The third-order nonlinear optical properties of a series of asymmetric pentaazadentate porphyrin-like metal complexes ([(R-APPC)M]Cln) with different conjugated R groups and metal centers have been investigated by degenerate four wave mixing experiments using 40 ps laser pulses at 532 nm. The molecular second-order hyperpolarizabilities (γ) for these complexes range from 1.1X10-31 to 1.2X10-30 esu and depend strongly on both the nature of the metal and the R group. The y values of the [(R-APPC)Cd]Cl complexes are 2-7 times larger than those of PbPc(CP)4 and SiNc, two of the most promising nonlinear optical materials for optical limiting applications. Measurements of the individual components of the χ(3) tensor indicate that the nonlinearity is predominantly electronic in origin when 40 ps laser pulses are used. © 1999 American Institute of Physics.