We compared the chemical reactivity of D-2d(23)-C-84 and that of Sc2C2@D-2d(23)-C-84, both having the same carbon cage geometry, in the photolysis of 2-adamantane-2,3'-[3H]-diazirine, to clarify metal-atom doping effects on the chemical reactivity of the carbon cage. Experimental and computational studies have revealed that the chemical reactivity of the D-2d(23)-C-84 carbon cage is altered drastically by endohedral Sc2C2 doping. The reaction of empty D-2d(23)-C-84 with the diazirine under photoirradiation yields two adamantylidene (Ad) adducts. NMR. spectroscopic studies revealed that the major Ad monoadduct (C-84(Ad)-A) has a fulleroid structure and that the minor Ad monoadduct (C-84(Ad)-B) has a methanofullerene structure. The latter was also characterized using X-ray crystallography. C-84(Ad)-A is stable under photoirradiation, but it interconverted to C-84(Ad)-B by heating at 80 degrees C. In contrast, the reaction of endohedral Sc2C2@D-2d(23)-C-84 with diazirine under photoirradiation affords four (Sc2C2@C-84(Ad)-A, Sc2C2@C-84(Ad)-B, Sc2C2@C-84(Ad)-C, and Sc2C2@C-84(Ad)-D). The structure Sc2C2@C-84(Ad)-C was characterized using X-ray crystallography. Thermal interconversion of Sc2C2@C-84(Ad)-A and Sc2C2@C-84(Ad)-B to Sc2C2@C-84(Ad)-C was also observed. The reaction mechanisms of the Ad addition and thermal interconversion were elucidated from theoretical calculations. Calculation results suggest that C-84(Ad)-B and Sc2C2@C-84(Ad)-C are thermodynamically favorable products. Their different chemical reactivities derive from Sc2C2 doping, which raises the HOMO and LUMO levels of the D-2d(23)-C-84 carbon cage.