Improved preparations of cobalt(III) tetrakis(N-methylpyridiniumyl)porphyrin (Co(TMpyP(4)) derivatives were developed to avoid Co(II) contaminants produced by the standard precipitant, iodide. Careful oxidation of solutions containing TMpyP(4), CoCl2, and HCl followed by precipitation with ClO4- gave a product, compound I, with an acceptable elemental analysis for a formula containing ClO4- as the only source of Cl. However, the H-1 NMR spectrum in DMSO-d(6) has signals for two nearly equally abundant species (a and b). Signals for only one of these species (a) are present in the new product [Co(TMpyP(4))Cl(H2O)](PF6)(4). Therefore, species a of compound I is a monochloro derivative; the elemental analysis of compound I did not readily distinguish between ClO4- and Cl-/H2O combinations. Solvent suppression NMR of the methyl signal of DMSO (natural abundance) by techniques that do not lead to signal saturation showed that this monochloro species a has one bound DMSO signal at a shift (-0.55 ppm) consistent with a DMSO bound through oxygen (DMSO). At 30 degrees C, this signal was broad; it was most readily observed at lower temperatures, indicating that the DMSO is in relatively fast exchange. Solvent suppression experiments showed that b was [Co(TMpyP(4))(DMSO)(2)](5+) and that DMSO exchange was slow. This (DMSO)(2) species was formed nearly exclusively from [CO(TMPyP(4))(H2O)(2)](ClO4)(5), which was best prepared in the absence of Cl- or NO3-. In water, [Co(TMpyP(4))X(H(2)0)](4+) (X = Cl- and NO3-) were not formed appreciably, but these 4+ species were precipitated preferentially over the 5+ diaqua species. Undeuterated DMSO was titrated into CDCl3 solutions of cobalt(III) chloro complexes of tetraphenylporphyrin (Co(TPP)CI), octaethylporphyrin (Co(OEP)Cl), and tetrakis(4-(trifluoromethyl)phenyl)porphyrin (Co-(TCF3PP)Cl). At 1:1 to 2:1 ratios of DMSO:Co(III) porphyrin, the DMSO signal was observed from -0.7 to 1 ppm. Further addition of DMSO shifted the DMSO signals downfield for all three porphyrins, but no separate free DMSO signal was observed. The pyrrole signals shifted downfield but gave no evidence of two slowly exchanging species. These results demonstrated that for these porphyrins the DMSO was in fast exchange. The downfield shift of the pyrrole signal indicated that, at ratios of DMSO to porphyrin >2, the CI,DMSO species were fully formed. In 100% DMSO-d(6), both Co(TPP)CI and Co(TCF3PP)CI have two species. Solvent suppression experiments as a function of temperature and added LiCl showed that one had (DMSO)2 and the other CI,DMSO axial ligation. As found for Co(TMpyP(4)), the DMSO exchange rate was slower for the (DMS0)2 than for the CI,DMSO species. The preference for Cl- binding Was found to be Co(TMpyP(4)) > Co(TCF3PP) > Co(TPP).