Editing Hypervalent molecule (section)

===Valence bond theory===
For hypervalent compounds in which the ligands are more [[electronegative]] than the central, hypervalent atom, [[resonance structures]] can be drawn with no more than four covalent electron pair bonds and completed with ionic bonds to obey the octet rule.  For example, in [[phosphorus pentafluoride]] (PF<sub>5</sub>), 5 resonance structures can be generated each with four covalent bonds and one ionic bond with greater weight in the structures placing ionic character in the axial bonds, thus satisfying the octet rule and explaining both the observed [[trigonal bipyramidal molecular geometry]] and the fact that the axial bond length (158 pm) is longer than the equatorial (154 pm).<ref>{{cite journal | title = A Simple Qualitative Molecular-Orbital/Valence-Bond Description of the Bonding in Main Group "Hypervalent" Molecules | journal = [[Journal of Chemical Education]] | year = 1998 | volume = 75 | pages = 910–915 | doi = 10.1021/ed075p910 | author1 = Curnow, Owen J. | issue = 7|bibcode = 1998JChEd..75..910C }}</ref>

[[image:penta phos.svg|thumb|500px | center | Phosphorus pentafluoride. There are 2 possible structures with an axial ionic bond, plus 3 possible structures with an equatorial ionic bond.]]

For a hexacoordinate molecule such as [[sulfur hexafluoride]], each of the six bonds is the same length.  The rationalization described above can be applied to generate 15 resonance structures each with four covalent bonds and two ionic bonds, such that the ionic character is distributed equally across each of the sulfur-fluorine bonds.

[[image:hexa sulf.svg|thumb|500px | center | Sulfur hexafluoride. There are 12 structures with the two ionic bonds in adjacent (''cis'') positions, plus 3 structures with the two ionic bonds in opposite (''trans'') positions.]]

Spin-coupled valence bond theory has been applied to [[diazomethane]] and the resulting orbital analysis was interpreted in terms of a chemical structure in which the central nitrogen has five covalent bonds;

[[File:Hypervalent diazomethane.png|thumb|150px| center|Chemical formula of diazomethane, showing hypervalent nitrogen]]

This led the authors to the interesting conclusion that "Contrary to what we were all taught as undergraduates, the nitrogen atom does indeed form five covalent linkages and the availability or otherwise of d-orbitals has nothing to do with this state of affairs."<ref>{{cite journal | title = Modern valence bond theory | journal = [[Chemical Society Reviews]] | year = 1997 | volume = 26 | pages = 87–100 | doi = 10.1039/CS9972600087 | author1 = Gerratt, Joe | issue = 2 }}</ref>