The transient nonlinear optical response of five novel unsymmetrical neutral nickel dithiolene complexes has been studied experimentally in the nanosecond regime at 532 and 1064 nm using the Z-scan technique. The nonlinear absorption and refraction parameters beta and gamma' respectively, the third order susceptibility chi((3)) of the solutions and the effective second-order hyperpolarizability gamma of the complexes were determined at both wavelengths. All dithiolene complexes studied were found to exhibit significant nonlinear refraction, larger than the corresponding nonlinear absorption. The second-order hyperpolarizabilities gamma values were found to be as large as 10(-27) esu. The origins of the observed nonlinear optical response are also discussed. (c) 2006 Elsevier B.V. All rights reserved.