AbstractThe predictions of Hückel's rule gave the incentive to numerous investigations which led ultimately to a new definition of aromatic character, and which have added greatly to our knowledge of the properties and reactivity of aromatic compounds. Increasing use was made of physical measurements as criteria for the aromaticity of a compound. The first member (n = O) in the series of Hückel's (4n + 2) π systems is the cyclopropenylium cation. The predictions regarding the stability of this system have been confirmed at least qualitatively by the synthesis of cyclopropenylium salts. Properties and reactions of cyclopropenylium compounds and cyclopropenones were investigat