Expanded Octet - Criticism

Criticism

Both the term and concept of hypervalency still fall under criticism. In 1984, in response to this general controversy, Paul von Ragué Schleyer proposed the replacement of 'hypervalency' with use of the term hypercoordination because this term does not imply any mode of chemical bonding and the question could thus be avoided altogether.

The concept itself has been criticized by Ronald Gillespie who, based on an analysis of electron localization functions, wrote in 2002 that "as there is no fundamental difference between the bonds in hypervalent and non-hypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent."

For hypercoordinated molecules with electronegative ligands such as PF₅ it has been demonstrated that the ligands can pull away enough electron density from the central atom so that its net content is again 8 electrons or fewer. Consistent with this alternative view is the finding that hypercoordinated molecules based on fluorine ligands, for example PF₅ do not have hydride counterparts e.g. phosphorane PH₅ which is an unstable molecule.

Even an ionic model holds up well in thermochemical calculations. It predicts favorable exothermic formation of PF₄+F− from phosphorus trifluoride PF₃ and fluorine F₂ whereas a similar reaction forming PH₄+H− is not favorable.