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 = eta(5)-C5H5) and oxidation and reduction of M(eta(6)-arene)(CO)(3); in group 7, oxidation of MCp(CO)(3) and reduction of [M(eta(6)-arene)(CO)(3)](+) in group 8, oxidation of [MCp(CO)(2)](-) and reduction of [M(eta(6)-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(eta(6)-arene)(CO)(3)](+), are stable in the absence of nucleophilic anions and can be used for electrosynthetic purposes or as organometallic redox tags. (C) 2012 Elsevier B.V. All rights reserved.