The first reactions of trimetallic nitride templated endohedral metallofullerenes (TNT EMFs) with carbon radicals generated from diethyl malonate catalyzed by manganese(III) acetate are reported. Two methano monoadducts, SC3N@C-80-A and SC3N@C-80-B, were isolated and characterized. SC3N@C-80-A contains two ester moieties, whereas SC3N@C-80-B contains only one ester group and a hydrogen atom on the central carbon of the addend. NMR spectroscopy of the two monoadducts suggests that the addition occurs regioselectively at a 6,6-ring juncture on the surface of the icosahedrally (1(h)) symmetric SC3N@C-80, forming the first 6,6-ring-bridged methano l(h) SC3N@C-80 derivatives. The measured (1)J((C,H)) = 147 Hz for the methano carbon with its hydrogen in monoadduct SC3N@C-80-B nearly perfectly matches the data for pi-homoaromatic systems, indicating an open [6,6]-methano structure. Geometry optimization also found that the "closed" [6,6)-methano structures were energetically unstable and always led to the open forms. Thus, an "open" [6,6]-methanofulleride structure is proposed, which was induced by the norcaradiene rearrangement, resulting in the cleavage of the cyclopropane ring and the formation of energetically stable open cage fullerene derivatives. These are the first examples of thermodynamically stable adducts of the "open" type at the 6,6-ring juncture of 1h SC3N@C-80, differing greatly from the "closed" 5,6-ring juncture adducts reported previously. In addition, bis-, tri-, and up to octaadducts Of SC3N@C-80 were detected by matrix-assisted laser desorption ionization time-of-flight mass spectrometry; this synthetic method was also applied to LU3N@C-80, producing adducts with up to 10 substituents on the carbon cage. These are the highest levels of substitution of TNT metallofullerenes reported so far.