An acidic ion exchange resin (Amberlyst 15 hydrogen form) (3 g) was filled in a Schlenk flask and degassed water (24 mL) was added. After the addition of potassium octacyanotungstate(IV) dihydrate (1 g, 1.71 mmol, 1 eq.) the solution was stirred for a few minutes. 18 mL of the solution were decanted off and transferred to another Schlenk flask and the solution was neutralized with 40% w/w [NBu4]OH in water (3.3 g, 5.10 mmol, 2.98 eq.). The water was removed under vacuum and the residue was taken up in degassed dichloromethane (5 mL), which was removed under vacuum afterwards. The crude product was washed with degassed diethyl ether (3x 5 mL) to remove traces of the octacyanotungstate(IV) acid and dried under high vacuum to remove traces of solvent. The product was obtained as a fine bright-yellowish powder and stored under argon.