Thermal and photochemical interaction of octacyanometallates(IV), [M(CN)(8)](4-) (M = Mo or W) in the presence of pyrazine

The interactions between [M(CN)(8)](4-) (M = Mo or W) and pyrazine (pz) in the solid state and in aqueous solutions have been analysed. In strongly ac idic solutions {pzH(+), [M(CN)(8)](4-)} ion pair formation is observed; the pyrazinium salts (pzH)(2)(H3O)(2)[Mo(CN)(8)].0.5pz . 3H(2)O and (pzH)(2)K( H3O)[W(CN)(8)].H2O have been isolated. The X-ray crystal structure of the l atter, and the spectroscopic properties of both, are described. The [W(CN)( 8)](4-) anion is approximately square antiprismatic (D-4d), with different H-bond environments around the N atoms. The ligand-field photolysis of [M(C N)(8)](4-) in the presence of pyrazine in neutral and alkaline solution res ults in the formation of tetracyanooxometallates(IV) in equilibrium with pe ntacyanooxometallates(IV) through the [M(CN)(7)(pz)](3-) anions as intermed iates. The formation of the [M(CN)(6)(pz)(2)](2-) ion, postulated in the li terature to be the final product of the alkaline photolysis, has definitive ly been excluded.