Substituent effects and the stability of chromarenes

The chemistry and air stability of the symmetric chromarenes (Ar2Cr) and the unsymmetric chromarenes of the type ArCr(C6H6) can be correlated with the sum of the Hammett σm) parameters of the substituents. It is proposed that the unsymmetrical chromarenes exhibit an internally compensating stabilisation effect analocaus to that of the chromium tricarbonyl moiety. This postulate is supported by NMR and polarographic evidence.