J. Phys. Chem. 1995, 99, 14571
Reply to “Comment on Sproul’s Evaluation of Electronegativity Scales” Gordon D. Sproul Department of Chemistry, University of South CarolinaBeaufort, Beaufort, South Carolina 29902-4602 Received: December 21, 1994 Bond type, while not a measurable property, remains a concept that has widespread value in describing chemical phenomena. Throughout his text, Wells’ indicates the various accepted subjective phenonena that he used to determine bond type: electrical conductance/resistance,melting points, and most importantly, structural characteristics. That he does not use electronegativity is clear: while electronegativity is mentioned in passing in the introduction, it is only on pp 289-290 where it is discussed. The discussion there relates to the inadequacy of electronegativity to predict bond lengths. Wells does not even mention the possibility of using electronegativity to determine bond types. Nelson is correct in stating that Wells considered the concept of electronegativity in his earlier editions, but in later editions he denigrates the concept. In the second edition,2 in addition to the disparaging bond length discussion (pp 57-61), Wells recapitulated Pauling’s description of electronegativity (pp 28 39), including the relationship between the differences in electronegativity and percentages of ionic character. In the third e d i t i ~ nbesides ,~ repeating the bond length difficulty (pp 5557), Wells gave an abbreviated description of the valence bond description of electronegativity (pp 29-35). Much of this discussion deprecates any relationship between electronegativity and dipole moment or between dipole moment and ionic character (pp 32-35, with numerous footnotes). By the fourth e d i t i ~ n although ,~ retaining his description of the inadequacy of electronegativity for describing bond lengths, Wells had omitted any significant reference to electronegativity. With Wells’ obvious disenchantment with the concept of electronegativity or its ability to provide useful relationships with physically measurable parameters, it is difficult to imagine him employing electronegativity to describe bonding which is so central to his discussion. What is clear throughout Wells’ book is his use of structural characteritics (isolated ions, rings, chains, separated molecules, etc.) to determine bond type. The particular configuration produced by the tripartite separation of binary compounds is itself Confirmation that Wells did not use electronegativity, to determine bond type. If Wells
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had used the difference in electronegativity values, Ax, to determine ionic character as Pauling had sugge~ted,~ a plot of Ax vs i,the averge electronegativities, should show a horizontal line at about 1.7 Pauling units, separating ionic compounds above from covalent compounds below. This is certainly not the case. Instead, the line separating ionic from covalent bond type is found to be at a steep angle, paralleling the left side of the isosceles triangle. A subsequent paper indicates why this is the case; two different values of electronegativity are necessary to separate compounds by bonding type.6 A plot of the lower electronegativity, XI,,, vs the higher electronegativity, Xhi, shows that these two values of electronegativity are quite different: on the Pauling scale, the line that separates metallic bonding from both ionic and covalent bonding occurs at 2.3 (13.5 eV on Allen’s scale), while the line separating ionic from covalent bonding occurs at 1.6 (9.6 eV on Allen’s scale). These distinctly different values of electronegativity that delineate the tripartite nature of triangular plots demonstrate that Wells did not use a set of electronegativity values to determine bond type. Further indication of Wells’ methodology for determining bond type is indicated by the boron family nitrides; he uses structural characteristics to ascertain bond types in these nitrides. Wells indicates that BN is a covalent compound because it “has been known for a long time as a white powder of great chemical stability and high melting point with a graphite-like layer structure”, but at high pressures and temperatures it attains the zinc-blende structure; AlN, GaN,and InN have the wurtzitetype structure.’ It is likey that I have mislabeled these latter three compounds as ionic. They properly should have been excluded as ambiguous, since Wells describes these under the heading of covalent nitrides, but indicates that they “have appreciable ionic character”. Indeed, for all except two scales, they actually fall within the covalent region.
References and Notes (1) Wells, A. F . Structural Inorganic Chemistry, 5th ed.; Oxford University Press: New York, 1989. (2) Wells, A. F. Structural Inorganic Chemistry, 2nd ed.; Clarendon Press: Oxford, 1950. (3) Wells, A. F. Structural Inorganic Chemistry, 3rd ed.; Clarendon Press: Oxford, 1962. (4) Wells, A. F. Structural Inorganic Chemistry, 4th ed.; Clarendon: Oxford, 1975. ( 5 ) Pauling, L. The Nature of the Chemical Bond, 1st ed.; Come11 University Press: Ithaca, NY, 1939. (6) Sproul, G. D. J . Phys. Chem. 1994, 98, 13221-13224. (7) Wells, A. F. Reference 1, p 835.
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0022-3654/95/2099-14571$09.00/0 0 1995 American Chemical Society
