August, 1959
THEELECTRONEGATIVITY OF GROUPS
1227
Although our value of 1.35 X lo-’ cm.2 sec.-l is not free from error due to the deviation of the slopes of the anodic and cathodic curves from their expected talues, yet it is considerably larger than those for metal cations. This eliminates [log Tanodic - 2 log Tcosthodic] log i-0 the possibility for the mercurous ion to be the difwill be equal to 2 log D’/z7r1/%/26,which enables the fusing species. determination of the diffusion coefficient of the speIt is therefore suggested that the diffusing species diffusion through the amino complex, if 6 is cies during the anodic passivation of mercury is known. From experiments conducted a t 75, 100, either oxygen or hydroxyl ions. A similar concluand 125 pa./electrode, an average value of 0.025 f sion was reached a t by Okamoto, Kabota and Na0.001 cm. for 6 was measured optically by means of gayamaI2 in their investigation on the corrosion of a cathetometer. The intercept of the anodic curve iron. These authors reported that oxygen is the was found to be -4.17 while that of the cathodic diffusing species through the oxide. The above line was -0.20. Therefore, D was calculated to be conclusion is further substantiated by the experi1.3.5 X lo-’ cm.2 set.-'. mentally observed fact that the thickening of the Diffusion coefficients of metal atoms through oxide film proceeds from the inside rather than from their oxides a t room temperature were calculated to the outside surface as indicated in part (I)’of this and 10-50 cm.2 sec.-l.ll range between series.
log i plot for the oxidation process (which is diffusion controlled), would make an intercept a t log i = 0 of 2 log nFCD1/~&/2. Therefore,
(11) M. T. Shim and W. J. Moore, J . Chem. Phys., 2 6 , 802 (1957); E. A. Secco and W. J. Moore, ibid., 26, 942 (1957).
(12) G. Okamoto, H. Kabota and M. Nagayama, J. Electrochem. Soc. Japan, 2 2 , 18 (1954).
THE ELECTRONEGATIVITY OF GROUPS BY A. F. CLIFFORD Contribution from the Richard Benbridge Wetherill Laboratory of Chemistry, Purdue University, Lujuyette, Indiana Received September 8, 1058
The electronegativity relationships presented in a previous paper have been consolidated and regularized to give the general relationship for all covalent substances MX,(s) yielding ions in water solution, pK.,/( I n) = 14.8 - 8.5A.2,where n is 1 to 4, X is any uninegative group and Ax is the difference in electronegativity between M and X. Likewise, for dinegan) = 48.7 37.5Ax. Each negative group except OH has been found tive groups in M,X,, the relationship is pK,,/(m to have a characteristic group electronegativity, usually close to the average of the electronegativities of its individual atoms, which is constant in compounds with metals of varying valence. The apparent electronegativity of OH decreases with increasing metal valence but is constant for constant valence. Group electronegativities for positive groups have also been determined and have been found to approximate the electronegativity of the metal in the group.
+
+
Although much work has been done on the relative electron-withdrawing power of organic groups and inorganic groups (such as -NOz) frequently associated with organic compounds, very little attempt has been made to estimate the group-electronegativities of inorganic groups, although there are some data in the literature from which inferences can be drawn. Thus, when the halogen is found to constitute the negative end of the NCBr molecule, but the positive end of the ICN molecule, it must be assumed that the electronegativity of the covalent cyano group lies between that of bromine (2.8) and that of iodine (2.5). It is interesting to note that the average (2.75) of the electronegativities of carbon (2.5) and nitrogen (3.0) does in fact lie between these two values. Likewise’ from the hydrolysis of NOzCl which yields HOCl and “ 0 2 , i t may be inferred t8hatthe NO2group is more electronegative than C1 (average of NO2 = 3.33). The hydrolysis of NOCl to HC1 and H N 0 2 cannot be taken as evidence that the NO group is less electronegative than CI, since, if HOCl and HNO were formed, reaction between them would be expected to produce HC1 and HN02. Since in a previous paper2 it was shown that for insoluble binary sub(1) E. 9. Gould. “Inorganio Reactions and Structure,” Henry Holt and Co., Ino., New York, N. Y..1955, p. 241. (2) A. F. Clifford, J. Am. Chem. Boc,, 79, 5404 (1957).
-
stances of considerable covalent character, the solubility in water solution t o give ions obeys the relationship p K B p= A
- CAX
where Ax is the difference in electronegativity between the elements in the binary compound and A and C are constants for any given series of substances, it was felt that for polyatomic inorganic groups forming insoluble “salts” of sufficiently covalent character a similar relationship might hold which would give a good indication of the electronegativities of the groups. The justification given in ref. 2 assumed that for the relatioiiship between pK,, and Ax to be linear, Ax must be less than 0.5. Nevertheless, abundant evidence was offered that the linearity of the relationship held for metal hydroxides in which the Ax between the metal and oxygen was as much as 1.5. However, if it be realized that in actual fact the more electronegative species is the hydroxyl group with an electronegativity characteristic o l the group and considerably less than that of oxygen itself, the discrepancy becomes much less. The Insoluble Halides and Hydroxides.-From the discussion in ref. 2, it can be seen that for the dihydroxides and dihalides for which the data are the most abundant the functions for the relation-
A. F. CLIFFORD
1228
ships between pK,, and Ax differ only in the intercepts, the slopes being almost identical. Consequently, by eliminating the data for Pt(OH)2, Be(OH)z and Sn(OH)2,it is possible to reconcile the data for both groups of compounds by means of equation 1 pKup = 46.4
- 24.4A~
(1)
the electronegativity of the hydroxyl group being 2.75. I n the same way, if the data for the cuprous compounds (as well as AuCl and AuI) be rejected, the relationship for all compounds MX becomes pK.p = 29.0
-
18.182
(2)
Vol. 63
FREEENERGIES
OF
TABLE I FORMATION A N D CALCULATED pKsp
VALUESFOR AuXa
THE AURICHALIDES AFfO P K SP
Au(OH)a -69.3“ 53.0 AuFa -69.6b -9.4 AuCls -11.6“ 24.5 AuBra - 5.9” 35.4 AuI~ 16.3d 46 a Reference 5. a A. A. Woolfe gives AHrO = -83.3 kcal. per mole for AuFa. The free energy was calculated from the free energy function given by Brewer.? Since the complex fluoride AuF4- has been shown* to be covalent, inclusion of AuF3 as a “covalent” halide was deemed justified. 0 Reference 3. By comparison with other noble metal halides Brewer’s estimate? of - 14 kcal. per mole for AH2$*for AuIl seems too high. Consequently a value of -20 kcal. per mole was used with Brewer’s free energy function to calculate AF2$Bo and the pKUpfor AUIBgiven in Table I .
-
However, the pKsp’sof the hydroxides (including the alkali hydroxides, which agree unexpectedly well) give the electronegativity of the hydroxyl group as 3.1 in this case. The data for the cuprous halides indicate the electronegativity of Cu(1) to and Ir+3. The free energy of formation of T i c & be 1.65, which is more consistent with the value of was calculated from the heat of formation given by 1.7 given for Cu(I1) in ref. 2. This relationship Woolfe and the (AF - AHZss)/T value of Brewer.’ From these values the electronegativity relationpredicts the pKsp’sfor AuCl and AuI t o be 12.7 and ship for MX3 is calculated t o be 22.8, respectively. pKsp = 52.9 - 35.5A~ (4) The pKSp’sof the trihalides considered in ref. 2 were all calculated from free energy data reported Fitting the pKsp’sof the trivalent hydroxides to eq. by Latimer.3 However, since in no single case was 4 gives an electronegativity value of 2.15 f 0.02 for the free energy of formation of the trivalent cation the hydroxyl group in M(0H)o. ( A u + ~Ir+a, , R h + 9 known with any accuracy, the Because of the paucity of good data on either the pK,,’s used were highly doubtful. halides or the hydroxides of the tetravalent eleExamination of the data available for the trihal- ments, the treatment of the substances MX4 can be ides sufficiently covalent to obey the electronega- only highly approximate. (The relationship given tivity relationship revealed that the auric halides in ref. 2 was assumed to be invalid in view of the are the most promising. The free energy of A u + ~ electronegativities for Th(1V) and U(1V) given by given by Latimer (103.6 kcal. per mole) was calcu- Gordy and Thomas9 which differed appreciably lated by him from the solubility of Au(OH)3 in from those used in ref. 2.) The Ksp’sof Sn(OH)4, 0.43 m nitric acid as reported by Jirsa and Jelineke4 U(OH)4 and Th(OH)4 were accepted as given by This calculation was made apparently on the as- Latimer.3 From the free energy for Pd(OH)4 and sumption that there would be no hydrolysis of Auf3 the potential for the Pd+2-Pd+4 couple given by in acid of this concentration. For an element as Latimer the pKsp of Pd(0H)r was calculated to be electronegative as gold, however, this seems a very 70.2. The pK,, for Ge(0H)c (ie., GeOz 2H2O) dubious assumption, and the ion in solution was was calculated from the free energies given by Latimore probably A U O H + or ~ even a more highly hy- mer and the experimental potential for the gerdrolyzed species. The p K s p of Au(OH)3 should manium couples reported by Everestlo from which therefore be larger than that (44.07) given by Lati- AFfO for Ge+4was calculated to be - 12.0 kcal./mole. mer. From these data, the curve for the tetrahydroxides Examination of the relationship given in ref. 2 was calculated to be for the trihydroxides
+
pKSp = 8G.1 - 27.4Ax
(3)
pKap = 133.1 - 4 3 . 8 A ~
(5)
takingxm = 3.5. reveals that the only trihydroxide falling far off the In order now t o establish the true XOH for this serline is Au(OH)~,and that because of too small a ies, and the true value of the pK,, intercept, pK,,. If Au(OH)~be now placed on the line with pKsp’sfor some tetrahalides were fitted to the curve. the other M(OH)3’s,its pKsp becomes 53.0. Now The chlorides, bromides and iodides of germanium accepting the Bureau of Standard’s value5 for the family elements could not be used because they are free energy of formation of A U ( C H ) (-69.3 ~ kcal. liquid rather than crystalline. The only series of per mole), the free energy of formation of A ~ + ~ ( a q crystalline ) tetrahalides promising to be covalent becomes 116.1 kcal. per mole. The pKsp’sof the enough to obey the electronegativity relationship auric halides calculated from this value are given in and for which sufficient data were available were Table I. the platinic halides. However, free energies of forThe pKsp’sof BiC13 and IrC13were calculated in mation were known neither for Ptf4 nor for the same manner by fitting Bi(OH)s and [‘/z IrzOa (6) A. A. Woolf, J. Chem.SOC.,4694 (1954). HZO] to eq. 3 to obtain the AFfO’s for Bi+3 (7) “The Chemistry and Metallurgy of Miscellaneous Materials.
+
(3) W . M. Latimer, “Oxidation Potentials,” 2nd Ed., Prentice-Hall, Inc., New York, N. Y., 1952. (4) F. Jirsa and H. J. Jelinek, 2. Elektrochem.,S O , 286, 535 (1924). (5) F. D.Rossini, U. S. Bureau of Standards, Research Paper 686, 1934.
Thermodynamics,” edited by L. L. Quill, McGraw-Hill Book Co., Inc., New York, N. Y., 1950, Chap. 6,p. 99. (8) W. Klemm, Bull. doc. chim. France, 1325 (1956). (9) W.Gordy and W. J . 0. Thomas, J. Chem. Phvs.,24,439 (1956). (10) J. C. S. Everest, J. Chen. Soc., 660 (1963).
*
THEELECTRONEGATIVITY OF GROUPS
August, 1959
Pt(OH)4. From the potentials given by Latinier AFfO for PtOz was calculated to be -17.5 kcal. From Gordy's electronegativity for platinum (2.l ) , the pKspfor Pt(OH), was calculated t o be 71.8 from eq. 5 (for which X O H = 3.5). If now the free energy of Pt(OH)4be taken as equal t o the sum of the free energies (PtOz 2Hz0), then AFfa(Pt+4) = 120.5 kcal. The pK,,'s for PtCl,, PtBr4and Pt14become then 28.1, 40.5 and 68.2, respectively. The electronegativity of the hydroxyl group for the series MX, becomes, therefore, 2.15 f 0.1, the uncertainty arising from the halides. The relationship then becomes
+
pK,, = 75.0
- 43.8A~
p K s p = 124.8 - 1OO.OAX
(7)
Additional information available for AlzS3 and AlzSe3,11for Biz&, Bi2Se3 and BizTe3lZand for In2S3laalso makes possible a re-evaluation of the relationship for the sesquichalcogenides, yielding the relationship pK,, = 235.1
- 171.7A~
(8)
The relationship developed for the monochalcogenides in ref. 2 was accepted without change. p K S p = 102.5 - 80.GAx
Comparison of the Relationships.-The
in equations 2, 1, 4 and 6 and in equations 7, 9 and 8 can now be seen to have an even greater degree of regularity than was indicated in ref. 2. For each series the value of the const,ants in the individual equations divided by the power of the activities in the corresponding I
