The first dimeric mixed-ligand zinc(II) complex of 1,3-dithiole-2-thione-4,5-dithiolate (dmit) [Zn(dmit)(2,2′-bpy)]2 (2,2′-bpy = 2,2′-bipyridine) (1), which exhibits a good third-order nonlinear refraction effect, was synthesized by a ligand exchange reaction between [(Et4N)2Zn(dmit)2] and [Cu(2,2′-bpy)(NO3)2]·2H2O. A Z-scanning measurement of complex 1 gives the third-order nonlinear refraction index n2 = 3.72 × 10−18 m2/W and the real part Reχ(3) = 1.01 × 10−12 esu. Crystal data for 1: monoclinic, P21/c (No.14), a = 11.556(2) Å, b = 9.141(2) Å, c = 15.020(3) Å, β = 109.55(3)°, Z = 4. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)