A series of acceptor-substituted nickel(diimine)(dithiolate) complexes were synthesized and characterized. Both ground-state and excited-state dipole moments were determined. These complexes have ground-state dipole moments (mu(g)s) in the wide range from 4 to 17 D, in contrast to the near constant mu(g)s reported earlier for a series of platinum(diimine)(dithiolate) complexes.(3) The excited-state dipole moments of the complexes, ranging between 10 and -9 D, are either smaller than or opposite to the ground-state dipoles. The relative strengths and directions of the ground-state and excited-state dipoles were found to be strongly influenced by the electronic accepters of the complexes. Ground and excited states dipoles of opposite sign were found for complex nickel(N,N'-ditolyl-3,4-hexanediimine)(tfd), 5 (tfd = 1,2-trifluoromethylethene-1,2-dithiolate). The opposite dipoles are related to the pronounced solvatochromic shift observed for the ligand to ligand charge-transfer (LLCT) band (544 nm in acetonitrile and 641 nm in toluene) of 5. On the basis of single-crystal X-ray structures, substantial pi-delocalization character is implicated for the central (N=C-C=N)Ni(S-C=C-S) unit of Ni(disn)(tfd), 3 (disn = diimnosuccinonitrile), and nickel(4-benzoyl-o-benzoquinonediimine) (tfd), 7. The pi-delocalization character has not been noted for other known M(diimine)(dithiolate) (M = Ni, Pd, Pt) complexes. A number of other unusual optical properties, such as large molar extinction coefficient (12 000-19 000 M-1 cm(-1)), weak solvatochromic shift, and small ground-state dipole moment (4-9 D), found for 3 and 7, are consistent with the pi-delocalization proposed.