Diamagnetic K4[M(CN)6], M = RuII, OsII are easily and quantitatively transformed into paramagnetic [M(CN)6]3– in methanol with the help of crown ethers. Hexacyanoosmate(II) is oxidized by air, whereas for hexacyanoruthenate(II), H2O2 is needed. These two cyanometallates, (K@18-crown-6)3[M(CN)6]·nSolv, were characterized both in solution and in the solid state. The compounds were obtained as single crystals, and their crystal and molecular structures were determined by X-ray diffraction analysis. In the proposed synthetic method, the use of methanol minimizes hydrolysis and considerably increases the yield and purity.