Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8 ]4+ (M=MoIV , WIV ) from Superacids.

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.