RADICAL INTERMEDIATES IN THE REDUCTIVE ISOMERIZATION OF OCTAHEDRAL MANGANESE CARBONYL BIPYRIDYL COMPLEXES - ELECTROCHEMISTRY, ELECTRON-SPIN-RESONANCE SPECTROSCOPY AND MOLECULAR-ORBITAL CALCULATIONS

The complexes fac-[Mn(CO)(3)L(bipy)](+) [L = CNBu(t) or P(OMe)(3), bipy = 2,2'-bipyridyl], cis,trans- and cis,cis-[Mn(CO)(2)L(L')(bipy)](+) [L = L' = CNBu(t), P(OEt)(3), P(OPh)(3) or PPh(3); L = CNBu(t), L' = P(OMe)(3)], fac-[Mn(CO)(CNBu(t))(3)(bipy)](+) and mer-[Mn(CO)L(3)(bipy)](+) [L = CMBu(t) or P(OR)(3), R = Me or Et] underwent one-electron oxidation at a platinum-disc electrode in CH2Cl2 and/or one-electron reduction in tetrahydrofuran (thf). The complexes mer-[Mn(CO)L(3)(bipy)](+) [L = P(OEt)(3) or CNBu(t)] were oxidised by [N2C6H4F-p][PF6] to give dications with ESR spectra characteristic of low-spin d(3) transition-metal complexes. The ESR spectroscopic studies of the sodium amalgam reduction of mer- and fac-[Mn(CO)(CNBu(t))(3)(bipy)](+), cis,trans- and cis,cis-[Mn(CO)(2)(CNBu(t))(2)(bipy)](+), mer-[Mn(CO){P-(OMe)(3)}(bipy)](+), or cis,trans-[Mn(CO)(2){P(OMe)(3)}(2)(bipy)](+) in the provide evidence for the intermediacy of neutral bipyridyl ligand-based radicals in the reductive isomerisation of the cis,cis to the cis,trans, and of the fac to the mer, cations. Extended-Huckel molecular-orbital calculations on the mer and fac isomers of [Mn(CO)L(3)(bipy)](+) and the cis,trans and cis,cis isomers of [Mn(CO)(2)L(2)(bipy)](+) (L = CNH or PH3) provide support for the formation of metal- and ligand-based radicals on oxidation and reduction respectively.