A convenient synthetic route to the ligand 3,7-dithianonane-1,9-diamine (dtnd) has been developed. A series of cobalt(III) complexes, [Co(dtnd)Cl₂]⁺, [Co(dtnd)O₃C]⁺, [Co(dtnd)(NO₂)₂]⁺, [Co(dtnd)(NO₂)Cl]⁺, [Co(dtnd)-(NO₂)Br]⁺, [Co(dtnd)(ox)]⁺(ox = oxalate), with a variety of anions have been prepared and characterised. The cis-complexes have the cis-β-configuration in contrast to the complexes of 3,6-dithiaoctane-1,8-diamine (dtod) which give only the cis-α-isomers. In the case of [Co(dtnd)Cl₂]⁺, both the cis-β- and the trans-isomers can be prepared. In the complex [Co(dtnd)(NO₂)₂]⁺, one nitro-group is readily replaced by Cl^– or Br^– on treatment with the appropriate mineral acid. A recent crystal-structure determination on the complex [Co(dtnd)(NO₂)Cl]Cl has shown that the nitro-group trans to the sulphur atom is displaced. The complex has the cis-β-configuration with the six-membered chelate ring in the chair conformation. The sulphur centres have the same chirality. N.m.r., i.r., and electronic spectra of the complexes are discussed.