The reaction of [Cu(sac)(2)(H(2)O)(4)] center dot 2H(2)O with 2-methylpyrazine (mpyz) leads two complexes, concomitant crystallization of a mononuclear complex [Cu(sac)(2)(mpyz)(H(2)O)(2)] (1) and a polymeric complex [Cu(sac)(2)(mu-mpyz)](n) (2). Both complexes have been characterized by elemental analyses, magnetic measurements, FT-IR and ESR, TG-DTA and single-crystal X-ray diffraction analyses. Single-crystal X-ray analyses show that complex 1 consists of discrete molecules in which the copper(II) ions exhibits a square-pyramidal coordination geometry. The individual molecules of 1 are connected into a hydrogen-bonded chain structure, which is further assembled to form a three-dimensional network by pi-pi stacking interactions. Complex 2 is an 1D coordination polymer in which copper(II) centers are bridged by the mpyz ligand. The chains are further assembled to form two-dimensional frameworks by pi-pi and C-H center dot center dot center dot pi stacking interactions.