Abstract5,7‐Dimethyl[1,2,4]triazolo[1,5‐a]pyrimidine (dmtp) has been used as a ligand for nickel(II) and cobalt(II) chloride, bromide, iodide and triiodide. The halides yield compounds of composition trans‐[M(dmtp)2(H2O)4](H2O)2X2, the triiodides having the composition trans‐[M(dmtp)4(H2O)2](dmtp)2(I3)2. The IR spectra of he halides are all mutually isomorphous, differing from those of the triiodides. Spectroscopic properties agree with octahedrally coordinated metal ions with MN2O4 and MN4O2 chromophores, respectively. The crystal structures of two representative compounds, i.e. the nickel iodide and the nickel triiodide compounds, are described.[Ni(dmtp)2(H2O)4](H2O)2I2 is triclinic, P−1, a 8.175(1) Å, b 8.263(3) Å, c 9.628(2) Å), α 85.60(2)°, β 81.84(2)°, γ 82.06(2)°, Z 1, dc 1.870 Mg.m−3, R 0.084 based on 2516 reflections. Ni2+ is octahedrally coordinated by two N(3)‐bonded dmtp ligands (trans‐oriented) with Ni‐N 2.080(6) Å) and four water molecules with Ni‐O 2.067(6) Å and 2.094(6) Å, respectively. The iodides and the remaining water molecules are retained in the lattice, through extensive hydrogen bonding. [Ni(dmtp)4(H2O)2](dmtp)2(I3)2 is triclinic, P−1, a 12.428(2) Å, b 14.852(2) Å, c 8.795(2) Å, α 103.27(2)°, (3 108.80(3)°, γ 89.73(2)°, Z 1, dc 1.944 mg.m−3, R 0.092 based on 3724 reflections. Ni2+ is octahedrally coordinated by two water molecules in a trans orientation with Ni‐O 2.13(1) Å and by four N(3)‐bonded dmtp ligands with Ni‐N 2.07(2) Å and 2.09(2) Å, respectively. The triiodide ions are linear with II bonds of 2.918(3) Å and 2.884(3) Å. The compound forms a lattice adduct with two extra dmtp molecules, with stacking interactions.