Lenglet Review on Zhang Electronegativity and Strength for Lewis Acid
Universite´de Rouen, LASTSM, IUT, 76821 Mont Saint Aignan Cedex, France
Active and Passive Elec. Comp., March 2004, Vol. 27, pp. 1–60
Although chemical bonding is a subject which is often dealt with in terms of quantum theory, there is much to be gained by using an approach based on semi-empirical parameters which relate to individual atoms or ions. The best known of these approaches,among chemists and physicists, is that based upon electronegativity, and this has become increasingly popular in the field of solid state chemistry. Electronegativity was devised by Pauling in 1932 . for providing a measure of the unequal sharing of electrons in a chemical bond. The concept was aimed at partial ionicity in covalent compounds. The original concept of atomic electronegativity (one datum per element) has moved towards a more general one taking into account the oxidation states and orbital hybridization (several data per element). Over the years, various other methods have been proposed for evaluating the electronegativity values of the elements as alternatives to Pauling’s thermochemical Scale [2-7]. The physical meaning of electronegativity is ambiguous. As discussed by Mullay , the various scales correspond to different concepts and consequently to different units: Pauling ,(energy) ½; Mulliken , energy; Allred and Rochow  and Zhang , force; Gordy , energy per electron; Sanderson  and Portier , dimensionless.
Zhang proposed an electronegativity scale for cations in specific valence states . This author defined the electronegativity of the element in valence states as "the electrostatic force exerted by the effective nuclear charges on the valence electrons
Xz = 0.241 n*( Iz /R) ½rc-2 + 0.775
where n* is the effective principal quantum number (n*=0.8 for n=1); r is the covalent radius, R the Pydberg constant and Iz (eV) the ionization potential.
According to the Lewis concept, an acid is an electrophile which accepts an electron pair and abase is a nucleophile that acts as an electron pair donor. Lewis acid–base interactions play animportant role in understanding chemical bonds, reactions and equilibrium; yet, this concept is often inadequate for solid state chemistry. Indeed, in a redox reaction, single-electron transfers are also commonly involved. A more general concept has been proposed by Usanovitch ;
– an acid is a species which combines with anions or is an electron acceptor,
– a base is one that combines with cations or is an electron donor.
Pearson, in his theory of Hard and Soft Acids and Bases (HSAB) , proposed a classification of acids and bases on their strengths and introduced the notion of covalence and its effects on the acid properties of an ion. He classified ions as hard, soft, or borderline acids or bases and specified that a hard acid (A) will react with a hard base (B) and a soft acid with a soft base to give stable combinations according to the reaction:
A(hard) + B(hard)=AB
log K = SASB + σAσB
where K represents the equilibrium constant and Si the acid=base strength; si is a covalency term.
In an oxide, the cation will be the acid, as it can accept a partial negative charge. Essentially, soft acids and bases are those of high polarizability. Although the HSAB principle has been useful in a variety of contexts , this system of classification is still only qualitative. The explanation given by Klopman  emphasized that soft acids and bases were largely covalently bound and hard acids and bases were ionically bound.
Zhang  has proposed a numerical scale for the acid strengths of cations. The advantage of such a scale is the predictive power for thermochemical and physical properties that obviously depend on the nature of the chemical bond established between the acid cation and the basic anion. In the case of inorganic compounds, this bond is called iono-covalent, meaning that this bond involves simultaneously electrostatic forces (ionic part) and covalent forces resulting from the combination of atomic orbitals of cations and anions.To express this duality, for a given cation, Zhang took into account its polarizing power for the ionic part and its electronegativity for the covalent part and defined a parameter Z called the acid strength of the concerned cation
Z = z/rk2 – 7.7Xz + 8.0
Z > 0:66 large electrostatic acids
Z < 0:66 border acids
Z < 0 large covalent acids.
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 [10, 12, 14, 15].
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METAL_OXYGEN BONDS IN OXIDES 53
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In 1984, Zhang proposed two papers on iono-covalency on the 2nd China National Congress on Catalysis:
1. Controlling factors of catalytic properties (Proceedings of the 2nd China National Congress on Catalysis,G014,Amoy,984.)
2. A mechanistic model for surface-adsorbate interaction (Proceedings of the 2nd China National Congresson Catalysis, G015, Amoy, 1984.)
Proceedings proposed in Chinese and could not be encompassed by Lenglet.