Storing and releasing Mg by C12 carbon ring

• A study of ground and excited states of magnesium carbon ring system employing accurate state-of-the-art quantum chemistry methods. • Varying charge on the guest Mg can be utilized to efficiently release Mg from the carbon ring. • To the best of our knowledge, it is the first paper reporting interesting quasi-cumulenic C-C bonds. Carbon allotropes accommodating guest atoms, like endohedral fullerenes, have been widely discussed in the literature owing to their potential applications. There is very little known on endocircular carbon rings containing guest atoms. We study a magnesium carbon ring system paying attention to the ground as well as to excited electronic states. As excited electronic states play a decisive role in applications, accurate state-of-the-art quantum chemistry methods are employed. Mg can donate up to two electrons to the carbon ring giving rise to a rich electronic structure in the various states which is analyzed and discussed. It is demonstrated how the varying charge on the guest Mg can be utilized to efficiently release the Mg from the carbon ring.