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

Octacyanometalates K(4)[Mo(CN)(8)] and K(4)[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+) [SbF(6)](−) (4) and [W(CNH)(8)](4+) [SbF(6)](−) (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.