Abstract The complexes of the [CoCN(O–O)(N)3] type (O–O represents CO32− or C2O42−, and (N)3 represents three NH3, one NH3 plus one en, or one dien) have been prepared starting from the [Co(CO3)3]3− complex, and their characterization has been achieved on the basis of the absorption and PMR spectra. Among these, each of the fac(N) isomers of the carbonato complexes shows a shoulder in the second absorption band and no shoulder in the first absorption band. In contrast, each of the corresponding fac(N) isomers of the oxalato complexes shows no such shoulder in the second absorption band, but does show a shoulder in the first absorption band. The same phenomena have been found in the spectra of the trans(N)-[Co(O–O)(mda)(NH3)]− complexes (mda stands for the methyliminodiacetate ion).