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Cobalt-() and -() complexes of pentadentate N 4 S ligands based on methyl 2-aminocyclopent-1-ene-1-carbodithioate with appended pyrazolyl groups (3,5-Me 2 C 3 HN 2 CH 2 ) 2 NCH(R)CH 2 NHC 5 H 6 C(᎐ ᎐ S)SCH 3 (R = H, Hmmecd; CH 3 , Hmmpcd) have been prepared and characterised by IR, 1 H NMR and electronic spectroscopy. Two of these compounds have also structurally been characterised by X-ray single crystal diffraction analyses. Cobalt() in [Co(mmpcd)]ClO 4 , 1, shows a five-coordinate, trigonal bipyramidal geometry while its cobalt() counterpart, [Co(mmpcd)Cl]ClO 4 , 2, reveals a six-coordinated distorted octahedral structure by the inclusion of a chloride ligand in its equatorial plane. In dmf or acetonitrile solution, 1 can bind dioxygen reversibly as indicated by EPR spectra recorded at cryogenic temperatures. Metal-dioxygen binding in 1 appears to be weak, possibly due to its trigonal bipyramidal structure and the presence of a sulfur donor in the ligand framework. Electronic spectra of the cobalt() complexes show two LMCT bands in the near UV region, tentatively assigned to S→Co III charge transfer.