The enthalpies of reaction of both the solvated and solid salts of the potassium 1, 6-methano [lO] annulene (M10) anion radical were measured with calorimetric techniques. These measurements show that the anion radical of M10 has a surprising degree of thermodynamic stability. The heat of formation of K+ M10-.,,, from gas-phase potassium and gas-phase hydrocarbon (-61.9 kcal/mol) is about 16 kcal/mol more negative than that for ...