Dwight A. Payne, Jr. and Frank Hall Fink' Tulane Universiv N e w Orleans, Louisiana
Electronegativities and Group IVA Chemistry
T h e chemistry of the elements of periodic group IVA are of considerable interest in a number of ways, but especially so in terms of pedagogy. Whether at the elementary level or that of advanced instruction the teacher of chemistry is involved in a fascinating game which remains basically constant, yet where rapid innovation and dexterity are rules rather than exceptions. A fresh look at the chemistry of the renresentative elements listed in neriodic erouu IVA is j k t one interesting example. The teacKer df inorganic chemistry should be obligated to present his subject as an intellectual and empirical form of iuvestigation rather than as a -ertles, rzactions, and bond tvpes. The comparative chemistry of the elements of periodic grouu. IVA and their groups offers much fuel relation to the other for this kind of endeavor. To begin at a point which for a long time has been deemed typical of inorganic chemistry, it is appropriate to gather and display some experimental data of the elements in group IVA. After we have a little - - -
'Part of the dissertation, A Study of Grwup IVA Organosulfur ChemGtry, by Frank Hall Fink, Tulane University, 1966. 2 R. T. SANDERSON, Chernieal Pe~iodicity,Reinbold Publishing Co., New York, 1960, pp. 22, 23, 26, 28, 30, and 34. a Ibid., Chapter 2, p. 16.
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starting material, we can then take a look at lrhat has been collected. The accompanying table2 represents only a hare beginning, but is enough for a start along the trail. Notice that only in the two kinds of radii is there a smooth or regular transition observed from carbon through lead. These distances are obtained by utilizing half of the atom-to-atom distance in nonpolar covalently bonded systems (i.e., element-to-element single bonds where there is no polarity) and in atomTable of Data First ionization energies in kcel/mole Average ionizstionlelectronto produce 'maximum oxidation state in kcd/mole Nonpolar covalent radii in ang~tromunits Metallic atomic radii in angstrom units Averam electron den-
C
Si
Ge
Sn
Pb
260
188
187
169
171
852
595
599
538
556
0.77
1.11
1.22
1.41
1.47
0.914 1.316 1.366 1.620 1.746
us;& the nonpblar covalent radii 3.11 Electronegativities, (Sanderson's SR con&ted to Pading's scale) 2.47
2.46
4.20
4.28
6.16
1.74
2.31
2.02
2.01
to-atom distance in metallic crystals where there is a coordination number of twelve. Generally, when one looks at properties representative of the kind of atom itself, one observes some kind of alternation in these properties in going from carbon to lead in the group IVA. Thus, when one considers what is actually contained in the atomic volume, defined by those radii listed in the table, alternation of the contained quantity is the rule. If, for example, one looks at either mass or electrical charge density one finds a very distinct alternation (see the fourth row in the table to note the alternation in electrical charge density between carbon and germanium), particularly between carbon and germanium. Since the electronegativities as defined by Sanderson3 are deduced by a consideration of actual electron (charge) density compared with a hypothetical ideal electron density, electronegativities also show distinct alternation from carbon through lead. Alternation also occurs in group IIIA, VA, and in other portions of the periodic table and is not uncommon. A look at first-ionization enernies (see table) s h m alternation coming at the heavy end of the average ionization energy per electron nation at germanium and at the heavy end of the g Whenever an atom is put into a ment other than one with like individual atoms, t,h resultine combination has net sarily, and not usually, resembling any of the individual atoms that make up the whole. We would thus not expect to be able to describe precisely the entity i terms of any one characteristic of any one kind of ato in the entity. However, just as we get useful result. from simple linear combinations of atomic orbitals in the treatment of molecular orbitals, so we can get very good and very useful results in a large number of cases in comparative chemistry by looking at the properties of the central atom in a series of analogous molecules. Where properties of central atoms appear to be reylar within a periodic group, we tend to observe regular rhanges in molecular properties where it is possible to compare an analogous series of molecules varied only by the kind of central atom. Likewise, where alternating properties are oh~ervedin the central atoms we can often favorably compare molecular properties in a series of analogous molecules on the basis of alternation. Of course it is not always easy to distinguishpredorninat- ' ing factors within a series, hut by carefully observing the available experimental data, rather than just collecting it, we can often do a better job of correlation and prediction than we may have first thought possible.. One can now consider some analogous series of compounds of the group IVA elements along with their properties and data in order to see what resonable correlations can be made. First look at the compounds corresponding to MH, (where h'I = C, Si, Ge, Sn, and Pb) and some of the simple alkyl derivatives of these compounds wherever there is sufficient data or information that can he used. For example the thermal decomposition temperatures have been measured and are found to decrease regularly from carbon to lead in the MH4 compounds: 800, 450, 285, 150, and O°C for CH,, SiH,, GeHn, SnH,.., and P ~ H. L resnectivelv. One minht correlate simple single bond lengths here as being of perhaps prime value in reflecting these decomposition values.
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This measurement, in general, probably reflects the I\?-H bond strength as related directly to bond length; in the C-H case, of course, very close approach is possible and a high bond energy is reflected in the decomposition temperature of 800°C, some 350°C above any of the other values. Also, there is greatest. similarity in the sizes of the remaining four kinds of atoms after carbon in the group IVA, and nearly the same difference in thermal stabilities are noted. If one now compares reactivity of this same series of compounds towkrd oxidizing agcnts and tbward positive hvdroeen com~ounds(such as water]. . " ,, e € ~ . one can observe a distinct alternation in these properties for the MH4 cdmpounds ot carbon through lead. One does not need quantitative reactivities in order to observe the obvious alternation. Note that the C-H bond is essentially nonpolar; there are also no orbitals okenerev levels in t,his molecule redily available to coordination with other valence electrons and methane is quite unreactive unless stimulated with a great
some degree, and presumably give to their molecules a part, at least, of this orbital availability; we find that, in general, all of the other MHI compounds are more reactive toward oxidizing agents and toward positive hydroaens than methane. Of course, there is then the distinct alternation in reactivity which is observed. silane 1s quite readily attacked at room b? both water and air (molecular oxygen), however& germane resembles methane very much in that neither -; 6li7X1e3 Tor ac~ds(positive hydrogen) attack at room O*. temperature and neither do oxmzing agents such as molecular oxygen. At SnH, we notice behavior more like germane, onlv to a lcsser extent. Stannane is more easily disturbred at room temperature h mild oxidizing agents, e . g x ( I I ) and by some of the positwe y ro gen compounds than is germane. Perhaps we have not enough information about plumbane, but it appears to resemble stannane in its reactivity with positiv hydrogen compounds and with mild oxidizing agents. Nonetheless there is an unmistakable alternation hetween methane reactivity and germane reactivity; there also are more subtle differences between the germane, stannane, and plurnhane. If one wishes to see more involvement in the alternation than just bond polarity, the electronegativity values of Sanderson offer an excellent correlation with the experimental chemistry of these compounds. The simple tetraalkyl substituted hydrides of group IVA also show these similar alternations in reactivities. Any series of the IVA tetrahalides offers another reflection of the influence of central atom properties on alternation of properties in the series of compounds. It is realized that the tetrahalides offer a more complex system than the hydrides in that morevalence electrons and orbitals may be involved. Even the molecular crystals of any of the four series of tetrahalides show a sharp discontinuity in crystal energies at silicon wherein the alternation suggests that the germanium com-
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pounds are more like the carbon compounds than are the silicon compounds. A comparison of the hydrolysis reactions of the tetrachlorides, for example, immediately shows alternation in the series. From carbon to silicon to germanium it is observed: the carbon tetrachloride is essentially inert toward water under any ordinary conditions; silicon tetrachloride is rapidly and irreversibly hydrolyzed . . at room temperature; and germanium tetrachlonde is much less reactive than the silicon tetrachloride, and hydrolysis can he reversed by increasing the concentration of hydrogen chloride. Admittedly, these tetrahalides represent more complexity than the simple hydrides, hut the predominance of alternation of properties of the group IVA atoms is clearly visible. Normal boiling points, which in themselves reflect a combination of properties of the atoms and of the bonding involved in and between molecules, alternate in each of the tetrahalide series. Only recently preparation has been announced4 of a new series of compounds of silicon, germanium, tin, and lead where only the central IVA atom differs in the series. These compounds also exhibit a novel kind of optical activity (not appropriate to discuss in detail in this paper). It has been found that the extent of molecular rotation at the wavelengths.investigated can be correlated with differences in the polarizability of the central group IVA atoms; these also show alternation. The reactivities of the compounds in hydrolysis and in reaction with molecular oxygen have been observed. These also alternate. The silicon compound resembles the tin compound, the germanium and lead compounds are similar and exhibit low reactivity and high chemical stability compared to the silicon and tin compounds. If the carbon analog were known, we feel confident that its properties would more closely resemble the germanium compound than any of the others. The compounds are shown below along with the summary of their preparations in the form of the preparative equations:
The product compounds were all quite soluble in anhydrous acetonitrile. Percolation of acetonitrile solutions of these compounds through about 30 cm of d-quartz in each case effected a t least a partial resolution. Primarily on the basis of the ultraviolet absorbance and optical rotatory dispersion data, the following structure was proposed:
where the four M-S bonds are in a tetrahedral arraugement. At least two of the bonds would be diierent through the influence of the metal electrical fields on the ligands and thus account for the optical activity. The rotations observed were also of a magnitude not previously reported. The essence of this recent work that is related to this paper is that chemical reactivities and other properties of this series show distinct alternation, with the germanium compound more similar to the lead compound and the silicon compound resembling the tin analog. If one stays away from molecules and reactions that are too complicated for the identification of predominant causative factors, quite plainly, alternation is the rule for group IVA rather than the exception. A systematic study of the chemistry can be made and predictions borne out by using this kind of comparative chemistry. I t is striking that Sanderson's concept of electronegativities, which are so very useful and which correlate so much periodic chemistry compared to other methods of studying comparative chemistry, has provided fine coordination between predictions and observations in our study of these group IVA compounds. We realize no one concept is a panacea for the interpretation and prediction of all of chemistry. Sanderson's concepts of electronegativity necessarily will suffer breakdowns in several instances and especially when one attempts to utilize the idea of equalization of electronegativities in some large molecules and ions, but as a tool of correlative and predictive value, it is one of the best to come along in some years. The person who is interested in teaching chemistry or writing textbooks for students would do well to consider Sanderson's ideas more closely. Periodic chemistry needs to be used and an excellent tool for so doing is close at hand. FINK,F. HALL,TURNER, J. A,, Amer. Chern. Soe., 88, 1571 (1966).
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PAYNE,D. A,, JR., J.
