The reactions of the open-cage fullerene, MMK-9, with an open 12-membered ring on its surface and silver(I) salts have been examined. The structure of MMK-9 itself has been determined by single-crystal X-ray diffraction. MMK-9 reacts with silver trifluoroacetate in air to form the dimer, IMMK-9(OCH3)(5)Ag(AgO2CCF3)}(2). Remarkably, five MeO groups have added to the surface of the open cage in a pattern that surrounds a pentagon immediately adjacent to the opening in the cage. Dioxygen has been implicated as the oxidant in this unusual addition of five groups to the open cage. Two silver ions connected to each other by a short argentophillic interaction reside at the core of the centrosymmetric dimer. The reaction of silver nitrate with MMK-9 yields the crystalline polymer, [{MMK-9(OCH3)(6)Ag(AgOCH3)}2-H2O],. This polymer consists of dimeric {MMK-9(OCH3)(5)Ag}(2) units that are connected into strands through silver ions, which are chelated by the amine functions of one open cage and bound in eta(2)-fashion to a pair of carbon atoms on an adjacent open cage.