A useful parameter describing magnetic coupling interactions has been inspected in transition metal dimers, in which two transition metal ions are bridged by extended bis-bidentate ligands. This parameter is the square of overlap integral between the spatial parts of magnetic orbitals in the broken symmetry state. The oxalato-bridged Cu (II) dimers, in which the Cu (II) atom is either in tetrahedral or square pyramid coordination environment, have been calculated with the density functional theory coupling the broken symmetry approach.

The geometry structure, dissociation energy, vibrational frequencies, and low-lying spin-state energy spectrum of Mn2+ are investigated by using ab initio CASSCF/ECP10MDF, complete active space self-consistent field/atomic natural orbital basis sets CASSCF/ANO-s, CASPT2/ECP10MDF, and second-order perturbation theory with CASSCF reference function/atomic natural orbital basis sets CASPT2/ANO-s levels of theory. For the ground state the dissociation energy of 1.397 eV calculated at the CASPT2/ANO-s level supports Jarrlod's experimental value of 1.39 eV. The equilibrium bond length and vibrational frequency are 2.940 Å calculated at the CASPT2/ANO-s level of theory and 214.4cm−1 calculated at the CASSCF/ANO-s level of theory, respectively. On the basis of the mixed-valence model, the Heisenberg exchange constant J−71.2cm−1 and the double-exchange constant B 647.7cm−1 are extracted explicitly from the low-lying energy spectrum calculated at the higher levels of theory. The magnetic competition between the weaker Heisenberg exchange interactions and the stronger double-exchange interactions makes the ground state a 12g+ state, consistent with electron paramagnetic resonance experimental observation, which explains unusual magnetic properties of Mn2+, quite different from the antiferromagnetic ground state of Mn2 and Cr2. On the other hand, the results calculated at the higher levels of theory show the consistent antiferromagnetic Heisenberg exchange interactions between 3d−3d for Cr2, Mn2+, and Mn2.

The construction of low-dimensional coordination polymers has received intense attention for a long time because of their structural diversity and unusual properties as well as their magneto-structural correlations.[1] Among them, some onedimensional (1D) expanded chains with exotic topologies have been reported, such as ladders,[2] ropes,[3] and clusterbased chains.[4] They provide an intermediate step from 1D chains towards 2Dnetworks, thus potentially helping the magnetic behavior at the border of 1Dto 2Dsystems to be understood. As a versatile bridging ligand, the azido ion could link two or more metal ions in various modes, for example, m-1,1 (end-on (EO)), m-1,3 (end-to-end (EE)), m-1,1,1, m-1,1,3, and so forth, thus giving rise to a variety of zero-to threedimensional polynuclear complexes.[5-9] As far as 1Dchains are concerned, many unusual alternating chains with different combinations of these modes have been obtained.[7,8]

Two new complexes, 1 and 2, with halo acetate ligands have been prepared from the reaction of ClCH2CO2H or BrCH2CO2H with the precursor Mn12Ac, and their structures and magnetic properties studied. The structures of complexes 1 and 2 consist of a central cubane-like [Mn4O4]8 unit held within a non-planar ring of eight Mn4 by eight µ3-O ions, sixteen XCH2CO2 units and four terminal water molecules, as well as included CH2Cl2 and H2O. Dierent arrangements of the four Mn4 relative to the eight Mn4 comparable complexes could play a role in the observed magnetic properties. Five and three periodic steps in the magnetic hysteresis loops are observed for both 1 and 2 at 2.2 and 2.6K, respectively.χmT values for complexes 1 and 2, where χm is the in-phase component of the AC magnetic susceptibility, exhibit a plateau in the 6-20K region, below which there is an abrupt decrease in χmT. Their χm signals measured at different frequencies are observed in the region 2-10 K, and exhibit a frequency dependency on the χm AC peaks, with a predominant peak in the 4-10K region. For these complexes, there is a difference between the zero-field-cooled (ZFC) and the field-cooled (FC) magnetization due to slow relaxation of the magnetization.

First published as an Advance Article on the web 25th October 2002 A novel 1D-coordination polymeric Co (II) complex [Co(btp)2(NCS)2]n [1, btp ¼ 1,3-bis(1,2,4-triazol-1-yl)propane] was synthesized and characterized. The coordination geometry of the cobalt ions is distorted octahedral, with the equatorial plane formed by the four nitrogen atoms of the four triazole groups and the axial sites occupied by two terminal trans thiocyanato ligands. The Co (II) ions are linked by the btp ligands, building up coordination polymers of infinite 1D-neutral chains.