Various types of metal complexes have received attention because of their large third-order nonlinear optical properties. Thin films of d(8) transition metal complexes with dionedioxime have been shown to form stacks perpendicular to the molecular plane, the central metal ions forming a quasi-one-dimensionaI chain of delocalized electrons. We have measured the imaginary part of the third-order nonlinear optical susceptibility, chi((3)), of one such material, Ni(dmg)(2), by using the pump-probe method. We give a detailed derivation of the analysis of Im omega((3)) from the differential transmission data and present results near the lowest energy resonance of this material. Maximum values of the continuous wave susceptibility chi(cw)((3)) reached -4 x 10(-5) esu at resonance, with a relatively slow relaxation time of the order of I ns. The differential transmission crossed over from negative (saturation) to positive already on the low-energy side of the absorption band.