GERMANIUM. XXII-. T H E DIHALIDES OF GERMAKIUM, T I N AND LEAD' BY F. M. BREWER?
Recent investigation of the dihalides of germanium renders it now possible to make a comparative study of the dihalides of germanium, tin and lead, elements in the fourth group of the Periodic Table. The dihalides of carbon and silicon are as yet unknown. Examination of the available data concerning the physical constants of these compounds discloses wide discrepancies in the results reported by varoius observers. The most trustworthy values? appear t o be the following: Meltzng Poznls Germanium
Dichloride Dibromide Di-iodide
Probably lowmelting solid I22O
Sublimes with decomposition above 240'
Tin 247'
Lead
232'
? 373O ? 488'
320'
solo
402'
Boding Points Germanium
Dichloride Dibromide Di-iodide
S o t known Volatilizes with some decomp. ca. 1 2 5 ' __
Tin
Lead
606' 619'
954O 920'
720'
(861" - 954')
These data show that, as regards thermal stability, the dihalides of germanium exhibit properties quite in conformity with the position of the element in this group. At elevated temperatures they dissociate much more readily than do the dihalides of tin and lead, the decomposition proceeding 2 GeXl = Ge GeX4 It is probable that the first stage in this dissociation is GeX? = Ge 2 X and that then the halogen acts upon the dihalide 2 X = GeXl CeXs
+
+
+
Contribution from the Department of Chemistry, Cornel1 University. 2 This article is based upon part of the thesis presented to the Faculty of the Graduate School of Cornell University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 3 A full list of references is given in Mellor's "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Yolumr YII. 1
1818
F. hl. BREWER
In this connection the thermal behavior of the tetrahalides of these three elements and of the two elements a t the head of the group, carbon and silicon, is of interest. Carbon tetra-iodide begins to decompose at so'. The tetrabromide dissociates somewhat when heated to boiling under atmospheric pressure. Carbon tetrachloride is a very stable substance which, when heated alone, breaks down only at elevated temperatures. The products in this case are mainly tetrachlorethylene and hexachlorethane, which indicates a primary decomposition
cc'14 = cc12
+ r12
followed either by polymerization or by further dissociation of the dihalides according to the equations given above. Much less is known of the behavior of the halides of silicon a t elevated temperatures. There is, however, no reason to expect any fundamental difference from the behavior which the corresponding compounds of carbon should be expected to exhibit except that, as is seen in other groups of the Periodic Table, the compounds of silicon would probably be more stable and less reactive. Coming to the tetrahalides of germanium, more data are available. Germanium tetrachloride does not dissociate' when heated to 9 jo". The thermal stability of germanium tetrabromide has not been directly studied but no dissociation was observed when the wmpound was distilled. (B. P t . 185.9')~. Germanium tetra-iodide, however, dissociates a t 440' into germanium diiodide and i ~ d i n e . All ~ of the tetrahalides of tin are stable a t their boiling points, but their behavior at higher temperatures has not been studied I t is, however, of interest to note that stannic iodide when dissolved in arsenic tribromide appears t o dissociate4 as follows : SnIi = Sn12 2 I.
+
Lead tetrachloride decomposes explosively5 into lead dichloride and chlorine a t 10s'. It is thus seen that the dihalides of germanium, tin and lead show increasing thermal stability and decreasing reactivity with rise of atomic number of either the halogen or the group element. The tetrahalides, on the other hand, show decreasing stability with rise of atomic number. When the dihalides of carbon and silicon are isolated, they willundoubtedly be found to be highly reactive. Even the polymerized form of carbon dtchloride, tetrachlorethylene, readily takes up additional chlorine, and it may safely be predicted that the unpolymerized form would show greatly enhanced reactivity. This belief is supported by the observation in this Laboratory that germanium dichloride unites with chlorine with almost uncontroliable speed, The gradual lessening of the activity of the dihalides with rise LLaubengayer and Tabern, J. Phys. Chem. 30, 1047 (1926) J. Am. Chcm. SOC.44, 299 (1922). 3 Dennis and Hance, J. Am. Chem. SOC. 44, 2854 (1922) 4 Walden: Z. anorg. Chem., 29, 379 (1902). SFriedrich. Ber., 26, 1434 (1893).
* Dennis and Hance,
OERJISXITJI XXIV
1819
of atomic number of the group element is shown by the fact that stannous chloride readily but quietly unites with chlorine in the cold, whereas lead dichloride takes up two additional atoms of chlorine only under special conditions, the resulting tetrachloride being quite unstable.
Beharzor toward Oxygen (Reducing Power) :-Here again the reactivity of the dihalide is less as the atomic number of the group element rises. Lead chloride, for example, is stable in the air and is only slowly decomposed when heated. Stannous chloride is fairly stable in air, and yields stannic chloride and an oxychloride when heated to its boiling point. Germanium dichloride is rapidly attacked by oxygen a t room temperature. The dibromide and diiodide are less readily affected by oxygen the latter being converted to germanium dioxide and germanium tetra-iodide when heated in air to 210'. .Is a direct corollary of the behavior toward oxygen, the reducing power decreases with rise of atomic number of the group-element, a solution of germanous chlsride having powerful reducing action, whereas lead dichloride shows this property almost not at all. Solubdzty Relatzonshzps and H ydroZyszs:-The dihalides of germanium are but slightly soluble in hydrocarbons, but they react with liquids which contain an actual or a potential hydroxyl group. This action is presumably of hydrolytic nature, yet whereas the action of a small quantity of water on germanium dibromide is to produce a voluminous solid precipitate which may be either white or pale yellow in color, and which is soluble in excess acid, there is no such precipitate formed with absolute alcohol. The diiodide reacts slowly with cold water, but is soluble in an excess. On the other hand, alcohol dissolves it readily, but the entire lack of color in the alcoholic solution indicates that possibly dissociation, but more probably alcoholysis, has taken place. Since recrystallization cannot be effected from either the colorless solution in water or that in alcohol, these solutions cannot be regarded as strictly analogous to the colorless solution of lead iodide in hot water. Stannous halides are soluble in water. There is evidence of hydrolysis, but the salts can be recrystallized from aqueous solution in a hydrated condition. In contrast to this, the lead salts are remarkable for their extremely low solubility in cold water, and their inordinately steep solubility curves, hydrolysis being negligible. With regard to non-polar solvents, stannous chloride is soluble to a small extent in benzene and some other non-hydroxylic solvents which are quite without action upon lead salts. Solubility relationships may therefore be said to bear out the more definitely polar character of the di-iodide. Thac is to say, the di-iodide forms the link with the metallic or salt-like halides, whereas the dichloride and dibromide are more closely allied to the halides of the non-metals and metalloids. As might be expected from certain theories of solution involving similarity of constitution between solvent and solute, the most effective solvents for germanous chloride and bromide appear to be the corresponding tetrahalides.
I820
F. M. BREWER
The solution of the colorless dibromide in germanium tetrabromide is yellow in color, and it is possible that the colorless solid is a polymer, while the yellow solutions contains the unassociated GeBra. This polymerization is seen in the case of stannous chloride which near its boiling point has a vapor density that indicates the presence of molecules of the polymer SnzC.14. The Significance of the Chloroform Series.-Of still greater significance are the chloroforms of the five elements. The reactions between the known dihalides of the fourth group and the corresponding hydrogen halides present a most fascinating, as well as a most comprehensive, picture of the valency relationships in the group. Chloroform is essentially a non-polar compound. I t is extremely volatile, is soluble in non-dissociating solvents, and gives a non-conducting solution in dissociating solvents. Bromoform is similar in all respects. Iodoform is rather less stable, decomposing when heated above its melting point. Its color suggests the possibility of an internal complexity not revealed by the other two compounds. Even so, there is no suggestion of electrolytic dissociation, or that, iodoform is anything but a neutral organic compound. The physical constaits of the silicon compounds are pamllel to those of the corresponding carbon derivatives. On the other hand, the chemical characteristics of these compounds implies a greater differentiation between the atoms attached to the carbon or silicon than would be expected from simple derivatives of methane. I t is only necessary to consider chloroform itself. Dumas first pointed out the relation of chloroform to formic acid, which, in its reducing power, and in its intimate connection with carbon monoxide, is in direct contrast to the saturated hydrocarbons. Since alkali converts chloroform into alkali formate, it is to be expected that alkaline oxidizing agents will be reduced by chloroform. But chloroform reduces potassium permanganate and potassium dichromate in acid solution almost as well. The oxidation products are phosgene and chlorine, which are essentially the oxidation products of carbon dichloride and hydrogen chloride respectively. These two compounds are probably the primary products of the thermal dissociation of chloroform vapor. Even more startling is the action of a concentrated solution of silver nitrate upon iodoform, which results in the cold in the formation of silver iodide and carbon monoxide. The latter may be regarded as a direct hydrolytic product of carbon di-iodide. Instances might in fact be multiplied to show that compounds of this type react in the majority of cases as if they underwent a primary dissociat,ion into a dihalide and hydrogen halide. As regards the analogous compounds of germanium, there is no question that this dissociation is the dominating characteristic of compounds of the chloroform type. The formation of germanium chloroform from germanium dichloride and hydrogen chloride does not appear t'o be reversible, but the hydrolysis of the product yields a solution possessing all the properties associated with germanous compounds. It is a powerful reducing agent, and yields the monosulphide, and the brown, hydrated monoxide with appropriate reagents.
GERMASIUY XXIV
IS2 I
In the case of germanium bromoform, thermal dissociation begins in the neighborhood of IO', and it has not been found possible to preserve the germanium analogue of iodoform above oo except in the presence of hydrogen iodide. This compound is all the more interesting since it again forms the true connecting link between the metillic and the non-metallic elements of the group. Since it has been discovered that tin and lead form volatile hydrides, there is no a priori reason why their partially substituted halogen derivstives should not preserve a non-polar continuity in physical properties, similar for instance to that which occurs with the tetra-chlorides. An examination of the available data shows, however, that this is not the case. The hypothetical addition product of hydrogen chloride and stannous chloride is not regarded as a nonpolar body, but rather as the parent acid of the chlorostannites, which are perfectly definite complex salts. By crystallizing stannous chloride from concentrated hydrochloric acid hydrates of the complex acid are said to be formed. The existence of SnIz . HI as a solid ph.3se has been demonstrated by indirect analysis, although the compound itself was found to be unstable. Finally, in the case of lead salts, the only connection is to be found in thc solubility relationsl$ps. The solubilities of the halides of lead are a t first decreased by the addition of the halogen hydracid to their solutions, owing to the common ion effect, but are eventually increased with increased concentration of acid owing to the formation of more soluble complexes. With the extensive range of variation that this series affords, yet marked nevertheless by such regular gradation, there can be little doubt that in thc nature and behavior of the germanium compounds lies a very important key to a generalized theory of chemical combination, KO attempt will be made to develop any such theory here, but there is one question involving general principles which may well be discussed at this point. The tetravalency of carbon in chloroform has long been tacitly accepted, and by analogy one would expect germanium chloroform to behave as though it contained tetravalent germanium. Actually il is much more intimately related to thc dichloride than to the tetrachloride, and the problem consequently arises R S to the true valency of germanium in this compound. Valency is an electronic function, and there are only two main possibilitie.. Electrons niay be transferred completely from one atom or group of atoms to another, combination being effected by simple electrostatic attraction between the charged systems so formed. This constitutes combination by polar or electro-valenay. In the case of non-polar valency, electrons may be influenced simultaneously by two or even more nuclei, in which case it is impossible to dissociate one nucleus from the other without altering the properties of both. An ion, on the other hand, can act as an independent entity. I t is therefore clear that the criterion which enables the distinction to be made between these two types of combination is that of electrolytic dissociation. Kevertheless it cannot always be said that a substance is per se an electrolyte, for ionic dissociation is determined by circumstantial conditions, and a compound
I822
F. 11. BREWER
which is dissociated in water niay give a non-conducting solution in a nonpolar solvent. I t will be obvious that non-polar valency niust include all those phenomena designated previously by such terms as co-valency, residual valency, or niolecular combination, and though the actual mechanics of non-polar valency is not yet understood, there cannot very well be any sharp differentiation between one kind of attachxiient and another in this general class. There must indeed be possible continuous variation in the strength of such attacfinients until the minimum limiting value is reached a t which ionic dissociation \vi11 occur. Regarded in t'his light, the question of the constitution of germanium chloroform is conaideribly siniplified. I t is not essential to decide between a formula GeC1,H (a derivative of tetravalent, germanium) and the molecular compound GeClz . HC1. The hydrogen and the chlorine niust be held by some attractive force, whether it is called residual valency or by any other name. and it is highly iniprobable that the t\yo chlorine atoms should be responsible for this attachment, with unsaturated germanium present. In other words, all four atoms are probably attached to the gcrnianium by non-polar bonds. To explain the reactions of the compound it will thcn be necessarJ- to show why two of the bonds are so very weak; and not only why they are weaker than the other two, but why they are weaker than the corresponding bonds in the chloroforms of csrbon and silicon, and finally why this differentiation is not shown to any extent in the tetrachlorides of all three elements. The answer depends upon two principles, one of which is reasonably obvious and generally accepted. I t is that in any group of the Periodic Tableand for this purpose the subgroups must be regarded as indepcndent of each other-the tendency to form positive ions increases with increasing atomic number. The tendency to form non-polar compounds will simultaneously decrease. The second principle is less obvious and its effect is of a secondary nature. I t appears therefore only in the comparison of those compounds which are sufficiently similar to render other influences approximately constant. Briefly, it niay be stated as follows:-that, other things being equal, there is an intimate correlation between symmttry and stability. Isolated cases have long been recognized. The stability of the inert gases is ascribed t 3 the symmetry of their electronic constitution. The stability of such structures as the paraffins of the benzene ring are fundamental axioms of organic cheniistry and the disturbing influence of single substituents in otherwise COIIIparatively inert molecules is well-known. But the principle is worthy of even wider application, and a noteworthy instance is seen in the case of germaniuni chloroform. In the first place, it explains why the reactions of the chloroform are essentially those of the dichloride plus hydrogen chloride. Every resction which has been studied so far has permitted the substitution for the hydrogen and one atom of chlorine either of two similar atoms or groups of atoms, or else of one doubly-bonded and therefore symmetrically placed single group. Either of these arrangements is more symmetrical than the original one, and
therefore the chloroform tends to lose hydrogen chloride so that they can bc est.ablished. It thus reacts as if it were dichloride. If this were not the case the bromination of germanium chloroforin should yield hydrogen broniide :iccording to the equation 2 R r I GeC1,H = GeCI3Rr 1 HBr But it was observed in this Laboratory that the evolved gas was hydrogen chloride, and that the resulting germanium coinpound contains both chlorinr and bromine. The only symmetrical product mhich could here be formed is (:eC'llRr?. The fact that hydrogen chloride is niorc stable than hydrogen hroniide no doubt assists. The relative stability of the chloroforins of carbon, silicon, and germanium is fully explained by the first principle involved, nzmely, that of increasing tendency to form the ionized, rather than the tetravalent or non-polar t'ype of compound, but the value of the second principle is again manifest when it is recalled that the series o f tetrachlorides is iiiaintnined from carbon to lead, 3s is the series of the trtrrih>-drides. Tet the chloroform series ceases to exist, at least a s a non-polar series, with the elenient gernianiuni. But for any given elenient, the tetrachloride a n d the tetrahydride, being syinmetricsl, should be more stable than the unsymnietrical chloroform. It is of interest to note that chloroform is reduced by zinc in alcoholic solution to the more symmetrical dichlorniethane. The isolation and investigation of the silicon and germaniuni analogues of this compound, though undoubtedly difficult, woiild afford an excellent test of the validity of the t heorj-. Itlinca. .\-m I-oTk.
