# Specific examples of all-main-group complexes with significant d-orbital contribution

It is usually said that d-orbitals are not usually used in bonding, even for so-called "hypervalent" ones, in compounds that are made purely of main group compounds.

However, after reading an article (or, strictly speaking, the preview of an article) about a calcium complex explicitly involving d-orbital contribution (see Krieck, S.; Görls, H.; Yu, L.; Reiher, M.; Westerhausen, M. J. Am. Chem. Soc. 2009), I began to wonder if there could be more d-orbital all-main-group compounds or not. The molecule depicted in the aforementioned paper is unstable in air and water, but maybe the unstability (i.e. high energy of main-group d-orbitals) could be reduced.

My guess is that, one can use the anti-bonding orbitals of a sufficiently weakly aromatic ring ("weakly" aromatic, in order to have relatively high energy) as ligands and putting many electrons in the coordination center (not necessarily metallic, and it should be reasonably electronegative, such as sulfur) so that the energies of the 4s and 3d orbitals of the coordination center reasonably match the anti-bonding orbitals of the ring and the usual Aufbau principle could be applied directly (3s2, 3p6, 4s2, 4d(1~5)).

The question is, are there any explicit examples of such complexes that are stabilised by a way similar to what I mentioned above?

• Can you define "significant d-orbital contribution"? I don't know of an example with the structure you describe, but calculations on pentamethyl antimony suggest d orbital particpitation is not negligible: actachemscand.org/pdf/acta_vol_47_p0368-0373.pdf – Andrew Jan 8 at 13:29
• Did you consider writing Dr. Krieck directly asking him? – pH13 - Yet another Philipp Jan 10 at 14:07
• "[...] Ca, Sr, Ba, and Ra use their inner(n−1) d-orbitals predominantly in covalent bonding contributions when sufficiently positively charged [...]. This leads to a number of peculiar structural features that bring these elements into the realm of ‘‘non-VSEPR $d^0$ systems’’ that encompass early transition elements and even lanthanides, and they have also been termed‘‘honorary d-elements’’ [4]" cited from media.wiley.com/product_data/excerpt/50/35273331/… – Karsten Theis Jan 11 at 0:56