Electrochemical oxidation of the metal-organic complex, nickel(II)–bis(1,3-dithiole-2-thione-4,5-diselenolate) or simply [Ni(dsit)2]2−, yields highly conducting salts, in which the stoichiometry and packing arrangement depends critically on the size of the counter-cations incorporated. Smaller counter-cations, e.g., tetramethylammonium and tetramethylphosphonium ions, yield salts with a 1:2 cation:[Ni(dsit)2] stoichiometry, while the larger tetraethylammonium ion yields a salt with 2:2 stoichiometry. In both these salts, the [Ni(dsit)2] units occur as tightly bound dimers, in which the coordination geometry of nickel is a unique, but not unprecedented square-pyramidal type. Moreover, the packing arrangement of [Ni(dsit)2]2 dimer units in both (Me4N)[Ni(dsit)2]2 and (Me4P) [Ni (dsit)2]2 is κ-type, similar to that found in the organic superconductor with the highest-known Tc (10.4 K), κ-(BEDT-TTF)2Cu(NCS)2. Both these salts possess good room temperature conductivities (σrt = 36 and 19 S.cm−1 respectively), but the temperature dependance of their conductivities is characteristic of semiconductors with small bandgaps (Eg = 0.11 and 0.13 eV, respectively). The 2:2 salt (Et4N)2[Ni(dsit)2]2, on the other hand, has a substantially lower room temperature conductivity, about six orders of magnitude smaller than that of the 1:2 salts. Its structure is characterized by [Ni(dsit)2]2 dimer units which are separated from each other by the tetraethylammonium cations, with no effective interaction between [Ni(dsit)2]2 dimers. It is suggested that dimeric metal-dithiolene and metal-diselenolene complexes may be potential building blocks in the structural design of κ-type organic conductors and superconductors.