A series of neutral (8-10), monomethylated (12-14), and dimethylated (17) 10-S-3 sulfuranes, derivatives of tetraazathiapentalenes fused with pyrimidine and/or pyridine ring, were prepared. These molecules are planar, and bond energies of the hypervalent N-S-N bond were evaluated by the temperature dependent restricted rotation of the pyrimidine ring caused by cleavage of one of the S-N bonds. The bond length is longer, and the energy is lower for the S-N bond fused with more electron-withdrawing heterocycles.