Magnesium is considered a metal

Magnesium: chemistry turned upside down

"Impossible" oxidation level: For the first time, chemists have succeeded in bringing the alkaline earth metal magnesium to the elementary oxidation level of zero. Instead of being positively charged twice as usual in compounds, the element in the newly created complex compound has no formal charge. The three magnesium atoms in the center of this molecule are, so to speak, the smallest piece of metallic magnesium, as the researchers report in the specialist journal "Nature".

Magnesium (Mg) is a metal with low electronegativity: it hardly attracts electrons and easily loses both of its outer electrons during chemical reactions. In connection with other elements it only occurs in nature as positively charged Mg2+-Cation in front. Magnesium can only form a complex bond with oxidation state 1 in combination with certain organic molecules. In contrast to other metals, however, it has never been possible to bring the magnesium to the zero oxidation state - i.e. in a compound in which the atom is without ionic charge and is elementary.

Complex molecule with an unusual inner workings

But this is exactly what a team led by Sjoerd Harder from the University of Erlangen-Nuremberg has succeeded in doing - almost by chance. The researchers actually intended to break magnesium-magnesium bonds in order to produce monovalent magnesium radicals. Sodium metal was used in this synthesis. The experimental setup stipulated that sodium should donate an electron to magnesium.

When the chemists allowed magnesium and sodium to react with the large organic molecule BDI, which consists of three hydrocarbon rings, dark, reddish-brown crystals formed. More detailed analyzes showed that the two metals in this newly formed complex reacted differently than expected: In the center there were two sodium atoms, each of which donated an electron to the previously twice positive magnesium atom.

Magnesium with zero oxidation level

This resulted in a previously never observed Mg (0) complex - a compound in which the central magnesium atoms have the oxidation state zero and are therefore in principle present as elemental metal. The magnesium centers in these complexes even carry a formal negative charge due to the unique magnesium-sodium bond, as the researchers explain.

As Harder and his team discovered, a kind of “ghost atoms” - zones of increased attraction that pull the electron pairs closer to the magnesium - are created between sodium and magnesium. This reacts completely differently to normal Mg2+-Links. While the normal Mg2+-Cations can accept electrons, the electron-rich Mg (0) reacts like a negatively charged anion by donating electrons.

Novel reducing agent

This was confirmed by further experiments in which the chemists brought the novel complex into contact with organic solvents such as toluene or benzene. The complex reacted like a strong reducing agent and gave off electrons when heated slightly. This led to a rearrangement in the complex, which resulted in another, new type of complex. In its center there were now three magnesium atoms lined up like pearls on a chain.

This trinuclear magnesium cluster also reacts like atomic Mg (0) and can be regarded as the smallest piece of magnesium metal. This new class of magnesium complexes turns magnesium chemistry upside down, as the chemists explain. At the same time, the extremely strong reducing effect of the complex opens up completely new application possibilities. (Nature, 2021; doi: 10.1038 / s41586-021-03401-w)

Source: Friedrich-Alexander University Erlangen-Nuremberg

May 17, 2021

- Nadja Podbregar