The reaction of ethanolic/water solutions of NiX2 . 6H(2)O (X = NO3-, ClO4-), bidentate amine N,N,N',N'-tetramethylethylenediamine (tmen) and N,N,N'-trimethylethylenediamine (trimen) with [Cu(oxpn)] (oxpn = N,N'-bis(3-aminopropyloxamide) and ammonium thiocyanate, in the presence of ammonium hexafluorophosphate when X is NO3-, leads to the high-yield formation of new heterotetranuclear (Cu-Ni)(2) complexes of general formulation [Cu(oxpn)Ni(mu-NCs) (H2O)(aa)](2)(X)(2) (1, X = PF6-, aa = tmen; 2, X = ClO4-, aa = tmen; 3, X = ClO4-; aa = trimen; 4, X = PF6-, aa = trimen). The crystal structures of 1, 2, and 3 were determined. Complex 1 crystallizes in the triclinic system, space group with a = 9.059(8) Angstrom, b = 12.604(2) Angstrom, c = 12.743(4) Angstrom, alpha = 95.27(6)degrees, beta = 99.22(2)degrees, gamma = 108.86(7)0, and Z = 2. Complex 2 crystallizes in the triclinic system, space group with a = 8.7586(7) Angstrom, b = 12.580(4) Angstrom, c = 12.6318(12) Angstrom, alpha = 94.630(14)degrees, beta = 98.486(8)degrees, gamma = 107.963(13)degrees, and Z = 2. Complex 3 crystallizes in the triclinic system, space group with a = 8.407(7) Angstrom, b = 12.779(4) Angstrom, c = 14.081(3) Angstrom, alpha = 63.25(2)degrees, beta = 88.62(3)degrees, gamma = 79.06(4)degrees, and Z = 2. The cationic parts of the three complexes are very similar, consisting of two [Cu(oxpn)Ni] moieties linked by two SCN- bridging ligands. The Cu(II)ions have square-pyramidal geometry with the S atom of the thiocyanato ligand in the apical position at ca. 3 Angstrom. The Ni-II ions are distorted octahedrally coordinated. Variable-temperature magnetic susceptibility studies were performed on 1-4 in the temperature range 2-300 K. Satisfactory fits to the observed susceptibility data were obtained by assuming isotropic magnetic exchange interactions and using the appropriate spin Hamiltonians considering the complexes first as two dinuclear entities and second as a single tetranuclear unit. In the former the susceptibility equation is easy to derive, but for the latter full-matrix diagonalization was needed. The J value for the Cu(oxpn)Ni entity isclose to -110 cm(-1) in all four cases. By using the full-matrix diagonalization method it was possible to derive both J(1), Cu(oxpn)Ni and J(2), Cu-SCN-Ni coupling constants. J(1) is very similar to that derived assuming a dinuclear entity (ca. -115 cm(-1)) and J(2) is very small but positive, similar to 0.03 cm(-1) (ferromagrnetic coupling). Theoretical calculations were performed for different J(2) values (from 10 to -10 cm(-1)) assuming a high J(1) coupling constant (J(1) = -115 cm(-1)). Such calculations revealed the influence of this J(2) value on the susceptibility curves at low temperatures. The EPR spectra measured at different temperatures (from 4 to 300 K) on polycrystalline samples of 1-4 show the typical band centered at g approximate to 2.2 due to the spin transition \1/2, -1/2> --> \1/2, 1/2> of the spin ground-state S-T = 1/2.