Synthesis, characterization, and reactions of tetrakis(nitrile)chromium(II) tetrafluoroborate complexes

Reaction of chromium(II) acetate with HBF4 in nitrile solutions led to air-sensitive, paramagnetic complexes of the general formula [Cr(NCR)(4)][BF4](2) (R = Me, Bu-t or Ph). In contrast to an earlier report of the synthesis of [Cr(NCMe)(4)][BF4](2) by electrochemical methods, these compounds are blue. The crystal structure of [Cr(NCMe)(4)][BF4](2) has been determined. The chromium(II) cation is surrounded by nitrile ligands in a square-planar manner. The BF4- counter ions are located at the apical positions of an elongated octahedron. Magnetic susceptibility measurements yielded mu(eff) = 4.9 mu(B) at 300 K which is characteristic for a d(4) high-spin complex. The paramagnetic complexes [Cr(NCR)(4)][BF4](2) are easy to prepare and can be used for the high-yield synthesis of several other chromium derivatives in low oxidation states like [Cr(py)(4)][BF4](2), [Cr(C5H5)(2)], [Cr(dppe)(2)][BF4](2) and [Cr(NO)(NCMe)(5)][BF4](2).