Thiocyanates of Molybdenum

Thiocyanates containing molybdenum as a cation are not known, but compounds similar to the complex cyanides, in which the metal figures as the central atom of a complex anion, have been prepared.

1. Compounds containing trivalent molybdenum of the, type R^•₃[Mo(SCN)₆].xH₂O. Many salts which appear to be of this type have been prepared by Sand and Maas, who formulate them with (OH) or (H₂O) of constitution thus: [Mo(SCN)₆.OH]R^•₃.xH₂O and [Mo(SCN)₆.H₂O]R^•₃.xH₂O. The potassium salt, K₃Mo(SCN)₆.4H₂O, may be prepared by the action of potassium thiocyanate on the double chloride, K₃MoCl₆. It yields orange-coloured crystals, readily soluble in water, and which readily lose water of crystallisation, becoming dark red in colour or, if heated to complete dehydration, almost black. The method of preparation used by Sand and Maas involved the electrolysis of potassium molybdate and thiocyanate in hydrochloric acid solution. The sodium and ammonium salts may be obtained by either of the two methods described, while insoluble salts of copper, nickel, silver, and zinc are obtained by addition of solutions of these metals to a solution of the complex alkali salt. Many metal-ammine derivatives and compounds with organic bases have been described.
   
   The alkali hexathiocyanates are not isomorphous amongst themselves, but each molybdenum salt is isomorphous with the corresponding chromium salt thus:
   
   

   K3Mo(SCN)6.4H2O K3Cr(SCN)6.4H2O	pseudo-hexagonal
   (NH4)3Mo(SCN)6.4H2O (NH4)3Cr(SCN)6.4H2O	rhombic
   Na3Mo(SCN)6.12H2O Na3Cr(SCN)6.12H2O	asymmetric

   
   
   The thallium salt, Tl₃Mo(SCN)₆, is formed as an amorphous, light yellow precipitate when thallium nitrate is added to a solution of the potassium salt.
   
   The molybdenum in these salts may be oxidised by ammoniacal silver nitrate and the equivalent of the molybdenum calculated from the amount of silver liberated; the results obtained indicate a trivalent molybdenum atom.
2. Compounds containing tetravalent molybdenum. When precipitated molybdenum disulphide is dissolved in a warm solution of potassium cyanide, a deep green liquid is formed which on concentration yields slender black needles of composition MoS₂(CN)₂.2KCN. The compound is also obtained by boiling a solution of potassium thio-molybdate with an excess of potassium cyanide until the red colour completely changes to green. When heated with acids it yields hydrocyanic acid and molybdenum disulphide. The green solution is precipitated with alcohol, a greenish oil separating which gradually solidifies. An examination of the reaction between potassium cyanide and molybdenum disulphide suggests that three different compounds may be formed according to the length of time allowed for the reaction, namely, Mo₂S₃.6KCN.5H₂O, which forms green silky crystals when the action goes for a short time; Mo₂SO(CN)₂.4KCN.4H₂O, obtained as beautiful reddish-brown needles by evaporating the reacting mixture in a vacuum; and Mo₃S₄(CN)₃.5KCN.7H₂O, which results in large black crystals (green by transmitted light) on allowing the preceding salt to remain in contact with the mother-liquor over sulphuric acid. The mode of formation and the properties of these compounds indicate the presence of tetravalent molybdenum, but in order to establish their true nature further investigation is desirable.
3. Compounds containing pentavalent molybdenum. When a solution of ammonium molybdate and ammonium thiocyanate is reduced electrolytically and the resulting liquid extracted with ether, all the red- colouring substances pass into the latter; on adding pyridine, a compound, of composition Mo(OH)₂(SCN)₃(C₅H₅N)₂, is obtained in the form of dark brownish-red feathery aggregates of crystals, which may be recrystallised from hot alcohol, and which melt and decompose at 182° C. Similar compounds with other organic bases have been described.