The formation of bisorotate(1-) complexes of the type [M(C5H3N2O4)(2)(H2O)(4)] center dot nH(2)O (M = Co, Ni, Zn and n = 2, 4) was achieved by the reaction of ammonium orotate with the corresponding M(II) ions. The crystal structure of [Co(C5H3N2O4)2(H2O)(4)] center dot 2H(2)O was determined by single crystal X-ray diffraction analysis. Each Co(II) ion in the monomeric CO(C5H3N2O4)(2)(H2O)(4) units adapts a slightly distorted octahedral geometry comprised of two monodentate orotate anions and four H2O ligands. Columnar packing of pyrimidine rings along the c axis leads to the formation of layers that propagate parallel to the b axis and the adjacent layers are linked by hydrogen bonds forming a 3D lattice. Complexes of nickel and zinc were assumed to contain monodentate bound orotate ligands as well on the basis of physical and spectroscopic data. (c) 2006 Elsevier B.V. All rights reserved.