The addition of a bulky N-heterocyclic carbene (NHC) to Sc2C2@C-3v(8)-C-82, affords twomono adducts (2a and 2b) quantitatively and regioselectively, representing the first examples of Lewis acid base pairs of metal carbide cluster fullerenes. 2b is likely a kinetically favorable labile product that cannot be isolated from the solution. The crystallographic results of 2a unambiguously demonstrate that one polarized C-C single bond is formed between the iaormal carbene site C2N of the NHC and a specific [5,6,6]-carbon atom out of 17 types of nonequivalent cage carbon atoms of Sc2C2@ C-3v(8)-C-82. Theoretical calculations demonstrate that the high regioselectivity, the unexpected addition pattern, and the quantitative formation of monoadducts are synergistic results from the cage geometry and electron distribution on the cage.