Metal–metal bonds inside fullerenes

Fullerenes featuring spherically hollow interiors provide an ideal matrix to accommodate metal ions to form endohedral metallofullerenes (EMFs). Benefiting from the confinement effect of carbon cages and intramolecular electron transfer, two or more metal ions can overcome the electrostatic repulsion between each other to be stabilized in the carbon cages, which makes EMFs an ideal model for studying metal-metal bonds. For the past few years, many unique metal-metal bonds involving f-block lanthanide (Ln) and actinide (An) metals have been found in EMFs by utilizing fullerenes as steric ligands. This feature article highlights the latest developments in experimental and theoretical studies relevant to the metal-metal bonds in EMFs, including homonuclear, heteronuclear and multicenter bonds, which could advance our understanding of the nature of f-block bonding. In particular, due to the strong magnetic coupling between the confined metal ions and the high thermal and chemical stability offered by the host fullerenes, EMFs bearing metal-metal bonds have great application prospects in fields such as single-molecule magnets and molecular spin qubits.