The Correction for Electronegativity Difference

The first values of covalent radii, as given in Table 7-2, were formulated before experimental values were available for F—F, O—O, and N—N single bonds. An electron-diffraction study of F2 by Brockway9 then gave the F—F distance as 1.45 A (a value substantiated by Rogers, Schomaker, and Stevenson, Table 7-3, who found 1.435 0.010 A), whereas the accepted radius of fluorine would require 1.28 A. Similar discrepancies were then reported for O—O, for which 

Gigufere and Schomaker10 found 1.47 0.02 A in H Ot (value from old radius 1.32 A), and N—N, for which they found 1.47 0.02 A in N H4 (value from old radius 1.40 A). Since the distances for many bonds between N, O, and F and other atoms are rather well given by the old radii, there are large deviations from additivity in the bond lengths. 

It was suggested by Schomaker knd Stevenson11 in 1941 that these deviations result from the partial ionic character of the bonds between unlike atoms. They proposed that the radii for N, O, and F be taken to be those given by the N—N, O—O, and F—F distances (Table 7-5), and that in general the interatomic distance for a bond A—B be taken to be the sum of the radii for the atoms A and B with a correction term —0.09 A xa — Xb, in which xa — Xb is the absolute value of the difference in electronegativity of the two atoms. 

I Similar agreement is also found for a great many other substances, such as hercamethylene tetramine (C—N 1.47 A), etc. the very extensive table that might be reproduced to show this agreement will be omitted. 

For example, the average value of the Si—C bond length found in the methylsilanes (Table 7-6) is 1.87 A. The sum of the single-bond radii of carbon and silicon is 1.94 A, which is 0.07 A larger. If this value is corrected for the 0.7 difference in electronegativity by the Schomaker-Stevenson term it becomes 1.88 A, in much better agreement with experiment. 

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As an increasing number of molecules with bonds between F, O, or N atoms and very electropositive elements like Si were studied in the following years, it became clear that the correction for electronegativity differences suggested by Schomaker and Stevenson was too small. Thus the experimentally determined Si-F bond distance in FSiHs was 13 pm shorter than estimated, the Si-O bond distance in O (SiH3)2 was 16 pm shorter, and the Si-N distance in N(SiH3)3 was 9 pm shorter than estimated. 

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