Pyrazole complexes of cobalt(II)

Cobalt acetylacetonates react with hydrazine hydrate in ethanolic solution to give bis-(3,5-dimethylpyrazolato)-cobalt(II). This and the corresponding salts of pyrazole and 3-methylpyrazole can also be prepared from cobalt(II) salts in basic solution. These compounds are believed to possess polymeric tetrahedral structures. In the absence of added base the reaction of pyrazoles with cobalt(II) salts in ethanol or water give pseudo-octahedral complexes [CoL4X2](L = pyrazole, 3-methylpyrazole; X = Cl, Br, SCN, NO3, or 1//2SO4) and pseudotetrahedral complexes [CoL2X2](L = 3,5-dimethyl-, 3-methyl-5-phenyl-, or 1,3,5-trimethyl-pyrazole; X = Cl, Br, or SCN. The electronic spectra of these compounds as well as those of [Co(pyrazole)6(NO3)2] and [CoL2(NO3)2](L = 3,5-dimethyl- or 1,3,5-trimethyl-pyrazole) are reported.