Review on the Debate of Zhang and Pauling on Electronegativity

Review on the Debate of Zhang and Pauling on Electronegativity.doc

 

 

American Huilin Institute

http://amhuilin.com,

 

Abstract: The main disadvantage of Pauling’s electronegativity is not considered the different valence of element and can not be used to the quantitative applications. Zhang proposed a electronegativity in valence states, for which Pauling failed to issue a reliable opinion. He proposed to discuss the properties of compounds of elements of different valence to illustrate if the Zhang electronegativity is useful. This paper cited several examples to release Pauling’s confusion. Results show that Zhang electronegativity or IC-model can obtain satisfactoy explanation of many chemical phenomena but Pauling electronegativity has powerless, even erroneous conclusion.

Keywords: electronegativity, ionocovalency, theoretical Chemistry

 

INTRODUCTION

 

      Pauling introduced the electronegativity concept defining it as “the power of an atom in a molecule to attract electrons to itself” [1]. And Pauling won the Nobel Prize in 1954. Pauling electronegativity concept is an approximate intuitively understanding the concept of chemical bonds, The main criticism of this approach is that it does not take into account of the different valence states,  leading to its concept ambiguous, and so it can not be applied to explain the quantitative chemical phenomena [2-10]. Mackay et al. [2] pointed out that the major difficulty in Pauling's electronegativity is that the attraction for an electron is clearly not expected to be the same for different valences of an element [2], they, therefore, edited the college textbooks incorporated the Zhang electronegativity. Zhang electronegativity is in different valences and defined as: “the electrostatic force exerted by the effective nuclear charge on the valence electron”: [3-5]

 

        X_z = 0.241 n*( I_z /R) ^½r_c^-2 + 0.775         

 

      However, according to Pauling’s review on Zhang electronegativity, Pauling was still in confusion and he wrote to Zhang that he was not able to form a reliable opinion. He needed more “discuss of some property of the elements, in various compounds, and indifferent valence states, in order to find out whether or not Zhang’s values are helpful in understanding the properties” [11].

To replay Pauling's concerns, Zhang published two papers “Electronegativities of elements in valence states and their applications” and “A scale for strengths of Lewis acids” [5], wherein 126 metal ion Lewis acids, in various compounds, and in different valence states.

    Over 45 years, Zhang electronegativity has been widely quantitatively used and developed to new ionocovalent theory [10], forming an international school [12]. Henry Taube, Nobel Laureate, reviewed in a letter to Zhang that "Electronegativity continue to be a useful concept, and becomes even more useful when it is treated as a function of oxidation state." [13]

 

EXPERIMENTAL RESULTS

 

     The following examples are cited to release Pauling’s confusion. Many chemical phenomena which involved the different valence state can be satisfactorily explained by Zhang electronegativity or ionocovalency, but Pauling electronegativity demonstrated incompetence, it can not be used for quantitative applications and even draw the wrong conclusions.

 

Carbon, Sulphur, P-elements and Hydrogen

There are some arguments about the values of electronegativities of carbon, sulphur, selenium, tellurium, iodine and hydrogen [14]. The Chart 1 shows IC values in the order:

Se²⁺ (3.146) > S²⁺ (3.121) > C²⁺ (2.998) > Te²⁺ (2.832) > I⁺ (2.530) > H⁺ (2.297)

The results are consistent with the observations that hydrides H2Se, H2S, H2C, H2Te and HI form H3O+ ions in water [15] .

As Thomas reviewed, the electronegativity of carbon and sulphur in most of the scale are almost identical. The key point, however, so far as their role as poisons is concerned, is that they differ markedly in the distance at which they sit on the nickel overlayers [16]. The calculations for these locations show that sulphur is very much stronger than carbon as a poison.

The results are also consistent with the experiment data of the dipole moment which indicates that the electron clouds on the C-S and C-I bond in the molecules CS₂ and CI₄ are close to the sulphur end and the iodine end, respectively [17]. From IC model data (IonocovalencyChart) we can see that S⁶⁺ has a greater ionicity than that of C⁴⁺: I_av (S⁶⁺ = 46.077, C⁴⁺ = 37.015), although they have the close spatial covalency, n*r_c^-1 (C⁴⁺ = 2.618, S⁶⁺ = 2.805) (Ionocovalency Parameters).

 

Retrieve of Pauling Erroneous Covalency Results

In study on the role of covalency in ferroelectric niobates and tantalites Villesuzanne et al. [18], the fact that Ta⁵⁺-O bonds are more covalent than Nb⁵⁺-O bonds is due to a larger radial expansion of Ta 5d orbitals, leading to a greater overlap with oxygen 2p orbitals. This effect is not accounted for in Pauling electronegativity scales [1], which give information on the energy difference between valence orbitals, not on their spatial overlap. The arguments led to the opposite assumption of reference [19]  concerning the covalency of Ta⁵⁺-O and Nb⁵⁺-O bonds from Pauling electronegativity Xp: Ta(1.5) < Nb(1.6).

      In their later paper, they proposed that the explicit calculation of the electronic structure give a larger covalency for Ta⁵⁺-O bonds than for Nb⁵⁺-O bonds. This result is retrieved in Zhang electronegativity scales for ions [2,3]. The results can be fairly well accounted in IC model [10]: The energies of Ta  5d and Nb 4d atomic orbitals are the same in EHTB parameters due to they have similar atomic ionicity I_av of  24.89 and 27.02 respectively (Ionocovalency Parameters). The bond lengths are equal due to they have similar linear covalency r_c^-1 of 0.745 and 0.745 respectively. The big difference is the spatial covalency, n*r_c^-1, in I(I_av )C(n*r_c^-1) = n*(I_av/R)^½r_c^-1. The Ta 5d orbitals, compared to Nb 4d orbitals, involved the greater spatial covalency, n*r_c^-1, (Ta⁵⁺ = 3.246, Nb⁵⁺ = 2.869), leading to a greater overlap with oxygen 2p orbitals and a greater IC: Ta⁵⁺ (4.393) > Nb⁵⁺ (4.043) and X_IC: Ta⁵⁺ (2.197) > Nb⁵⁺ ( 2.053).

Mössbauer Parameters d and Δ

As the IC model, n*(I_av/R)^½r_c^-1. is defined as ionocovalent density of the effective nuclear charges at covalent boundary, it strongly related with the Mössbauer parameters d and Δ. [20,21]. The value of the isomer shift,d, depends particularly on the density of s electrons at the nucleus. Therefore, in iron-57 an increase in electron density causes a negative isomer shift; since d electrons tend to shield the nucleus slightly from the s electrons the value of d falls as the number of d electrons in the iron atom falls. Mean values of d [22], Z* and IC for some oxidation states of iron are shown in Figure 1:

Table 4.2. IC, Z* and d for Iron-57.

Iron-57	FeI	FeII	FeIII	FeIV	FeV
d /mm s-1	2.3	1.5	0.7	0.2	-0.6
Z*= n*(Iav/R)½	2.624	3.245	3.997	4.896	5.684
IC=n*(Iav/R)½rc1	2.253	2.786	3.431	4.203	4.879

 

Inert Pair Effect (6s2 Elements)

The IC model based on the VB approximation intuitively appealing and determined by covalent radius and ionization energy is in accord with the relativistic effects with which contributions to the unusual chemistry of the heavier elements are two principal consequences. First, the s orbitals become more stable. The second, d and f orbitals expand and their energies are less.

For the inert pair effect in Tl(I), Pb(II), and Bi(III), the Relativistic effects can give a qualitative verbalize: “The s orbitals of the heavier elements become more stable than otherwise expected” [23]. In IC model, as shown in Table 2, the effect is attributable to the fact that the bond property in this case is controlled by the ionic function I(I_z, I_av). They are more stable in ionic compounds than in the entirely covalent form. Their IEs for forming higher covalent bonds are too much higher to form a stable hybridizing ionicity I_av:

 

Table 2. Atomic Parameters of Tl, Pb and Bi.

 

Cations	Tl+	Tl2+	Tl3+	Pb2+	Pb3+	Pb4+	Bi3+	Bi4+	Bi5+
Iz	6.11	20.4	29.8	15	32	42.3	25.6	45.3	56
Iav	6.11	13.26	18.77	11.21	18.14	24.18	16.63	23.72	30.18
XIC	1.16	1.59	1.75	1.45	1.74	1.94	1.69	1.95	2.15
IC	1.89	2.92	3.31	2.58	3.44	3.78	3.16	3.81	4.27

 

 

DISCUSSION AND CONCLUSION

 

      Cherkasov et al. concluded in their work [8] that electronegativity is the ability of an element to attract electrons on formation of chemical bonds which depends on its nature and valence state. Pauling’s definition, therefore, requires refinement of concept ambiguous.

      Fenhmm posted [24] that Electronegativity should be a potential which correlates the energy or force with respect to its environmental distance. Yonghe Zhang proposed a new finding [25] : everything exists in Ionocovalent potential, the ionic energy harmonized with the covalent environment. It correlates with quantum potential and spectroscopy [10]:

I(Z*)(n*r_c^-1) = Ze²/r = *(I_av/R)^½r_c^-1

The ionocovalent potential (IC) and its derivers IC-electronegativities can quantitatively describe possibly all chemical observations and have much more versatile and exceptional applications than the traditional electronegativity scales.

      So we can finally conclude that electronegativity should a function of oxidation state, or a function of the valence state that follows the rule: everything exists in Ionocovalent potential

 

REFERENCE

 

[1] Pauling, L. J. Am. Chem. Soc. 1932, 54, 3570.

[2] Mackay, K. M.; Mackay, R. A.;Henderson W. "Introduction to Modern Inorganic Chemistry",6th ed., Nelson Thornes, United Kingdom, 2002, pp 53-54.

[3] Zhang, Y. J. MolecularScience 1 (1981) 125.
[4] Zhang, Y. Inorg Chem. 21 (1982) 3886.

[5] Zhang, Y. Inorg Chem. 21 (1982) 3889

[6] Portier, J.; Campet, G.; Etoumeau, J. and Tanguy, B. Alloys Comp.,1994a, 209, 59-64.

[7] D. Bergmann and J. Hinze. Angew,Chem. Int. Ed. Engl. 1996, 35, 150-163.

[8] A. R. Cherkasov, V. I. Galkin, E.M. Zueva, R. A. Cherkasov, Russian Chemical Reviews, 67, 5(1998) 375-392.
[9] Lenglet, M. Iono-covalent character of the metal-oxygen bonds inoxides: A comparison of experimental and theoretical data. Act.Passive Electron. Compon.2004, 27, 1–60.

[10] Zhang, Y. Ionocovalency and Applications 1. Ionocovalency Model andOrbital Hybrid Scales.Int. J. Mol. Sci. 2010,11,4381-4406  

[11] Pauling, L. a personal letter to Zhang, February 6, 1981.

[12] InternationalIonocovalency Schools –

[13] Taube, H. a personal letter to Zhang, October 3, 1984.

[14] Li, Z.-H.; Dai, Y.-M.; Wen, S.-N.; Nie, C.-M.; Zhou, C.-Y. Relationship between atom valence shell electron quantum topological indices and electronegativity of elements. Acta Chimica. Sinica. 2005, 14, 1348.

[15] Dalian Technology Institute. Inorg. Chem. (in Chinese); 3rd ed.; High Education Press: Beijing, China, 1990; pp. 638, 804.

[16] Thomas, J.M. Principles and Practice of Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 1996; p. 448.

[17] Xu, G.-X. Material Structure (in Chinise); People’s Education Press: Beijing, China, 1961; p. 160.

[18] Villesuzanne, A.; Elissalde, C.;Pouchard, M. and Ravez, J. J. Eur. Phy. J. B. 6 (1998) 307.
[19] Ravez, J.; Pouchard, M.; Hagenmuller, P., Eur. J.Solid State Inorg. Chem., 1991, 25, 1107.

[20] Reguera, E.; Bertran, J.F.; Miranda, J.; Portilla, C. Study of the dependence of Mossbauer parameters on the outer cation in nitroprussides. J. Radioanal. Nucl. Chem. Lett. 1992, 3, 191–201.

[21] Reguera, E.; Rodriguez-Hernandez, J.; Champi, A.; Duque, J.G.; Granado, E.; Rettori, C. Unique coordination of copper in hexacyanometallates. Zeitschrift für physikalische chemie 2006, 12, 1609–1619.

[22] Heslop, R.B. Jones, K. Inorganic Chemistry; Elsevier Scientific Publishing: Amsterdam, Netherland, 1976; p. 31.

[23] Pyykkö, P. Relativistic Effects in Structural Chemistry. Chem. Rev. 2002, 3, 563–594.

[24] Fenhmm (talk) 22:47, 17 November 2013 (UTC), Talk:Electronegativity - Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Talk%3AElectronegativity

[25] Science Letter, February 22, 2011