Zhang Lewis-Acid Strength is Derived from Ionocovalent Function

Yonghe Zhang ionocovalent theory applications (10)  


Yonghe Zhang

Inorg. Chem., 21, 3889.


Based on his ionocovalent theory Zhang proposed “A Scale for the Strengths of Lewis Acids” [1]: “The Lewis acid strengths could be composed of some electrostatic and some covalent properties. The electrostatic force between a positive charge and a negative charge is approximately proportional to Z/rk2, where Z is the charge number of the atomic core (i.e. the number of valence electrons) and rk is the ionic radius, that operate in such a way that the stability of the complex rises with the increase in charge of the metal ion and fall with an increase in its radius. Apart from electrostatic force there appears to be covalent force. Since the σ-bond is formed by sharing of an electron pair between the metal ion and the ligand, its strength is bound to increase with the tendency of the cation to take electrons, i.e., with increasing electronegativity of the metal ion involved. The electronegativity values we have adopted here are the ones in valence states that we proposed previously [2]. The othe electronegativity values as shown by f values of ref 11 in Table I, could not obtain very good results. The quantity Z/rk2 was calculated, with use of ionic radii mainly from Shannon [3] and Dean [4]. Thus the ideal results, as shown in Figure 1, are obtained by plotting Z/rk2 against Xz. The equation for classification is derived as


Z/rk2 – 7.7Xz + 8.0 =0                         (3)


and we define the function Z as the scale for strengths of Lewis acids by


Z = Z/rk2 – 7.7Xz + 8.0 =0                       (4)


The values of the scales for the strengths of 126 metal ion of Lewis acids are calculated from equation 4 and listed in Table 1.”

The Z value gives a quantitative order of relative Pearson hardness or softness for the various Lewis acids and agrees fairly well with the previous classifications [5-9].

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

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

[3] ShannonR.D. (1976). Acta Crystallogr., Sect. A32,751.

[4] Dean, J.A. (1973). “Lange’s Handbook of Chemistry”, 11th ed., McGraw-Hill, New York, pp.3-118.

[5] Pearson, R. G. (1963). J. Am. Chem. Soc., 85, 3533; (1968). J. Chem. Educ., 45, 581.

[6] Pearson, R. G. (1973). In: Dowden (Ed.), Hard and Soft Acids and Bases. Hutchinson and Ross Inc.,Stroudsburg.

[7] Klopman, G. (1968). J. Am. Chem. Soc., 90, 223.

[8] Yingst, A. and McDaniel, D. H. (1967). Inorg. Chem., 6, 1076.

[9] Ahrland, S. (1968). Chem. Phys. Lett., 2, 303; (1966). Struct. Bond., 1, 207

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