Slow diffusion of an ethanolic solution of copper(II) into an aqueous solution of partially deprotonated malonodihydroxamic acid (H₄mdh) affords the pentanuclear complex of formula {[K(H₂O)₂]₂[Cu₅(mmh)₄]}_n (1) (H₃mmh = malonomonohydroxamic acid) which is formed by 12-MC-4 metallacrown units with the fifth copper(II) ion being placed at the center of the square metallacrown unit. The dianionic pentacopper(II) planar entity interacts with diaquapotassium(I) counterions through the carboxylate-oxygen atoms resulting in a neutral three-dimensional structure. Magnetic susceptibility measurements in the temperature range 1.9–300 K for 1 show the occurrence of relatively strong antiferromagnetic interactions within the pentanuclear copper(II) core leading to two degenerated low-lying spin doublets at T < 50 K. X-band EPR spectra at low temperatures give additional support to the ground spin doublet nature. The topology of the pentanuclear copper(II) unit of 1 together with good ability to mediate magnetic interactions between the copper(II) ions of the diatomic N–O(oximato) and monoatomic O(oximato) bridges which connect the metal centers within the pentanuclear core account for this magnetic behaviour.