Why can sulfur have an expanded octet but not oxygen?

Question

In my search to understand the bonding in structures like $\ce{SF6}$ I found many sources that said it was because sulfur has d orbitals to accommodate an expanded octet, which made sense to me. But I also found sources like the paper by Reed and Weinhold (1986) that say d orbitals contribute very little to the bonding in $\ce{SF6}$. An alternative explanation is that $\ce{SF6}$ has three-center/two-electron bonds. What I don't understand is why compounds like $\ce{SF6}$, $\ce{ClF5}$, $\ce{PCl5}$ exist for third row element centers, but not $\ce{OF6}$ if it has nothing to do with d orbitals? References would be great.

Answer 1

Neither oxygen nor sulfur "expands its octet". Instead, sulfur in compounds such as $\ce{SF6}$ delocalizes the bonds formed with the usual octet over multiple linkages. As described in this answer, this delocalization in a neutral molecule comes with occupying ligand-based orbitals that are roughly nonbonding with a large central atom, but antibonding with a small central atom because of ligand-ligand interactions. Thus forming a compound with delocalized bonding and increased coordination number is more favored for third period elements than for second period elements.

We can get such coordination with second-period elements if the ligand-based orbitals noted above are unoccupied, for then the net ligand-ligand overlaps with the remaining orbitals are bonding. This usually requires a cationic species such as the six-coordinate carbon complex reported by Yamaguchi et al.[1], pictured below from the reference.

References

1. Torahiko Yamaguchi, Yohsuke Yamamoto, Daisuke Kinoshita, Kin-ya Akiba, Yun Zhang, Christopher A. Reed, Daisuke Hashizume, and Fujiko Iwasaki. "Synthesis and Structure of a Hexacoordinate Carbon Compound", J. Am. Chem. Soc. 2008, 130, 22, 6894–6895. https://doi.org/10.1021/ja710423d

Answer 2

Hopefully the pictures answers the question as to why oxygen cannot form an octet, when covalently bonding with other atoms, such as if it was possible for it to form OF6, but sulfur can, to form SF6. But a brief added explanation below:

It's to with the working out oxygen and sulfur's electon configuration, and then also representing this configuration using arrows in boxes. Realising that oxygen only goes up to the 2nd quantum principal shell number, so 2s and 2p, and ends here.

Sulfur has electrons in 3s and 3p, but has an additional 3d sub shell as well, that's currently empty. It can excite some its paired electrons into this empty orbital, spreading them out, so each can now form bonds and expand an octet to 12 electrons.

Oxygen has no empty orbital in quantum 2, so cannot excite and 'spread' its electrons, so cannot bond these to form an expanded octet.

Hope this helps.

Hannah S