Portier et al. review on Zhang Electronegativity and Strength for Lewis Acid

Portier et al. review on Zhang Electronegativity and Strength for Lewis Acid

 

Josik Porties, Guy Campet, Jean Etournear, M.C.R.Shastry and Bermard Tanguy

Lab de Chimie du Solide du CNRS, Univerite de Bordeaux I, 33405 Talence Cedex (France)

Journal of Alloys Compounds, 209 (1994) 59-64,

(Received July 27, 1993)

 

 

Portier et al. reviewed [1]: “Pearson’s classification is qualitative. Zhang [2] 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, (a) its polarizing power for the ionic part and (b) its electronegativity for the covalent part.

The polarizing power of a cation is suggested to be equal to Z/rk2, where Z is the charge and r the ionic radius. It corresponds to the electrostatic forces exerted by the nucleus on the counter-anion.”

Pauling [3] introduced the electronegativity concept defining it as “the power of anf atom in a molecule to attract electrons to itself”. He also proposed a scale going from about 4 for fluorine, the most electronegative element, down to 0.7 for cesium, the least electronegative. The values were based on thermochemical data.

One of the criticisms of the Pauling’s scale is that it does not take into account the valence state of the ion, apart from being quite relative. For instance, the electrophilic nature of Mn in (MnO4)- is quite different from the Mn in MnO. The obvious difference is due to the fact that the formal charge of Mn is +2 in the latter compound. Zhang proposed a new electronegativity scale [4]. He defines electronegativity as: “the electrostatic force exerted by the effective nuclear charge on the valence electron” and is calculated as

 

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.

Zhang observed that the polarizing power (P) of the cation should vary linearly with electronegativity. He classified all cations on the basis of the ideal equation [2]

 

P – 7.7 Xz + 8.0= 0

 

Then, he defined a parameter (Z) call the acid strength of the concerned cation as

 

Z = Z/rk2 – 7.7Xz + 8.0

 

 

[1] Josik Porties, Guy Campet, Jean Etournear, M.C.R.Shastry and Bermard Tanguy, Journal of Alloys Compounds, 209 (1994) 59-64,

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

[3] L. Pauling, J. Am. Chem. Soc. 54 (1932) 3570.

[4] Y. Zhang, Inorg Chem. 21 (1982) 3886.

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