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It is well known that, even in periods 3 and higher, the bonding in so-called "hypervalent" main-group molecules nearly always do not involve d-orbitals, and I've heard that it's because the d-orbitals are just too high in energy.

I now have a question: if the energetic difference is the only problem, then can bonds involving d-orbitals exist ("exist" as in "actually be observed") in excited states of "hypervalent" molecules?

For a more concrete example- do the bondings in sulfonate groups in Brilliant Blue FCF and indigo carmine get "elevated" from S(-O)3 to S(=O)2(-O) (charges omitted and S=O bonds involving sulfur's 3d orbitals) as they absorb red-light (opposite of blue) photons, and, if they do, to what extent?

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One candidate involves calcium, of all things. This paper by Krieck et al. [1] reports the synthesis of a dimeric calcium(I) complex $\ce{[(thf)3Ca\{μ-C6H3-1,3,5-Ph3\}Ca(thf)3]}$ in which the calcium atoms are bonded to each other and there are two unpaired electrons in antibonding orbitals of the aromatic ring system.

DFT calculations show that the orbitals containing the unpaired electrons have a significant contribution from calcium 3d orbitals which have the same symmetry as the ring antibonding orbitals. A model of the compound is given below.

Sandwich Complex with Unprecedented Organocalcium(I)

References

  1. Krieck, S.; Görls, H.; Yu, L.; Reiher, M.; Westerhausen, M. Stable “Inverse” Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(Thf)2Mg(Br)-C6H2-2,4,6-Ph3] and [(Thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(Thf)3]. J. Am. Chem. Soc. 2009, 131 (8), 2977–2985. DOI: 10.1021/ja808524y.
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  • $\begingroup$ The preview says that "the compound is extremely sensitive to air and moisture". I can't get past the paywall so I can't read the full text but the molecule definitely looks unstable. $\endgroup$ – Just A Young Artist Dec 29 '19 at 13:53
  • $\begingroup$ Unstable/sensitive, but still there. Magnesium also forms organic complexes using the +1 oxidation state (with no d-orbital bonding), and these too are air- and moisture-sensitive. $\endgroup$ – Oscar Lanzi Dec 29 '19 at 15:26

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