Abstract The title compound C 18H 30ClCu 4N 12O 6 has been synthesized and its crystal structure and magnetic properties determined. The compound contains a tetranuclear copper(II) complex in which two dimeric units are bridged by two pyrazolate ligands and a perchlorate anion which is bound to the tetrameric unit by hydrogen bonds in the asymmetric unit. In each dimeric moiety the two copper(II) ions are bridged by the alkoxo group of the ligand and an additional pyrazolate ligand, these dimers are then bridged by two pyrazolate ions to form the tetranuclear complex. Magnetic susceptibilities for the compound in the solid state are measured over the temperature range 4.5–306.8 K. The experimental magnetic susceptibilities were fitted to the Bleaney–Bowers equation for a spin-coupled S=1/2 system in which the singlet–triplet separation is 2 J. The title compound shows weak antiferromagnetic coupling with a best fit J value −98.5 cm −1. The alkoxo oxygen bridge is one of the principal pathways for the super-exchange interaction and the weak antiferromagnetic coupling of the complex is interpreted in terms of the countercomplementary effect of the different bridging ligands which participate in the super-exchange interactions. The ab initio restricted Hartree–Fock molecular orbital calculation has shown that the pyrazolate bridge contributes to magnetic interaction countercomplementarily to the alkoxide bridge.