CHARGE-TRANSFER SALTS OF CARBONYLMETALLATES AS OUTER-SPHERE ION-PAIRS IN PHOTOCHEMICAL AND THERMAL ELECTRON-TRANSFER

Highly colored, crystalline salts result from the combination of the carbonylmetalates Co(CO)4-, Mn(CO)5- and HFe(CO)4- with perphenyl-phosphonium and sulfonium cations. The unique colors are associated with the interionic charge-transfer interactions of carbonylmetalates acting as electron donors and the onium cations as electron acceptors by the independent evaluation of their anodic and cathodic redox potentials E(p)a and E(p)c, respectively. Such charge-transfer salts exist as unperturbed contact ion pairs in the solid state and in solution, as is shown by their carbonyl (IR) spectra which establish the presence of Co(CO)4-, Mn(CO)5- and HFe(CO)4- in T(d), D3h and C3v symmetry of the undistorted anions. The rich photochemistry that accompanies the charge-transfer activation of these outer-sphere ion pairs is described and it is mechanistically placed in the context of photoinduced electron transfer followed by the facile reactions of phosphoranyl(sulfuranyl) and 17-electron carbonylmetal radical pairs. The analogous thermal annihilation of the iodonium salt Ph2I+ Mn(CO)5- proceeds via an adiabatic electron transfer that is facilitated by an isoergonic driving force based on the enhanced reduction potential of Ph2I+.