Co-ordination compounds of the new ligand 1,8-bis(3,5-dimethyl-1 -pyrazolyl)-3,6-dithiaoctane (bddo) with MCl₂(M = Fe, Mn, Ni, Co, Zn, Cu, or Cd), MBr₂(M = Mn, Co, Ni, or Zn), Cu(BF₄)₂, and CuX (X = BF₄, NCS, Cl, Br, or I) are described. The general formula for the divalent metal is [M(bddo)X₂] and for copper(I), [Cu₂(bddo)X₂]. With CuCl₂ two modifications were obtained. The green modification of [Cu(bddo)Cl₂] crystallises in space group P2₁/n with a= 9.019(2), b= 28.671(5), c= 8.431(2)Å, β= 113.65(2)°, R= 0.055, and R′= 0.066 for 1 578 unique reflections [I > 2σ(I)]. The compound consists of Cu(bddo)Cl₂ units. The copper atom is co-ordinated by two pyrazole nitrogens and two chloride atoms, in trans positions, in a distorted square-planar geometry. The red modification of [Cu(bddo)Cl₂] crystallises in space group Pbcn with a= 9.397(4), b= 15.093(4), c= 15.142(4)Å, Z= 4, R= 0.069, and R′= 0.089 for 864 unique reflections [I > σ(I)]. This compound consists of CuCl₂ units linked together by ligand molecules, thus forming chains with distinct C₂ symmetry perpendicular to the chain axis. The copper atom is co-ordinated in a distorted-tetrahedral geometry by two pyrazole nitrogens and two chloride atoms in cis positions. The sulphur atoms do not participate in the co-ordination, although molecular-mechanics calculations show that the ligand bddo is not sterically hindered to form tetradentate mononuclear chelates, i.e. with a MN₂S₂ chromophore. The structures of the other divalent metal halides were established as being very similar to that of the red modification. For [Cu(bddo)(BF₄)₂] semi-co-ordination of one or both tetrafluoroborates is indicated by the i.r. spectrum. Solid-state ¹³C n.m.r. spectra of the copper(I) compounds indicate that the S atoms show significant shifts, suggesting co-ordination. In the thiocyanate and iodide compounds both thioether sulphurs co-ordinate in an identical manner, whereas in the chloride and bromide compounds they co-ordinate in a different manner.