K4[W(CN)8]•2 H2O was prepared according to the literature procedure. Tetrabutylammonium chloride, tetrabutylammonium hydroxide 40% w/w in water, Amberlyst 15 hydrogen form, methyl triflate and methyl fluoride were purchased from commercial sources and used without further purification. Sulfur dioxide (Attention: toxic gas with boiling point of -10 °C, high vapor pressure) was dried over calcium hydride and stored in a stainless-steel gas cylinder. AsF5 was prepared by literature procedures and stored in a stainless-steel gas cylinder. AsF5 is a highly toxic and corrosive gas (boiling point -53 °C, Attention: high vapor pressure) that should be only handled by trained personal with suitable protective equipment. For condensing in methyl fluoride (boiling point -78 °C, vapor pressure of 33 bar at room temperature (!)) as well as AsF5, the gas cylinders were placed in cold ethanol baths to reduce the overpressure. Strong alkylating agents (e.g. methyl triflate) are potentially carcinogenic.