The influence of the encapsulation of an ion inside the C 60 fullerene cage on its exohedral reactivity was explored by means of DFT calculations. To this end, the Diels–Alder reaction between 1,3-cyclohexadiene and M@C 60 (M=Li + , Na + , K + , Be 2+ , Mg 2+ , Al 3+ , and Cl − ) was studied and compared to the analogous process involving the parent C 60 fullerene. A significant enhancement of the Diels–Alder reactivity is found for systems having an endohedral cation, whereas a clear decrease in reactivity is observed when an anion is encapsulated in the C 60 cage. The origins of this reactivity trend were quantitatively analyzed in detail by using the activation strain model of reactivity in combination with energy decomposition analysis

We are grateful for financial support from the Spanish MINECO FEDER (Grants CTQ2013-44303-P, CTQ2016-78205-P and CTQ2014-51912-REDC to I. F. and CTQ2014-54306-P to M. S.), the Netherlands Organization for Scientific Research (NWO), and Catalan DIUE (projects 2014SGR931, ICREA Academia 2014 prize, and XRQTC to M. S.). The FEDER grant UNGI10-4E-801 has also funded this research.