Pauling's Electronegativity Problem and Zhang's Improvement

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The main disadvantage of Pauling electronegativity [1-10] 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. Pauling proposed[11]to discuss the properties of compounds of elements of different valence to illustrate if the Zhang electronegativity is useful. Over decades, let us see what is the result. 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 [12]. 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 [13] . 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 [14]. 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 [15]. From IC model data (IonocovalencyChart) we can see that S6+ has a greater ionicity than that of C4+: Iav (S6+ = 46.077, C4+ = 37.015), although they have the close spatial covalency, n*rc-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. [7], 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 [16], which give information on the energy difference between valence orbitals, not on their spatial overlap. The arguments led to the opposite assumption of reference [17] 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 [1,8]. 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 Iav of 24.89 and 27.02 respectively (Ionocovalency Parameters). The bond lengths are equal due to they have similar linear covalency rc-1 of 0.745 and 0.745 respectively. The big difference is the spatial covalency, n*rc-1, in I(Iav )C(n*rc-1) = n*(Iav/R)½rc-1. The Ta 5d orbitals, compared to Nb 4d orbitals, involved the greater spatial covalency, n*rc-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 XIC: Ta5+ (2.197) > Nb5+ ( 2.053).

Mössbauer Parameters δ and Δ

As the IC model, n*(Iav/R)½rc-1. is defined as ionocovalent density of the effective nuclear charges at covalent boundary, it strongly related with the Mössbauer parameters δ and Δ. [18.19]. The value of the isomer shift,δ, 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 δ falls as the number of d electrons in the iron atom falls. Mean values of δ [20], Z* and IC for some oxidation states of iron are shown in Table 1:

Table 1. IC, Z* and δ for Iron-57.

Iron-57	FeI	FeII	FeIII	FeIV	FeV
δ/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” [21]. 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(Iz, Iav). 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 Iav:

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	2.68	3.44	3.78	3.16	3.81	4.27

REFERENCE

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