Electron-Transfer Reactions of Re(CO)(5): Atom-Transfer-Concerted Mechanism vs Bimetallic Intermediate Formation

Flash photochemically generated Re(CO)(5) reacts with halide complexes, Cu(Me(4)[14]-1,3,8,10-tetraeneN(4))X(+), Cu(Me(2)pyo[14]trieneN(4))X(+), and Ni(Me(2)pyo[14]trieneN(4))X(+) (X = Cl, Br, I) and ion pairs, [Co(bipy)(3)(3+), X(-)]. The rate constants for the electron transfers have values, k approximately 10(9) M(-1) s(-1), close to expectations for processes with diffusion-controlled rates. Reaction intermediates, probably bimetallic species, were detected in electron-transfer reactions of Re(CO)(5) with Cu(Me(6)[14]dieneN(4))X(+), (X = Cl, Br, I). In the absence of the halides X(-), the electron-transfer reactions between Re(CO)(5) and these complexes are slow, k < 10(6) M(-1) s(-1). The results are discussed in terms of inner-sphere pathways, namely an atom-transfer-concerted mechanism. The mediation of bimetallic intermediates in the electron transfer is also considered.