Light metal atoms such as Li, K (electronic state S-2(1/2)) or Ca (S-1(0)) encapsulated in a C-70 cage considerably modifies the chemical properties of the fullerene surface due to metal-to-cage electron transfer. H-doped and anion ozonide systems were also considered to validate the electron transfer hypothesis. The relative stabilities of the eight isomers of the C70O3 molozonide series at room temperature depend on the identity of the endohedral guest, as was the preferred channel for thermal decomposition. No electron transfer was observed for the complex N@C-70 where the fullerene acts as an inert container for the S-4(3/2) radical. (C) 2014 Elsevier B.V. All rights reserved.