Translator George B. Kauffman Colifornia State University Fresno, California 93710
I
C~~S~CIIS US Molecular Compounds
Through the fundamental works of von Laue ( 1 ) and W.
L. and W. H. Bragg f2J, the space lattice theory of crystals has received a more certain experimental basis. In the simpler cases, we are now in a position to report with great certainty the mutual grouping of atoms in crystals. Of the elements, only carbon, viz., in the form of the diamond, has until now been thoroughly investigated. As is well known, W. L. and W. H. Bragg have obtained the surprising result that in the diamond the structure occurs in such a manner that each individual carhon atom is symmetrically surrounded in space by four other carhon atoms. I t was now obvious to assume that the crystallographic forces which determine the cohesion and the definite mutual arrangement of the atoms in the diamond correspond to our valence forces. In fact, this conclusion has been drawn. In the following we shall now show that our valence concepts are also sufficient for understanding the structure of the crystals of chemical compounds, if only, in addition to primary valence forces [Hauptualenzkriiften], we also make use of Werner's secondary valence forces [Nebenualenzkraften]. The final result of our considerations will he that crystals of chemical compounds are joined directlv to the so-called "molecular comnounds" [~ofekiilverbindungen].'In fact, one can view molecular compounds, especially those which are constructed of the same molecules, as transition members between ordinary valence compounds (compounds of the first order) in the gaseous or dissolved state and the crystals of chemical compounds. As an example of our theoretical investigation let us choose common salt [Kochsalz]. W. L. and W. H. Bragg have shown that the common salt crystal is constructed in such a manner that a Na atom and a CI atom are located alternately in the corners of a cube.2 Now if one constructs a portion of a common salt crystal with the help of our ordinary stereochemical atomic models, then one ohtains the surprising result that the Bragg concept includes the fact that in the common salt crystal each individual Na atom is surrounded symmetrically in space by six C1 atoms and that each C1 atom is surrounded symmetrically in space by six Na atoms, i.e., that we have a symmetrical octahedral grouping of Na and C1 atoms around each other. In order to make this structure of the common salt crystal chemically intelligible, we proceed from the modern theory of double s a h 3 According to this theory, two metal halides combine in such a manner that the halogen atoms of one halide are honded with their secondary valences to the metal atom of the second halide, whereby the more negative metal atom acts as the central atom. The following examples may elucidate this4
minum chloride, ferric chloride, and many other halides in the gaseous state under conditions of not too low a density or high a temperature are mainly himolecular, corresponding to the formulas AIzCla, FezCls, etc. In these polymolecular systems we should not perchance assume metal bonds according to older views; rather molecular compounds of a special type in which the components are chemically identical, are present here. According to the coordination theory, for such polymeric metal halides, we must consider formulas of the following type
No definite decision can he made between these formulas with the present state of our k n ~ w l e d g e . ~ But in any case we see that the molecules come together to form a polymolecular system in such a way that the halogen atoms provide the linkage of the molecules. Thus, metal atoms form central points around which the halogen atoms are grouped. Furthermore, from the formulas cited we recognize that halogen atoms too can he honded to more than one metal atom so that it seems as if halogen atoms also can act as centers (around which the metal atoms are grouped). That this view is justified may be deduced from the following facts. Numerous mercuric halide douhle salts of the type MeCl,GHgCl2 exist; to these the formula [C1(. . .HgC12)6]Me is assigned since it is assumed that the C1 atom of the metal halide MeCl is coordinatively honded to six mercury atoms. But of special interest to us are the iodine-silver compounds6 [JAgzlNOa and [JAg31(N03)2, whose theoretical significance is further supported by the existence of the compounds [PAg61(N03)3 and [AsAgs](NO3)3, in which Translation of "Die Kristalle als Molekiilverbindungen," Z. amrg. allgem. Chem., 92,376 (1915). Compounds of higher order or, in modern terminalogy, coordination compounds. The constituents of the sodium chloride crystal are now known to be ions rather than atoms. 3For Werner's theory of double salts, see Z.onorg. Chem.. 3, 261 (1893) (translated into English in Kauffman, G. B., "Classics in Coordination Chemistry, Part I: The Selected Papers of Alfred Werner," Dover Publications, Inc., New Yark, 1968, pp. 28-31); Vierteljahr.ssehrift der Ziireher Naturforsehenden Gesellwhafi, 41,254-269 (1896), andZ. anorg. Chem., 19,158-118 (1899). In modern natation, K[ZnCIs], K[AuClrl, and K2[PtCIeI, respectively. 51n dimerie aluminum chloride, each aluminum atom exhibits a coordination number of four with a tetrahedral configuration C1 C1\ /c1 *,A], CI' CI' C1 6Although most coordination £s consist of a metallic species surrounded by several nonmetallic species, a few compounds with the reverse situations are known. Far discussions af the compounds listed by Pfeiffer as well as other compounds with nonmetallic coordination centers, see Jones, M. M.. "Elementary Coordination Chemistry," Prentice-Hall, Inc., Englewood Cliffs. N. J., 1964, pp. 38-39 and references therein.
\AIL
But the polymeric metal halides in particular also belong among the double salts. It is well known that alu-
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the metailoids phosphorus and arsenic are present as central atoms. Since we thus see that in double salt-like structures metal atoms as well as halogen atoms can function as centers, the structure of a common salt crystal immediately becomes intelligible to us. In a common salt crystal we have nothing but a symmetrically constructed molecular compound of the douhle salt class, which is intimately joined to the polymolecular metal halides. In the common salt crystal, each Na atom is a center for the addition [Anlugerung] of C1 atoms (in the manner of ordinary double salts), and each CI atom is a center for the bonding of Na atoms (in the manner of the J, P, and As compounds cited above) (3). The fact that according to Bragg each Na atom is surrounded symmetrically in space by six CI atoms and each C1 atom is surrounded symmetrically in space by six Na atoms is in complete agreement with the spatial concepts of the coordination theory. In the common salt crystal the Na as well as the C1 atoms have coordination number 6. According to these considerations, in the common salt crystal the Na and C1 atoms are linked to each other by secondary valences in addition to primary valences, i. e., in such a manner that from each atom a total of six valences emanates, of which-corresponding to the valence [Wertigkeit] of the sodium and chlorine-one each is a primary valence. At first it mieht seem that this tvDe " ". of valence distrihution produces a certain asymmetry of crystal structure. But this is not the case. rA.1 Werner (4) and J. V. Duhsk? (5) in some beautiful examples, have recently shown that in symmetrically constructed molecular compounds the difference between primary and secondary valences disappears since an exchange of affinity [Affinitdtsausgleich] apparently takes place between the individual valences. Thus, according to Werner the two cobalt atoms in the cohalt-ammines7
.CIMe
Br,
Br are identical with the compounds CI,
Cd:
,.BrMe
CI' ''~r~e a fact which can be expressed as follows8
According to all this, such an exchange of affinity hetween primary and secondary valences is also t o be assumed in the case of common salt. The "douhle salt theory" is thus able to explain fully the complete symmetry of the structure of common salt crystals. I helieve that in principle it is always possible to make the crystal structure of compounds chemically intelligible on the basis of such considerations of the coordination theory and to show that the crystals in question may be joined or subordinated to quite definite classes of molecular compounds (6). Since ordinary molecular compounds are thus regarded as preliminary stages of crystal formation, the chemistry of compounds of higher order gains a significance even greater than has hitherto been suspected. In closing, it may be pointed out that the structure of zinc sulfide crystals is just as easily intelligible from a chemical standpoint. According to Bragg, in zinc sulfide crystals the zinc atoms are surrounded symmetrically in space by four sulfur atoms, and the sulfur atoms are surrounded symmetrically in space by four zinc atoms. Accordingly, the zinc sulfide crystals are closely related to the douhle sulfides. In a further article9 I still wish to show on the basis of a series of experiments that the view that I have presented is suitable for the explanation of many phenomena in the border area between chemical isomerism and polymorphism.
are in reality completely equally chained so that these compounds are better written
Zurich. Chemisches Universititsloborotorium. Received by the editor on J u n e 9,1915.
Pfeiffer's Citations
further, according to Dubsk9, the douhle salts
Friodrich. lP.1 Kniooine. and IM. vonl Laue. Ann, der Phvsik. 41. 971 1-9891 (isis!; M V& isue.id X s. "in der iingen, Physih. zeit&hrilt. 15, 75.1-77j
I l l IWI
(19141; M. von Law and F. Tank. Vwh. dcr Deutschenphya. Ges. 1914: Ann. der Phvsik. dl. 1W3 1-10111 119131: M. van Laue. Ann. dar Phvsik. dl. 1561
"en represents the bidentate ligand ethylenediamine, H1NCH2CH2NHs. 8The configuration of the cadmium atom is tetrahedral so that cis-trans isomerism is not possible. 9 A second article by Pfeiffer on the topic "Die Kristalle sls Molekiilverbindungen" appeared in 2.Anorg. Allgem. Chem., 97, 161-174 (1916).
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1 Journal of Chemical Education
1914, Abt. W. o. 20.3 . H. and W. L. (2) W. L. Bragg. Roe. Sor. Combridge Phil. Soc., 17. 43 1-17] 119131; W Bragg, Pror Royal Soc. London. 88, 428 1-4381 (1913); Z Anom Chem., 90, 153. 169. 182. 1s.219.235, 246. 270. 277(19111. (3) In this eonneetion also see the recently decribed alkali chloride douhle salts LiCI, CsCland LiCI. 2CsCI:E. Korrene 2.Anorn. Chsm.. 91.194 1-2081. 11915). (4) B e r . 46.3674 1-3'6831119131. IS! J. pmkf. Chom. 1219rl.61 1-1181(19141. (6) The theory developed here permits u- to predict that close relationships must exist between optically active molecular compounds end optically acfivecryntals.
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