I was taught to use Occam's razor whenever in doubt over an octet expansion, being told it never happened when, for example, dative bonds could resolve the issue with a regular octet. This seems to fit what users here say about the recent paradigm shift on "hypervalent" molecules that "it is now known, that for almost all molecules that were described this way the contribution of d orbitals is negligible (<1%).", but could anyone explain exactly why an element wouldn't expand it's octet unless it absolutely has to? What advantage is there to not involving 'd' orbitals over involving them in bonding? Or is it just Occam's razor and I'm misunderstanding this maxim.
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2$\begingroup$ No. There is no octet expansion, the concept has been disproved. The d-orbitals simply have a too high energy to be involved in bonding. $\endgroup$ – Martin - マーチン♦ Aug 27 at 13:35
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$\begingroup$ Then what theory has superseded it so I can be more informed, how are moleules like sulfur hexafluoride explained? $\endgroup$ – Andrea Rowlatt Aug 27 at 15:04
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2$\begingroup$ In short: Molecular Orbital Theory (but a modern interpretation of Valence Bond theory comes to the same conclusions). Related: Can an atom have more than 8 valence electrons? If not, why is 8 the limit? Unfortunately I have to add that the accepted answer (by Terry) is also beyond wrong. Also: How are the hybrid orbitals of sulfur hexafluoride shaped? What is the highest possible expanded octet? $\endgroup$ – Martin - マーチン♦ Aug 27 at 15:18
