Reexamination of the magnetic properties of Cu2(dca)4(2,5-me2pyz) {dca = dicyanamide; me2pyz = dimethylpyrazine}:: isolated spin-1/2 dimers versus long-range magnetic ordering

The magnetic properties of Cu-2(dca)(4)(2,5-me(2)pyz) have been reexamined. The extended structure of Cu-2(dca)(4)(2,5-me(2)pyz) can be viewed in terms of Cu-2(2,5-me(2)pyz)(4+) dimer units interconnected via mu(1,5)-dca ligands. The bulk magnetic susceptibility chi(T) and the isothermal M(H) of Cu-2(dca)(4)(2,5-me(2)pyz) are shown to be well described by an isolated dimer model. This finding was confirmed by carrying out a spin dimer analysis based on tight-binding calculations, which shows that the 2,5-me(2)pyz ligand provides a substantial spin exchange interaction between the Cu2+ ions while the dca ligands do not.