Notes
167
with 10 M HNOs. In qualitative tests, unfired Vycor showed a higher capacity for adsorption of zirconium and niobium and a greater potential for separating the two elements than did fired. Unfired Vycor has been used to remove niobium-95 from nitric acid solutions containing zirconium carrier and zirconium-95. A stock solution was prepared which contained 20 g/1. Zr, ~ 1 0 ~ c/min per 10 M HNO 3
6M
~
8M
5o
o
t
I
I
I
I
12
20
28
36
44
THROUGHPUT, rnl
Fta. 1.--Separation of niobium-95 from zirconium-95 in nitric acid solutions. Adsorbent:
0'5 g of 100-200 mesh unfired Vycor glass powder (No. 7930 Coming Glass Works). Column size: 8 mm high x 10 mm dia. Zr, c/min per ml: ,--2'5 x 10~. Nh, c/rain per ml: --~4"5 x 10s. Solution flow rate: 0.2 to 0.7 ml/min.
ml Zr-95, ~ 2 x 106 c/rain per ml Nb-95, and 50 g/1. 100 mesh unfired Vycor glass. Prior to use, 10 ml of solution was decanted from the glass and passed twice through a column 13 mm high by 10 mm dia. containing 1 g of finer than 100 mesh unfired Vycor. Next it was passed twice through a second identical column. The activity in the effluent was 95 to 99 per cent zirconium, and < 1 per cent of the zirconium had been removed from the solution. Acknowledgements--The author expresses his appreciation to the personnel of the Analytical Chemistry Division of Oak Ridge National Laboratory for analytical support. Chemical Technology Division Oak Ridge National Laboratory Oak Ridge, Tennessee
J. G. MOORE
Electronegativities of carbon, silicon, germanium, tin and lead--H (Received 21 February 1961 ; in revised f o r m 16 June 1961)
IN a recent paper ~1~in this Journal, R. S. DRAGO attempted to show that the electronegativities of silicon, germanium, tin, and lead are the same or decrease slightly. In an earlier paper, ts~ we presented extensive evidence for the alternation of electronegativity values within Group IV-B. We wish to comment upon the claims of DR. DRAOO. ~tl R. S. DItAGO, d. Inorg. Nucl. Chem. 15, 237 (1960). lzl A. L. ALLREOand E. G. ROCHOW,J. Inorg. Nuel. Chem. 5, 269 (1958).
168
Notes
We reported ~2~an electronegativity value of 2.45 for lead and DRAc,o commented: TM "Lead, the larger element with the smaller ionization potential, is more electronegative than silicon. The following paradox arises: The more electronegative element lead is more metallic in its properties and reactions than the less electronegative element silicon. There is obviously something wrong." The difficulty here is that a mental picture of metallic lead is being brought into the issue, although the question originally was confined c~ to the sp 3 valence states of the Group IV elements. Indeed, a considerable discussion of orbital electronegativity~> was necessary in order to make this important point. Lead in the zerovalent or bivalent state is quite different from its congeners, by reason of the inert pair of s electrons. This tendency to hold onto its s electrons deprives lead of a tetracovalent diamond-structure allotrope which we might compare with elementary silicon. Since the ionization potential and electron affinity values for valence states must be employed in the method of MOLLWO~Nand many of these values are not available, we did not calculate electronegativities from ionization potentials. DV,AC,O writes: ~1~ "Although the electron affiity of lead is not accurately known, a reasonable estimate gives lead a smaller electronegativity value than silicon. The series then becomes C > Si > Ge ~ Sn ~ Pb." The publication of the source and magnitude of the "reasonable estimate" would be informative, tS~ DRAC,O mentions, apparently in support of a decreasing order of electronegativity, the order, C >~ Si > Ge ~ Sn > Pb, obtained from an empirical relationship ~4~ involving stretching force constants. We discussed this scale of electronegativity but did not draw conclusions from it since the relationship predicts c4~ the electronegativity values 3.1 for gold, 2-2 for copper, 1"0 for mercury relative to 2"55 for carbon and 1.0 for calcium. Considerable space in the recent paper c~ by DRAC,O is devoted to a discussion of the " e r r o r " which arises in the calculation of electronegativity of lead from the application of Equation 1, E(A-A) 4- E(B--B) :, =
E(A-B)
--
2
(I)
where E(A-B) is the energy of the bond A-B, E(A-A) and E(B-B) are the energies of the homoatomic bonds and A is the "extra ionic resonance energy." We did not employ Equation 1. Instead, the relationship ~8~ A = --H/
(2)
n
was employed in the calculation of the electronegativity of lead. (For a discussion of Equation 2, see references 5, 6, and 7.) A recent paper ~8~ by one of us demonstrates that Equations 1 and 2 give approximately the same values for A for each of eighteen bonds. The value of the lead-lead bond energy, required in Equation 1, is not known at present, and we did not employ any estimate of it here or in our thermochemical calculations, a point which seems to have been missed. DRAGO attempted ca~ to explain several phenomena involving compounds of the Group IV-B elements in terms of orbital overlap. It is our impression that rigorous calculations of overlap energies of bonds involving Group IV-B elements have not been made. Proof that the low energy of the lead-chlorine bond, relative to the tin-chlorine bond, is due to less "overlap," as claimed in reference 1, rather than to less ionicity, must await the solution of formidable wave equations. Alternatively, ~3~We note that in an earlier paper (R. S. DRAGO,J. Phys. Chem. 62, 353 (1958)), quoted frequently in reference 1, that Dr. DRAGO writes: "The spectral data which are necessary for the calculation of the promotion energies for thallium and for the elements of the carbon family (other than carbon) have not been reported. The value of thallium was estimated from a derived relationship between the second ionization potential of gallium and indium and the promotion energies of these atoms. In the case of the elements of the carbon family it will be assumed that since the third ionization potentials are nearly the same, the promotion energies probably also are. These assumptions are very crude and more experimental work is required before a more reliable decision can be made." c4~W. GORDY,J. Chem. Phys. 14, 305 (1946). ~5~L. PAUUNO, The Nature of the Chemical Bond, Chap. 2, Cornell University Press, Ithaca, New York (1939). ~e~M. HMSSINSKY,J. Phys. Radium, 7, 7 (1946). ~v)H. O. PI~ICHARDand H. A. SKINNER,Chem. Rev. 55, 745 (1955). ta~ A. L. ALLRED,J. lnorg. Nucl. Chem. 17, 215 (1961).
Notes
169
one could argue that the bonding orbitals of lead do not overlap well with the bonding orbitals of chlorine and that the spacial distribution of the valence electrons of lead in lead tetrachloride, compared with tin in tin tetrachloride, would be determined more by the nucleus of lead. The attraction of valence electrons by nuclei is the essence of electronegativity. In other words, decreasing overlap may parallel increasing electronegativity. DRAC,O also stated c1~ that "repulsion of chlorine non-bonding electrons with the non-bonding electrons of the Group IV e l e m e n t . . , leads to bond weakening for many Ge and Pb compounds." This statement could appropriately accompany experimental measurement and/or rigorous calculation of the magnitude of the repulsion energy of non-bonding electrons. A claim ~1~ was made that the application of Equation 1 in the determination of the "covalent" bond energy of Ge--C1 the ionic resonance energy would be low due to the energy of repulsion of non-bonding electrons on germanium with the non-bonding electrons of chlorine. It should be pointed out that the delectrons of germanium may also lower the energies of the germanium-germanium bonds. The effects, if any, of non-bonding electrons should largely cancel with the application of Equation 1. For the five series, MC14, MBr4, MI4, MH3C1 and MHsBr, plots of the nuclear quadrupole coupling constant as a function of the electronegativity difference, Z~ --Zx, were linear, t2~ In reference 2 the linear relations were "attributed either to the electronegativity order carbon > germanium > tin > silicon, or to a different electronegativity order (say of PAULING, carbon > silicon > germanium = tin) and different relative contributions of s-hybridization and double bonding for silicon and for germanium and tin." We consider now, as then, that the former explanation is highly probable. DRAGO attempts to explain the chemical shifts of the series, M(CH3)4, in terms of changes in the hybridization of carbon within the series. His argument follows: The weaker the M43 bond, the larger the s character in the carbon hybrid orbital of the C - H bond, the less the shielding of hydrogen. DRAGO quoted the AH ° values 233.7 kcal/mole, 200.3 kcal/mole, and 192'7 kcal/mole, for SiC14~g~ Sicg~ + 2Cla(g), GeC141g~ -* G e ~ + 2Clalg), and SnCl4~gj ~ Sn~g) + 2Clalg~, respectively; and we report caj proton chemical shifts of 189.8 c/s for tetramethylsilane, 184"7 c/s for tetramethylgermane, and 188.0 for tetramethyltin in dilute solutions of carbon tetrachloride with respect to water at 40 Me. One may readily note that the explanation in terms of bond strengths is not consistent with these data. It has been pointed out that "there is a possible error in the extrapolation of the Sn(CH3)4 value in ALLRED'Sand Rocnow's paper. The possible error in this number, plus possible contributions from other factors affecting chemical shift, make this a most unconvincing argument for Si, Ge, and Sn. ''~'~ A new 0"5 ~o solution of Sn(CHa)4 in CC14 has therefore been prepared, and the chemical shift has again been measured, in a different laboratory. The previous extrapolated value was found to be correct. The "other factors" are unknown to us. One may correlate the chemical shifts of the tetramethyl derivatives of the Group I ¥ - B elements with the electronegativities of these elements in terms of simple inductive effects or of inductive effects coupled with isovalent hybridization. For a discussion of the relationship between isovalent hybridization and electronegativity, see reference 10. An increase in the electronegativity of M in the linkage H - C - M , leads to an increase in the s character of the orbital from carbon to hydrogen, tl°~ Therefore, according to this argument, an increase in the electronegativity of M causes decreased shielding of the proton and a chemical shift to a lower magnetic field. Indeed, our pape rt~ took the pragmatic view that if electronegativity is "the power of an atom in a molecule to attract electrons to itself" (PAULING'Sdefinition), and if N M R chemical shift measures electron density, then it is a means of comparing electronegativities. DRAGO'S statement that " a large nuclear attraction for the bonding electrons is the essence of a large electronegativity''~1~ would seem to reinforce that view. Lastly, by quoting out of context the statement of a colleague, DRAGO indicates c1~ that we are "foolhardy" to try to explain chemical behaviour by using the results of our measurements. (~ We take the view that it is the purpose of chemical theory to explain and predict chemical facts. The established chemical facts (2~ are that germanium in sp a hybrid states behaves as a more electronegative element than silicon, and no new experimental evidence of any sort is reported in DRAGO'Spaper. We believe (1) that there is no such thing as intrinsic electronegativity of an element, (2) that the concept of orbital electronegativity leads to the relative values of electronegativity for spa states of C 2.60, Si 1"90, Ge 2.90, Sn1"93 and Pb 2-45, based on a wide variety of physical and chemicalevidence, ~ R. S. DRAOO,private communication to A. L. A. ta0) H. A. B~NT,J. Chem. Phys. 33, 1258 (1960).
170
Notes
(3) that nothing is achieved by an adherence to the view that despite all evidence the electronegativities of the elements in any group should decrease monotonically, (4) that such a view belies the fact that many heavy elements (such as Ir, Pt, Au, Hg, Pd, and Ag) are more electronegative than their lighter congeners, and (5) that such deviations from a general trend actually lend individuality to the elements, and so should be emphasized as a means of encouraging further study and explanation.
Department of Chemistry Northwestern University Evanston, Illinois Department of Chemistry Harvard University Cambridge, Massachusetts
A. L. ALLm~D
E. Ca. Rocl-IOW