally, the whole of which has the symmetry of a rhombohedron. In conclusion, it is obvious that these models offer the following advantages. (1) They are conveniently alterable. (2) They are prepared from models used in organic chemistry, which are now on the market, or from common materials a t relativelv small ex-
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t This point has been well presented by J. W. BENNETT, "Liaison in organic and inorganic chemistry," J. CneM. Enuc.. 10, 20-4
pense. (3) They serve to stimulate the interest of the student of inorganic chemistry in the coordination theory by demonstrating its proposals more completely and more understandably than can be done with blackboard and chalk because they include the third dimen-
sion. (4) They show that not only position and ionization isomerism, but also geometrical and optical isomerism, exist in inorganic compounds, and correct a sometimes prevalent notion that these phenomena are peculiar to organic compounds.
