The Ni(II) complexes of 1,4,7,10,13,16-hexathiacyclooctadecane (18S6, hexathia-18-crown-6), 2,5,8-trithianonane, 1,5,9-trithiacyclododecane (12S3, trithia-12-crown-3), and 1,5,9,13,17,21-hexathiacyclotetracosane (24S6, hexathia-24-crown-6) have been synthesized and characterized by single-crystal X-ray diffraction and electronic spectroscopy. Each of these complexes contains a high-spin [Ni(thioether)6]2+ cation with octahedral microsymmetry. Comparison of these complexes shows that within a conserved coordination sphere the different ring sizes (1) change the Ni-S distances by up to 0.05 ?, (2) affect the ligand field splitting by up to 10%, and (3) greatly influence the stability of the resulting complexes. Crystallographic data: [Ni(18S6)](picrate)2, C24H28N6O14S6Ni, monoclinic, space group C2/c (No. 15), a = 21.079 (6) ?, b = 8.685 (2) ?, c = 18.399 (5) ?, β = 93.63 (2)°, Z = 4; [Ni(12S3)2](BF4)2, C18H36B2F8S6Ni, orthorhombic, space group Pbca (No. 61), a = 19.369 (9) ?, b = 12.500 (5) ?, c = 11.862 (2) ?, Z = 4; [Ni(24S6)](BF4)2, C18H36B2F8S6Ni, monoclinic, space group C2/c (No. 15), a = 13.744 (5) ?, b = 17.021 (5) ?, c = 13.801 (5) ?, β = 113.25 (3)°, Z = 4.