Disclosure of an important letter from A.L.Allred

——  ABOUT  ALLRED-ROCHOW  ELECTRONEGATIVITY

American Huilin Institute

Electronegativitiy(EN) is an important parameter in understanding molecular properties. It has unique and valuable applications to the interpretation of a vast area of common chemistry.

Pauling first defined electronegativity and suggested methods for its estimation in 1932 (1). Over the years, various other methods have been proposed for evaluating the electronegativity values of the elements. Only one method, proposed by Allred and Rochow (2), has been widely accepted as an alternative to Pauling’s thermochemical scale.

Nevertheless, as Zhang pointed out in his paper published 31 years ago (3-5), the concept of  electronegativity is not precisely perfect, and the scale is found to violate the experiment fact. The effective nuclear charges adopted in the Allred-Rochow method are from the simplified Slater rule (6) which ignored the variations in atomic orbitals with increasing nuclear charges. Therefore this method cannot yet correspond or correlate to some chemical facts. For instance, its electronegativities of the post-scandide elements (from Ga to Kr) are too high and those of the post-lanthanide elements (from Tl to Rn) are too low.

Not relied on the amendment of the prohibition affection but based upon the experimental data, ionization potentials, Zhang has calculated an ion electronegativity scale. So a wide range of chemical phenomena such as the ligand field stabilization, the first filling of p orbitals, the transition-metal contraction, and lanthanide contration can be reflected. And when Zhang proposed a scale for strengths of Lewis acids, using the ion electronegativity as a parameter, it would be seen that the values could have considerable predictive power (4).

Pauling and Allred-Rochow originally defined electronegativity as “the power of an atom in a molecule to attract electrons”. However they actually did not take account the different valence states even though the attraction of the atom to electrons is decided by the environment of the atom in the molecule. The higher the charge number of an element in a compound, the more strongly its atom attracts electrons. Using this approach Zhang has more physically defined the electronegativity of the element in valence states as the electrostatic force exerted by the effective nuclear charges on the valence electrons.

Fortunately，after Zhang's papers had been published, Zhang has been pleased to receive a letter from Allred in January 4, 1983 (7).  In the letter Allred admitted that “ the “Xa values of Zr—Pd, Hf—At including Pb, and —Lu are not mine. Slater’s Rules fail to correlate with ionization energy data for these elements.”   And what is even worse is that “unfortunately, Little and Jones in a J. Chemical article extended the electrostatic force calculations to these elements and a number of textbook writers have used their results but have referenced the Allred-Rochow paper.”

   Unfortunately, for some reason Zhang did not promulgate the story to the society. Over 31 years, the fact that these data failed to correlate with the experiment fact still remains unchanged in the textbooks and continues to be published.
 

     (1)   L. Pauling, J. Am. Chem. Soc., 54, 3570 (1932).
(2)   A. L. Allred and E. G. Rochow, J. Inorg. Nucl. Chem., 5, 246 (1958)
(3)   Y. Zhang, Inorg. Chem., 21, 3386 (1982).
(4)   Y. Zhang, Inorg. Chem., 21, 3389 (1982).
(5)   Y. Zhang, J. Molecular Science 1, 125(1981)
(6)   J. C. Slater, Phys. Rev., 36, 57 (1930)
(7)   A. L. Allred, a personal letter, January 4, 1983.