One-electron electrochemistry of parent piano-stool complexes

Whereas the oxidation of ferrocene and other sandwich complexes has been widely utilized in both chemistry and biochemistry, the one-electron chemistry of half-sandwich complexes is still underdeveloped. Here we review the known electrochemistry of the “parent” half-sandwich complexes containing only carbonyl groups and one planar carbocyclic ring. The greatest number of electrochemical studies of this so-called piano-stool series has been conducted on the middle transition metals: in group 6, oxidation of [MCp(CO)3]− (Cp = η5-C5H5) and oxidation and reduction of M(η6-arene)(CO)3; in group 7, oxidation of MCp(CO)3 and reduction of [M(η6-arene)(CO)3]+; in group 8, oxidation of [MCp(CO)2]− and reduction of [M(η6-arene)(CO)3]+; in group 9, oxidation and reduction of MCp(CO)2. An electron-transfer sequence involving 17e−/18e−/19e− metal centers has often been found. The fact that the LUMOs of the 18-electron complexes are formally metal-ring anti-bonding explains the general weakening of the metal-ring bond upon reduction, and provides a rationalization for the lowering of the metal-ring hapticity that has been reported for several hypervalent members of this family. Reduction past the 18-electron configuration generally occurs at quite negative potentials. Oxidation of the 18-electron complexes occurs principally at the M(CO)3 moiety, but the SOMO of the 17e− complex generally has a higher degree of metal-ring covalency than in the 19e− case. The 17-electron radicals often undergo metal–metal coupling reactions to form stable dimers, some of which have sufficiently weak metal–metal bonds that they partly dissociate to the corresponding monomers in solution. Oxidation of the 18-electron complexes occurs over a wide range of potentials, e.g. −1.35 V vs FcH for [FeCp(CO)2]−/0 compared to 1.16 V for [ReCp(CO)3]0/+. The radical cations of some first-row complexes, e.g. [MnCp(CO)3]+ and [Cr(η6-arene)(CO)3]+, are stable in the absence of nucleophilic anions and can be used for electrosynthetic purposes or as organometallic redox tags.

One-electron oxidation or reduction of 18e− half-sandwich metal carbonyl complexes has been effectively probed by electrochemistry. 17e− complexes frequently form metal–metal bonded dimers and may undergo fast CO substitution by a donor ligand. 19e− complexes tend to lose a carbocyclic ring, undergo metal-ring hapticity changes, or dimerize at the ring. Simple M.O. descriptions of the HOMO and LUMO orbitals of the 18-electron complexes provide a guide to the electronic and structural behavior of the redox products.Figure optionsDownload high-quality image (175 K)Download as PowerPoint slideHighlights
► Electrochemistry has been reported on a large number of half-sandwich metal carbonyl complexes.
► Cationic 17e− radicals may undergo fast CO substitution by a donor ligand or, especially with heavier metals, form a directly metal–metal bonded dimer.
► Neutral 17e− radicals undergo rapid metal–metal bonded dimerization.
► 19e− products loose a carbocyclic ring, undergo metal-ring hapticity changes, or dimerize at the ring.
► The HOMO and LUMO orbitals of the 18-electron complexes provide a guide to the
► electronic and structural behavior of the redox products.