The magnetic coupling mechanism for an oxalato-brided Cu(Ⅱ) dimer has been analyzed using the density functional theory and the broken symmetry approach. The spin densities on two Cu(Ⅱ) atoms have the same values but with the opposite signs. The coupling between the two magnetic centers is antiferromagnetic, and the spin delocalization from the center toward its neighbors is significant. The strength of the coupling reduces while the envirnment around Cu(Ⅱ) atom charges from planar to tetrahedral geometry or square pyramid. The frontier orbitals are mainly composed of d-like orbitals of Cu(Ⅱ) atom and p- like orbitals of ligand. This constitution ofthe frontier orbitals favors the electron transfer from the oxalato ligand to the magnetic centers, leading to a strong antiferromagnetic coupling.

Key words: BRIDGE BOND, COPPER COMPLEX, DINUCLEAR COMPLEX, OXALATO COMPLEX, ELECTRON TRANSFER REACTION