One-electron electrochemistry of parent piano-stool complexes

doi:10.1016/j.ccr.2012.07.004

Coordination Chemistry Reviews

Volume 257, Issues 9–10, May 2013, Pages 1459-1471

https://doi.org/10.1016/j.ccr.2012.07.004 Get rights and content

Abstract

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.

Graphical abstract

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.

Highlights

► 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.

Section snippets

Scope and introduction

Owing to the fact that an electron-transfer is one of the principal chemical reactions, investigations of ‘redox chemistry’ have played a significant role in delineating the properties and reactions of sandwich and half-sandwich organotransition metal complexes [1], [2]. The present review addresses the current state of knowledge of the consequences of one-electron oxidation or reduction of an important set of half-sandwich complexes, namely structurally simple compounds having as ligands only

Electronic structure considerations

The basic M.O. schemes of metallocenes and M(CO)₃-containing half-sandwich complexes have important similarities [10]. Both have six ligand-based orbitals lying below a set of three predominantly metal-based orbitals, all of which are filled for d⁶ (18-electron) complexes. Above the filled orbitals is a pair of unoccupied, largely metal-based, orbitals that comprise the LUMOs (see Scheme 2). As noted by Albright et al. [10], [11] the “two (e_g) above three (t_2g)” orbital model is strongly

Electrochemical studies

Reports on the electrochemistry of parent piano-stool complexes are common, in some cases going back a number of years. Although some of the older papers will be referred to here, more comprehensive coverage of the early work can be found in other references [1], [2]. The present review is intended to cover the current state of understanding of the general subject. We have attempted to offer oxidation and reduction potentials, either E_1/2 for reversible systems or E_peak for irreversible

Conclusions and prospects

Electron-transfer involving a sequence of 17e⁻/18e⁻/19e⁻ metals is general for simple piano-stool metal carbonyl complexes. Reduction of the 18e⁻ complex usually occurs at fairly negative potentials and results in radicals that may undergo changes in the hapticity of the carbocyclic ring, loss of ring or carbonyl ligand, or ligand-based dimerization processes. A number of systems which experience lowering of the carbocyclic ring hapticity give a net two-electron reduction. The overall results

Acknowledgement

The author is grateful to the National Science Foundation for support, most recently under NSF CHE-0808909.

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ReviewOne-electron electrochemistry of parent piano-stool complexes

Abstract

Graphical abstract

Highlights

Section snippets

Scope and introduction

Electronic structure considerations

Electrochemical studies

Conclusions and prospects

Acknowledgement

Organometallics

J. Am. Chem. Soc.

Organometallics

J. Am. Chem. Soc.

Organometallics

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Inorg. Chem.

Organometallics

Chem. Eur. J.

J. Am. Chem. Soc.

Organometallics

Electron Transfer and Radical Processes in Transition-metal Chemistry

Organometallics

Accounts Chem. Res.

Chem. Rev.

Pure Appl. Chem.

Inorg. Chem.

Chem. Rev.

Angew. Chem. Int. Ed.

Inorg. Chem.

Organometallics

Organometallics

Organometallics

Orbital Interactions in Chemistry

Problems in Molecular Orbital Theory

Chem. Phys.

Chem. Rev.

J. Chem. Phys.

Organometallics

Organometallics

Chem. Eur. J.

Organometallics

Organometallics

Chem. Phys. Lett.

Organometallics

Coord. Chem. Rev.

Mol. Phys.

Inorg. Chem.

J. Organomet. Chem.

J. Chem. Soc. Dalton Trans.

J. Chem. Soc. Dalton Trans.

Organometallics

Eur. J. Inorg. Chem.

Inorg. Chim. Acta

J. Chem. Soc. Dalton Trans.

Inorg. Chem.

J. Am. Chem. Soc.

Inorg. Chem.

Chem. Rev.

J. Am. Chem. Soc.

Inorg. Chem.

Inorg. Chem.

J. Chem. Soc. Dalton Trans.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Review
One-electron electrochemistry of parent piano-stool complexes