Chemistry - Can Hückel's rule be applied to inorganic compounds?

Solution 1:

Huckel's rule can be applied to "inorganic" cyclic molecules but only if they have a delocalised pi system. They also have to have flat (or close to flat) rings.

Recently some N5 rings which 6 pi electrons have been made, these are "inorganic" aromatic rings.

Also borazine has been the subject of considerable debate, it has been discussed on stack exchange before. It is the subject of a interesting paper: Islas, R.; Chamorro, E.; Robles, J.; Heine, T.; Santos, J. C.; Merino, G. Borazine: to be or not to be aromatic. Struct Chem 2007, 18 (6), 833–839. DOI: 10.1007/s11224-007-9229-z.

Solution 2:

This rule can also be applied to all-metal compounds, which due to the electron deficiency can have both pi- and sigma-aromaticity. See, e.g.: Wikipedia article on Metal Aromaticity or the paper: Boldyrev, A. I.; Wang, L. All-Metal Aromaticity and Antiaromaticity. Chem. Rev. 2005, 105 (10), 3716–3757 DOI: 10.1021/cr030091t.

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Solution 3:

The tetrasulfur dication, $\ce{S_4^{2+}}$, has six pi electrons in its four-membered ring and shows the symmetry expected of an aromatic ring (reference: Elemental Sulfur and Sulfur-Rich Compounds II, edited by Ralf Steudel). It has even been obtained as a salt, $\ce{S4(SO3F)2}$, suggesting enhanced stability.