Structural Effects on Molecular Dipoles and Solvatochromism of Nickel(diimine)(dithiolate) Complexes

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
(μgs) in the wide range from 4 to 17 D, in contrast to the near constant μgs reported earlier for a series of platinum(diimine)(dithiolate) complexes. 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 acceptors 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 π-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 π-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 π-delocalization proposed.