How does hypervalency (expanded octet) affect the stability of a molecule?

This was a question I had a hard time answering. All I could say was "it must result in increased stability, otherwise hypervalency would not occur." Does anyone have a better answer?

Links
http://en.wikipedia.org/wiki/Hypervalency
http://en.wikipedia.org/wiki/Expanded_octet
http://student.ccbcmd.edu/~cyau1/121TutorialExpandedOctetSp2006.pdf

Question

How does hypervalency (expanded octet) affect the stability of a molecule?

This was a question I had a hard time answering.  All I could say was "it must result in increased stability, otherwise hypervalency would not occur."  Does anyone have a better answer?

Links
http://en.wikipedia.org/wiki/Hypervalency
http://en.wikipedia.org/wiki/Expanded_octet
http://student.ccbcmd.edu/~cyau1/121TutorialExpandedOctetSp2006.pdf

anonymous · Answer

I am too looking for a good answer to this question.Expanded octets can be stable and only occur in certain elements.

anonymous · Answer

I know that you wind up hybrid orbitals:$$sp^3d \quad or \quad sp^3d^2$$But that says nothing about "how hypervalency affects stability".  Obviously the extra bonds must lead to an overall decrease in the energy of the system, or they would not form.

anonymous · Answer

could it have something to do with the geometry of the specific molecules?

anonymous · Answer

Atoms generally strive for octet stability (8 electrons in the valance shell), but some of the main group elements can have more than 8 electrons in the valence shell.  This certainly affects the molecular geometry of the molecules containing these atoms.