Four selenium containing dmit-type ligands (dmit=1,3-dithiole-2-thione-4,5-dithiolate) possessing 1,3-diselenole substructure, namely dmist (1,3-diselenole-2-thione-4,5-dithiolate), dsist (1,3-discienole-2-thione-4,5-diselenolate), dmis (1,3-diselenole2-selone-4,5-dithiolate), and dsis (1,3-diselenole-2-selone-4,5-diselenolate), and their nickel complexes were synthesized and characterized. The synthesis of the ligands involved lithiation reaction of 1,3-diselenole-2-selone as a key step followed by introduction of additional chalcogen atoms. The molecular structures of (n-Bu4N)[Ni(dmist)(2)], (n-Bu4N)[Ni(dsist)(2)], and (n-Bu4N)[Ni(dsis)(2)] were investigated by means of X-ray analyses, and a systematic enlargement of the anion size with the number of the introduced selenium atoms was revealed.