CHARGE-TRANSFER CRYSTALS OF OCTACARBONYLDIFERRATE - SOLID-STATE STRUCTURE AND OXIDATION-REDUCTION OF AN IRON-IRON-BONDED ELECTRON-DONOR

The dinuclear carbonylmetalate [Fe-2(CO)(8)](2-) forms deeply colored crystals A(2)[Fe-2(CO)(8)] with various phosphonium and pyridinium cations (A(+)) by metathesis in aqueous solution. Diffuse reflectance absorption spectroscopy and X-ray crystallography establish the colors to derive from anion-to-cation charge-transfer (CT) within the triple ion [A(+), Fe-2(CO)(8)(2-), A(+)] extant in the solid state. Partial electron transfer in the charge-transfer salts is established by the significant shortening of the iron-iron bond in the [Fe-2(CO)(8)](2-) moiety when it is ion paired with the N-methyllutidinium acceptor. In solution, the charge-transfer salts can undergo interionic electron transfer, and they lead to the dimeric A(2) and a series of iron carbonyl products that are uniquely dependent on the solvent (water, ether, and acetonitrile).