The negative solvatochromic and enzymichromic properties of 4-carbamidopyridinium cyclopentadienylide, 3, are presented. Tle 78 nm bathochromic shift of the long wavelength absorption band of 3 upon going from water to the horse liver alcohol dehydrogenase (HLADH)-NADH ternary complex is consistent with a low dielectric for the hydrophobic binding pocket of this enzyme.