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I've been searching some things regarding the ability of sulfur to accomodate negative charge and reached the conclusion that it's due to its size and its ability to expand its valence shell.

I can't fully understand the expansion part though. It is mentioned in various sites and books that sulfur uses the 3d orbitals due to their similar energy level with 3p orbitals in order to achieve this "expansion". However, 4s are supposed to be filled first. I even came across this question: How does expansion of electron shells work? but the answer given didn't really helped me. So why do we consider the expansion to happen with 3d orbitals, while 4s is filled first?

Αντώνιος Κελεσίδης
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    I have a feeling we should make some banner next to "top questions" list saying "Octet expansion is outdated and deprecated theory" – Mithoron May 15 '18 at 00:13
  • I've also seen that being mentioned. So what is accepeted regarding this phenomenon? – Αντώνιος Κελεσίδης May 15 '18 at 08:23
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    That there's no such phenomenon? – Mithoron May 15 '18 at 17:49
  • How do we justify the ability of sulfur to create more bonds then? And regarding its ability to accomodate negative charge better than oxygen? – Αντώνιος Κελεσίδης May 15 '18 at 23:16
  • https://chemistry.stackexchange.com/a/5242/9961 I suggest to ignore rest of answers there. – Mithoron May 15 '18 at 23:56
  • I hadn't found this thread. Thank you. So, the only accpepted justification of sulfur "controlling" a negative charge better than oxygen, is simply it size? – Αντώνιος Κελεσίδης May 16 '18 at 08:45
  • I really hate linking to that post, but @Mithoron is right, that one answer is ok. (There are two more that are not awful.) The top answer is just terrible. Anyway, I don't know what caused Oscar to delete his answer, but he linked to a paper that you might want to check out: Alan E. Reed and Frank. Weinhold (1986). "On the role of d orbitals in sulfur hexafluoride". J. Am. Chem. Soc. 108(13), 3586–3593. https://doi.org/10.1021/ja00273a006 – Martin - マーチン May 17 '23 at 21:05

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