Conventional reactions of the versatile multidentate ligand 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (HmtpO) with metallic(II) salts lead to three novel multidimensional complexes [Cu(HmtpO)(2)(H(2)O)(3)](ClO(4))(2)·H(2)O (1), {[Cu(HmtpO)(2)(H(2)O)(2)](ClO(4))(2)·2HmtpO}(n) (2) and {[Co(HmtpO)(H(2)O)(3)](ClO(4))(2)·2H(2)O}(n) (3). In each compound, the triazolopyrimidine ligand shows a different and unusual coordination mode, giving rise to structures with diverse topologies and dimensionality. Compound 1 is a monomeric complex, in which HmtpO shows both N3-monodentate and N1,O71-bidentate modes. 2 is a bidimensional framework with the ligand showing a N3,O71 bidentate-bridging mode. The structure of 3 consists of 1D chains, in which HmtpO displays a N1,N3,O71-tridentate-bridging mode. It should be noted that these coordination modes of the HmtpO ligand are unique in the case of compounds 2 and 3. On the other hand, the magnetic properties of the polynuclear complexes 2 and 3 have been studied showing weak ferromagnetic and antiferromagnetic behaviour, respectively.