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

Octacyanometalates K₄Mo(CN)₈] and K₄W(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.