J. Phys. Chem. 1995, 99, 14570
14570
COMMENTS Comment on Sproul’s Evaluation of Electronegativity Scales Peter G. Nelson School of Chemistry, University of Hull, Hull HU6 7RX, UK Received: October 18, 1994; In Final Form: February 2, 1995 Sproul’ has evaluated scales of electronegativity 01) by examining how well a plot of Ax against separates compounds according to the type of bonding in them (ionic, covalent, or metallic). A problem with this procedure is that bond type is not a measurable property.2 This is illustrated by the widely different scales of ionicity that have been proposed, differing from one another by up to an order of magnit~de.~ Sproul used Well’s assignments of bond type, as given in the fifth edition of his textbook! Sproul states that Wells avoided the use of electronegativity when making these, and only used “measurable physical and chemical parameters”. However, on the pages Sproul cites (pp 289-290) Wells does not say this? and I cannot find where he does. Indeed, in earlier editions he gives the impression that he did use electronegativity in making some of his assignments: and it is difficult to see how he could make statements like “While BN is presumably
a covalent compound, the bonds in AlN, GaN,and InN probably have appreciable ionic ~haracter”~ without doing so. A further problem is that Sproul takes Wells’s use of tenns like “molecule” to indicate covalent bonding, whereas Wells cautions against making this connection, arguing that the bonds in SiF4 are “probably as nearly ionic” a’s those in AlF3.8 According to some calculations, even such molecules as PF5 and SF6 are >50% i o n k g By taking all molecules to be (’50%) covalent, Sproul curtails the ionic region on his plot and makes the dividing line between ionic and covalent bonding increase with 2.
References and Notes (1) Sproul, G. D. J . Phys. Chem. 1994,98, 6699. (2) Nelson, P. G. J . Chem. Educ. 1994,71, 24. (3) Meister, J.; Schwarz, W. H. E. J . Phys. Chem. 1994,98, 8245. (4) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984. ( 5 ) On pp 289-290 of ref 4 he only questions how far electronegativity can explain differences between bond lengths and sums of covalent radii. ( 6 ) Reference 4, 2nd ed., 1950, pp 9-12, 28-39; 3rd ed., 1962, pp 29-35. (7) Reference 4, 5th ed., p 835. (8) Reference 4, 2nd ed., pp 9-10. (9) Cioslowski, J.; Mixon, S. T. lnorg. Chem. 1993,32, 3209.
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0022-3654/95/2099-14570$09.00/0 0 1995 American Chemical Society
