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.


2 Answers 2


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.

  • $\begingroup$ 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
  • 1
    $\begingroup$ Yes, you are right @Oscar. I will edit my answer. $\endgroup$ May 14, 2019 at 14:01
  • 1
    $\begingroup$ @Oscar no worries. You always learn something new everyday : ) $\endgroup$ Apr 25, 2020 at 16:52
  • 1
    $\begingroup$ @Oscar I used the option hyperlink to embed links into the text. $\endgroup$ Apr 25, 2020 at 16:58
  • 1
    $\begingroup$ FYI I add a new reference for Mg(0). $\endgroup$ Dec 15, 2021 at 13:43

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.

  • $\begingroup$ "...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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.