Cyanamidonitrate-cobalt(II) complexes of pyrazole and imidazole ligands.: X-ray structure of [Co((NO)2NCN)2 (pyrazole)4] and [Co(2-methylimidazole)4]((NO)2NCN)2

bis[2-(Dimethylaminomethyl)pyrrolyl]tantalum chloride (1) was prepared by treatment of the TaCl5 with 2 equiv. of [2(dimethylaminomethyl)pyrrolyl]lithium. The solid state structure of 1 shows the seven-coordinated tantalum atom, with a geometry corresponding to a pentagonal bipyramidal structure, in which two chlorine atoms occupy the axial position with the angle of 174.8(2)°. Furthermore, variable-temperature 1H NMR spectra infer the fluxionality of the two chelating pyrrolyl ligands with an estimated energy barrier of 13.8 kcal mol-1. Compound 1 shows no catalytic activity toward ethylene polymerization even activated with MAO.