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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, magnesium) and +1(for all AE metals lighter than radium).

  1. Be(0)

A 2016 paper gives an insight to the existence of a zero valent beryllium complex compound. Previously the stability of Be(0) compounds had been computationally predicted; the referenced paper reports the first actual synthesis.

[...]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. Mg(0)

This more recent (2021) addition has been obtained as an organic complex containing the $\ce{Na2Mg2^{2+}}$ cluster by Rösch et al. The authors also report a compound containing the mixed $\ce{Mg(0)}$-$\ce{Mg(I)}$ species $\ce{Mg3^{2+}}$.

  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). In [2018] Wu and Lerner (source) reported a barium(I) in a complex graphite intercalation compound.

  1. Ba(III, IV, V)?

Various studies have suggested that elements can assume oxidation states outside their usual range under GPa-level pressures. Luo et al. (2018) have performed such calculations with barium fluorides and predict the possibility of $\ce{BaF3,BaF4,BaF5}$. These proposed compounds await experimental verification.


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. $\endgroup$ May 14, 2019 at 9:37
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    $\begingroup$ Yes, you are right @Oscar. I will edit my answer. $\endgroup$ May 14, 2019 at 14:01
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    $\begingroup$ @Oscar no worries. You always learn something new everyday : ) $\endgroup$ Apr 25, 2020 at 16:52
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    $\begingroup$ @Oscar I used the option hyperlink to embed links into the text. $\endgroup$ Apr 25, 2020 at 16:58
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    $\begingroup$ FYI I add a new reference for Mg(0). $\endgroup$ Dec 15, 2021 at 13:43
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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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  • $\begingroup$ "...the similarity between Mg and Zn." I think this is overplayed. In the +2 oxidation state you see some divergence, e.g. in the crystal structure of the oxides and basicity of the hydroxides. They are due to the zinc ion having lower-energy acceptor orbitals making it a stronger Lewis acid. In lower oxidation states where Lewis acidity is not so important, the Mg-Zn similarity is stronger. $\endgroup$ Dec 15, 2021 at 14:15

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