Pyrazoles and imidazoles as ligands: Part I. Some simple metal(II) perchlorates and tetrafluoroborates solvated by neutral pyrazole and imidazole

A number of new solvates containing pyrazole (Pz) and imidazole (Iz) as ligands, of the general formula M(ligand)n(anion)2 is reported. In these compounds M = Mg, Mn, Fe, Co, Ni, Cu, Zn and Cd, the anions are ClO4− and BF44−, and n is usually 6; n = 4 in the case of the Cu compounds and for Zn in combination with Iz. The compounds are characterized and identified by chemical analysis and physical measurements. With the aid of ligand‐field spectra in combination with X‐ray powder patterns, octahedrally coordinated metal ions are found in compounds of stoichiometry M(ligand)6(anion)2. Both Pz and Iz are placed in the spectro‐chemical and nephelauxetic series. The spectrochemical series appears to be (with increasing Dq): CH3CN < Iz < NH3 < Pz < en, whereas the nephelauxetic series (with decreasing B) has the sequence: CH3CN > NH3 > Iz > Pz > en. The magnetic moments for the transition‐metal compounds fall within the range usually observed for octahedrally coordinated ions. Both paramagnetic resonance measurements and the deviation of the magnetic moments from the spin‐only value, indicate a considerable amount of covalency in the metal‐ligand bond. Infrared spectroscopy evidently shows pure ionic ClO4− and BF4− for all solvates, except those of Cu(II) where the anions are coordinated to the metal ion. Except for the N‐H modes, no large shifts occur in the ligand infrared absorptions with respect to the free ligand. Compared with the gaseous ligands, the N‐H stretch in the solvates is decreased, whereas the bending modes are increased in frequency. This is attributed to ‐ weak ‐ hydrogen bonding with the anions in the lattice of these solvates. Metal‐ligand stretching frequencies for both ligands occur in the 165‐330 cm−1 range, and follow the Irving‐Williams sequence of metal ions.