The real part of third-order optical nonlinearity of an azo dye (2-(2-thiazolylazo)-5-diethylaminophenol TAEP) and those of its copper-and cobalt-substituted compounds are measured by using femtosecond optical heterodyne optical Kerr gate technique at the wavelength of 647 nm. All the values are found to be negative. Those of Cu- and Co-substituted compounds are found to be larger than that of the metal-free dye by two and three orders of magnitude, respectively. A negative gamma (eff) as large as 10(-29) esu is found in Co complex. We use a three-level model to characterize the nonlinear process, and suggest that pi -electron delocalization in metal complex leads to an enhancement.