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Are there compound of alkaline earth elements showing unusual oxidation numbers? For the sake of the question, every oxidation number different from +2 counts as unusual, most exciting would be oxidation numbers greater than +2.

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This answer is an extension to @Ian Bush answer. Not only magnesium, but every group 2 element has a lower oxidation state of +0(beryllium) and +1(for other metals).

  1. Be(0)

A 2016 paper gives an insight to the existence of a zero valent beryllium complex compound i.e it has been verified through computational method but it is yet to be synthesized.

[...]The team added a single ligand i.e CAAC = cyclic (alkyl)(amino)carbene to $\ce{BeCl2}$ in benzene solution to give $\ce{(CAAC)BeCl2}$, and obtained the final product, $\ce{[Be(CAAC)2]}$, by reducing the beryllium with KC8 in the presence of a second equivalent of CAAC ligand.

  1. Be(I)

High resolution infrared emission spectra of beryllium monohydride and monodeuteride have been recorded. The molecules were generated in a furnace-discharge source, at 1500 °C and 333 mA discharge current, with beryllium metal and a mixture of helium and hydrogen or deuterium gases.(source)

  1. Mg(I)

It has been described in @Ian Bush's answer. For more information, refer to these sources(1 and 2). Some examples are $\ce{Mg2RuH4, Mg3RuH3, and Mg4IrH5}$ having the $\ce{Mg-Mg}$ bond and magnesium diboride containing the metastable $\ce{Mg2^{2+}}$ ion(Credit @Oscar Lanzi).

  1. Ca(I)

A Ca(I) Sandwich Complex $\ce{[(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3]}$ is being described in this 2010 paper.

  1. Sr(I)

The high-resolution infrared spectrum of gas-phase $\ce{SrF}$ was obtained in emission with a Fourier transform spectrometer.[...] (Source)

  1. Ba(I)

3 isotopes of barium in $\ce{BaF}$ (Source)


I did not had to google each and every element to search for lower oxidation state. The wikipedia article of oxidation element gave a list of all posible O.S of element. Information regarding unusual O.S of any element can be found in the footnote and thus only clicked the relevant elements for the answer.

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  • $\begingroup$ How does $\ce{MgB2}$ contain magnesium-magnesium bonds? When I look at sketches of the structure it looks like there are boron-boron and boron-magnesium bonds (the latter possibly having ionic character to make up the electron deficiency in the boron) instead. $\endgroup$ – Oscar Lanzi May 14 at 9:37
  • $\begingroup$ See this reference for the ionic bonding. $\endgroup$ – Oscar Lanzi May 14 at 9:51
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    $\begingroup$ Yes, you are right @Oscar. I will edit my answer. $\endgroup$ – Nilay Ghosh May 14 at 14:01
  • $\begingroup$ Tganks @nilay. The quoted reference does not, however, mention any magnesium(I) species. It renders magnesium fully ionized with two positive charges. Most of the compensating negative charge in the normal state is in metallic bonding, but switches to one-half metallic, one-half ionic Mg-B bonding in the superconducting state. $\endgroup$ – Oscar Lanzi May 14 at 15:11
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Magnesium(I) compounds are known, but they are of the form $\ce{[Mg-Mg]^{2+}}$ rather than a bare $\ce{Mg+}$ - so more like Mercury than Sodium. See https://pubs.rsc.org/en/content/articlelanding/2011/dt/c0dt01831g#!divAbstract, and in a way this furthers the similarity between Mg and Zn. Similar compounds are also possible for the heavier metals.

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