Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)₈]⁴⁺ (M=Mo^IV, W^IV) from Superacids

Abstract

Octacyanometalates K₄ [Mo(CN)₈ ] and K₄ [W(CN)₈ ] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)₈ ]⁴⁺ [SbF₆ ]^- ₄ and [W(CNH)₈ ]⁴⁺ [SbF₆ ]^- ₄ are the first examples of eight-coordinate homoleptic metal complexes containing hydrogen isocyanide (CNH) ligands. The complexes were crystallographically characterized, revealing hydrogen-bonded networks with short N⋅⋅⋅H⋅⋅⋅F contacts. Low-temperature NMR measurements in HF confirmed rapid proton exchange even at -40 °C. Upon protonation, ν(C≡N) increases of about 50 cm^-1 which is in agreement with DFT calculations.