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

Abstract

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 µ_eff = 4.9 µ_B at 300 K which is characteristic for a d⁴ 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.