A series of mono Schiff-base M(II) (M = Ni, Pd, Pt) complexes based on the crystal data are designed. Density Functional Theory (DFT) method was employed to investigate the second-order nonlinear optical (NLO) properties of all metal complexes. The results show that the second-order NLO properties of metal complexes are intensively sensitive to the exchange of donor/acceptor because of the differences of the extent of charge separation and the intraligand charge transfer (ILCT) processes. The metal ions do not offer direct contributions to second-order NLO responses because they are as electron bridges for transferring electrons in CT processes. (c) 2006 Elsevier B.V. All rights reserved.