Octacyanometalates K 4 [Mo(CN) 8 ] and K 4 [W(CN) 8 ] are completely protonated in superacidic mixtures of anhy-drous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH) 8 ] 4+ [SbF 6 ] À 4 and [W(CNH) 8 ] 4+ [SbF 6 ] À 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 À40 8C. Upon protonation, n(C N) increases of about 50 cm À1 which is in agreement with DFT calculations.