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From the dianion [Cu(pba)]2", with pba = l,3-propylenebis(oxamato), have been synthesized the two compounds ACu-(pba)(H20)3-2H20, A =(1) and Ni (2). The crystal structure of 1 has been solved at room temperature. 1 crystallizes in the orthorhombic system, space group Pnma, with a = 12.945 (1) A, b = 21.250 (4) A, c = 5.2105 (8) A, and Z = 4 (MnCu units). The structure consists of ordered bimetallic chains with octahedral Mn(II) and square-pyramidal Cu(II) ions bridged by oxamato groups. The Mn-Cu intrachain separation is 5.412 A. The chains are linked together through hydrogen bonding. The shortest interchain metal-metal separations are -Mn = Cu-Cu = 5.211 A. The magnetic properties have been investigated down to 1.2 K for 1 and down to 3 K for 2. Both compounds exhibit the magnetic behavior typical of ordered bimetallic chains with large intrachain antiferromagnetic interaction and very weak interchain antiferromagnetic interaction. The values of the intrachain interaction parameters have been deduced from the magnetic data and found as J = -23.4 crrf1 11in 1 and -82.7 cm"1 in 2 (fí = -yEA-Sj+t)• The mechanism of the interchain interaction in 1 has been discussed, and a strategy has been suggested to avoid the three-dimensional antiferromagnetic ordering of the ferrimagnetic chains. This strategy could lead to one of the first genuine molecular ferromagnets;

The three compounds MnCu(obbz)-5H20, MnCu(obbz)• 1H20, and NiCu(obbz)-6H20, hereafter abbreviated as MnCu-5H20, MnCu-lH2Ó, and NiCu respectively, have been synthesized, obbz stands for oxamidobis(benzoato). The crystal structure of NiCu has been determined. NiCu crystallizes in the monoclinic system, space group P2\/n, with a = 7.764 (1) Á, b = 12.559 (3) Á, c = 20.083 (3) Á, ß = 97.56°, and Z = 4. The structure consists of isolated heterodinuclear units with octahedral Ni(II) and square-planar Cu(II) ions bridged by an oxamido group. NiCu is not isomorphous to or to MnCu-lH20, neither of which has been obtained in the form of single crystals suitable for X-ray work. To get information on their structure, we have studied their XANES and EXAFS spectra at both Mn and Cu edges, as well as their powder X-ray patterns. MnCu-5H20 and MnCu-lH20 have very similar structures, with four noncoordinated water molecules in the former compound. Cu(Il) is in elongated tetragonal surroundings and Mn(II) in octahedral surroundings. The radius of the first shell around Mn(II) is equal to 2.17 (2) Á for MnCu-5H20 and 2.16 (2) Á for MnCu-lH20. Those data are consistent with the following structure: Mn(II) would be surrounded by six oxygen atoms; two of these atoms MnCu-lH20 belong to an oxamido group, one to a carboxylato group, and one to a water molecule in the equatorial plane; the oxygen atoms in the apical positions would belong to other carboxylato groups, forming a twoor three-dimensional network. The magnetic properties of NiCu were studied in the 4.2-300 K temperature range. They are typical of an isolated heteropair with a doublet-quartet energy gap 37/2 equal to -156 (3) cm"1. The magnetic properties of MnCu-5H20, down to 4 K, are characteristics of alternating bimetallic chains with a minimum in the versus T plot, being the molar magnetic susceptibility per MnCu unit and T the temperature. At 2.3 K, for MnCu-5H20 shows a sharp maximum due to a three-dimensional antiferromagnetic ordering. In contrast, the versus T plot for MnCu-lH20 diverges when T approaches the critical temperature Tc = 14 K. Below T0, MnCu-lH20 exhibits a spontaneous magnetization. The field dependences of the magnetization M for MnCu-5H20 and MnCu-lH20 have been compared. For both compounds, the saturation magnetization corresponds to a S = 2 resulting spin per MnCu unit. Moreover, MnCu-lH20 at 4.2 K exhibits an = f(H) hysteresis loop characteristic of a soft magnet.The powder X-band EPR spectra of MnCu-5H20 and MnCu-lH20 have been recorded at various temperatures down to 15 K. The comparison of the line widths suggests that the mechanism of the magnetic ordering observed for MnCu-lH20 is of exchange interaction nature rather than of dipolar nature. The perspectives in the field of the molecular-based ferromagnets are discussed.