The mononuclear copper(II) complexes CuL1 and CuL2 have been prepared with two tetradentate Schiff base ligands, abbr. (LH2)-H-1=N,N '-bis(salicylidene)-1,3-diaminopropane and (LH2)-H-2 = N,N '-bis(salicylidene)-1,4-butanediamine, and then transformed into the heterotrinuclear complexes, [(CuL1)(2)Hg(SCN)(2)] and [(CuL2)(2)Hg(SCN)(2)], in the presence of the Hg(SCN)(2) salt. Both heterotrinuclear complexes have been characterized by means of elemental analyses, IR and electronic spectra, and their structures determined by X-ray analysis. The title compounds Hg[Cu(C17H16N2O2)SCN](2) (1) and Hg[Cu(C18H18N2O2)SCN](2) (2), both heterotrinuclear centrosymmetric Cu(II)-Hg(II)-Cu(II) complexes, crystallize in the triclinic P (1) over bar space group and the monoclinic P2(1)/c space group, respectively. In both compounds the central Hg atom, which is located on an inversion centre, has a distorted octahedral coordination geometry. The coordination geometry of the inversion-related terminal Cu atoms of the compounds is square-pyramidal. The ligands and the complexes were screened for antibacterial and antifungal activities by the disc diffusion and microtitter plate techniques using DMF as the solvent. The minimum inhibitory concentration (MIC) values were calculated. It has been found that the antimicrobial activities of the complexes are higher than those of the free ligands. (C) 2007 Elsevier Ltd. All rights reserved.