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Electronic structures and related properties of a defective endohedral metallofullerene U2@C61-Def[5,6] are studied using density functional theory. It is shown that unlike the U2@C61-Def[6,6] which has a nonet ground state, the U2@C61-Def[5,6] has a quintet ground spin state with a lower total energy than the U2@C61-Def[6,6]. This is due to the antiferromagnetic coupling of the net spin electrons of the U2 unit with the net spin of both the cage and the adatom. Compared with the U2@C60, the [5,6]-type defect demonstrates almost no change in the energy gap, while the [6,6]-type defect does show a reduction. The electronic states and energy gaps of the endohedral metallofullerenes can therefore be engineered in a controllable manner by introducing different adatom-type defects. [ABSTRACT FROM AUTHOR]