The octet rule and hypervalence: two misunderstood concepts

The octet rule proposed by Lewis states that the valence shell of an atom in a molecule always has eight electrons in its valence shell when each pair of shared electrons is counted as contributing fully and equally to the valence shell of both bonded atoms irrespective of the polarity of the bonds. However, as a result of attempts to allow for bond polarity an atom has been said to obey the octet rule if it has no more than eight electrons in its valence shell. This misinterpretation of the Lewis octet rule has led to much controversial discussion in connection with the concept of hypervalent molecules which have more than eight electrons in the valence shell of the central atom in their Lewis structures. Whereas hypervalent molecules clearly do not obey the Lewis octet rule it has been proposed that they do not violate the octet rule when it is expressed in its modified form because molecular orbital calculations of hypervalent molecules with strongly electronegative ligands, such as SF6, have been interpreted to show that there are fewer than eight electrons in the valence shell of the central atom. The electron localization function (ELF) allows the populations of the bonding and nonbonding domains in a molecule to be determined to a reasonably good approximation in many cases. ELF calculations of hypervalent molecules with less electronegative ligands such as PMe5, PMe3=CH2 AsMe5, and TeMe6, have shown that the population of the valence shell of the central atom is greater than eight, so these molecules do not obey the modified octet rule. We conclude that the octet rule is only valid in its original form as proposed by Lewis. We also conclude that hypervalence is not a useful concept as hypervalent molecules are not different in any significant way from Lewis octet molecules, except that in many, but not all, cases they have a coordination number higher than four. (C) 2002 Elsevier Science B.V. All rights reserved.