The aquatris(4-methylimidazole)orotatocobalt(II), [Co(HOr)(H2O)(4-Meim)(3)] (1) and bis[aquatris(4-methylimidazole)orotatonickel(II)] pentahydrate, [Ni(HOr)(H2O)(4-Meim)(3)](2) center dot 5H(2)O (2) have been synthesized and characterized by means of elemental analysis, thermal analysis, IR, UV-Vis and mass spectroscopic studies. The crystal structure of 2 was also determined by single crystal X-ray diffraction. The Ni(II) complex contains the isomers Lambda and Delta of [Ni(HOr)(H2O)(4-Meim)(3)], self-assembled through complementary pi-pi and C-H center dot center dot center dot pi interactions, leading to an intricate two-dimensional array. The Co(II) and Ni(II) ions in [M(C5H2N2O4)(H2O)(C4H6N2)(3)] are chelated to the deprotonated N3 pyrimidine atom and to the carboxylate oxygen atom of the bidentate orotate dianion, and to the three nitrogen atoms of the 4-Meim ligands, and their octahedral geometries are completed by an aqua ligand. The thermal decomposition pathway of the complexes has been predicted by the help of thermal analysis (TG, DTG and DTA) and mass spectroscopic data. (C) 2006 Elsevier Ltd. All rights reserved.