Carbon-sulfur bond cleavage in tris(.mu.-thioether)ditungsten(III) complexes in reactions with anionic nucleophiles, including hydride: synthesis and structure of [PPh4][Cl3W(.mu.-THT)2(.mu.-S(CH2)4Cl)WCl3] and Na[Cl3W(.mu.-SEt2)2(.mu.-SEt)WCl3].cntdot.3THF