Introduction to Tungsten

Chemical Properties:

Tungstates Formed by dissolving WO3 in NaOH solution and subsequently by cation exchange. The normal tungstates. e.g.Na2WO4,2H2O, contain tetrahedral WO4^2-ions. Most metal salts other than alkali metal salts are insoluble. Acidification of a solution of WO4^2-ions gives polymeric anions-iso-polytungstates-containing WO₆ octahedra joined corner to corner to corner or edge to edge; [HW₆O₂₁]^5- ,[H2W12O42]^10- and[W12O39]^6- are well-established. If the acidification is carried out in the presence of another oxyanion, heteropolytungstates in which MO6 or MO4 groups (M=heteroatom) are incorporated into the polyanion are formed. E.g.[FeW12O4]^5- and [PW12O4]^3- .It seems unlikely that free tungstic acids of any of these forms are sable. Further acidification of isopolytungstates gives WO3,2H2O, Heteropolytungstates are used to form Wcontaining catalysts by heating.

Tungsten,(w) The main ore of tungsten is wolframite (Fe,Mn)WO4; scheelite (CaWO4) and solzite (PbWO4) are also important. The concentrated ores are fused with NaOH and, after water extraction, WO3 is precipitated with acid and reduced to the metal with H2. The metal is bcc. The fuse metal is lustrous silver-white and is only attacked slowly even by HNO3-HF. Dssoleds in KNO3-NaOH or Na2O2, attacked by O2 at red-heat. The metal isused extensively in steel alloys, in electric lamp and heating filaments and in electric contaces. Tungsten carbides are very hard and are used in cutting tools. World production WO31981 52000 tonnes.

Tungsten is a typiacl element of transition element Group VI and shows oxidation states from +6 to-2 and, particularly in its oxides, forms many non-stoicheiometric compounds. There is little aqueous chemistry except that of complex oxy-anions and some complex halides. The hexahalides are moleular but lower halides are polymeric and the lowest halides show extensive W-W bonding (more than Mo). Carbonyl and phos-phine derivatives are typical low oxidation state compounds. Complexes are formed, particularly by O-and S-ligands in higher oxidation states and by P-ligands in low oxidation states. Complex cyanides are well established.

Tungstic acid See tungstates.

Tungsten oxides Yellow WO₃ is the final product from heating tungsten or other tungsten oxides in excess oxygen. It is used in yellow glazes for ceramics. Brown WO₂ (WO₃+H₂) and many intermediate phases containing shear structures* (e.g.W₁₈O₄₉) are formed by reduction of WO₃. Mixed metal oxides are formed by fusing WO₃ with other metal oxides.

Tungsten blue Partially reduced WO₃ or tungstates.

Tungsten carbides W₂C and WC are obtained by heating W powder with C. Both compounds are extremely hard and are used for making cutting tools and dies. Ternary carbides are also used in cutting tools and other metallic carbides, borides, silicides or nitrides are added to improve the mechanical properties of the mixture.

Tungsten alloys The most impotant alloys are the tungsten steels whichcontain up to 18% of tungsten. Stellite, an extremely hard alloy of W with Cr and Cu, is used for high-speed cutting tools. Electric contacts for switch-gear are made from Cu and Ag-W alloys.

Tungsten bronzes Coloured compounds MnWO₃ (M= unipositive metal 0<n<1) prepared by reducing tungstates with, e.g., Na. The compounds contain W^v1and W^v

Tungsten carbonyl, W(CO)₆ Bery similar to molybdenum carbonyl* but less readily substituted.

Tungsten halides The known fluorides are colorless WF₆(W+F₂), b.p.17¡æ,mixed tungsten(VI) chloride fluorides (e.g. WF₅Cl) and derivatives of WF₆ (e.g. WF₅OMe), tetrameric WF₅ (W+WF₆) and WF₄ (heat on WF₅). Complex fluorides of W(VI) and W(V) contain ions such as [WF₇]^-and [WF₆]^-. The knowm chlorides are WCl₆(W+Cl₂) an olefine metathesis reagent, green WCl₅(W+Cl₂), involatile WCl4(WCl₆+Al), WCl₃ and WCl₂(both WCl₆+H₂). WCl₃ and WCl₂ both contain metal-clusters [W₆Cl₁₂]Cl₆ and [W₆Cl₈]Cl₄ respectively. Complex chlorides containing [WCl₆]^- ,[WCl₆]^2-,[W₂Cl₉]^3- are known ([W₂Cl₉]^3- contains three bridging Cl and a W-W bond), All of the halides form extensive ranges of complexes with various ligands. Hydrolysis of the higher oxidation state fluorides and chlorides gives oxide halides and these compounds may also be prepared directly from oxides and halogenating agents. WOF₄, [WOF₅]^-, [WOF₄]^2-, [WO₂F₃]^3-, WOCl₄, WO₂Cl₂,WOCl₃ and [WOCl₅]^2- are known and their chemistry is similar to that of the halides. Tungsten bromides, WBr₆, WBr₄, WBr₃, WBr₂ and iodides WI₄, WI₃ and WI₂ are similar to the chlorides.

Tungsten, organic derivatives Generally similar to the molybdenum derivatives but higher oxidation state alkyls, e.g. WMe₆ (WCl₆+LiMe) are known.