We report on the highly compact nitroxide-substituted nitronyl nitroxide 1 and iminonitroxide 2; they have isoelectronic structures with trimethylenemethane. These diradicals are stable under aerated conditions at room temperature and have large positive exchange interactions: J/k(8) = +390 K (H = 2,JS(1/2)center dot S-1/2) for 1 and J/k(B) approximate to +550 K for 2.