Hydrogen-bonded complexes of cyclopentadiene with the strong bases ammonia, trimethylamine, and dimethyl ether have been isolated and characterized for the first time in argon matrices at 16. K. Coordination of the acidic alkyl hydrogen to the electron donor was evidenced by distinct red shifts of the CH2 stretching modes of cyclopentadiene in the infrared spectrum. An additional NH . . . pi interaction was evidenced by the red shift of an olefinic C-H stretching mode. Ab initio calculations yield a complex with NH3 located above the ring, oriented by both of these hydrogen-bonding interactions. The calculated interaction energy of the complex is 2.40 kcal/mol, with the energy being divided equally between these two interactions. This study represents the first example of an sp(3)-hybridized carbon on a hydrocarbon taking part in a C-H . . .N(O) hydrogen bond.