The unsymmetrical complexes Ni(dmdt)(dmio) and Ni(dmstfdt)(dmio) (dmdt = dimethyltetrathiafulvalenedithiolate, dmstfdt = dimethyldiselenadithiafulvalenedithiolate, dmio = 1,3-dithiol-2-one- 4,5-dithiolate) were prepared and characterized analytically and spectroscopically. The complexes exhibit strong third-order nonlinear optical response in the visible and near-infrared spectral regions.