Redox coupling cyclopentadienyl complexes

The Lever s model has also been extended to sandwich and half-sandwich complexes with 7T-cyclopentadienyl or 7r-arene ligands [66, 67, 69]. The parameter for the 7r-ligands has been defined [69] on the basis of the low spin Fe hH redox couple, by Eqs. (21) or (22), for homolep-tic sandwich [Fe( 7r-L)2] or mixed sandwich ]Fe( 7r-Li)( 7r-L2)] complexes, respectively. 

Cyclopentadienyl analogs of the bi- and trimetallic chain complexes were prepared recently (69). These compounds, as well as the carborane-capped compounds, exhibit reversible 1-electron redox couples, indicating that the electrons are added to and removed from delocalized molecular orbitals rather than localized metal centers. 

The low spin Ee /Ee redox couple was initially proposed by Lever et al. [23, 24, 31] as the most appropriate standard for sandwich complexes bearing two cyclopentadienyl (Cp) or derived ligands, as such compounds are known in a large number and exhibit metal-based reversible or quasi-reversible redox processes that are fairly independent of the solvent/electrolyte system. Eor homoleptic complexes the value for the El parameter would be given by Eq. 50.6 and for mixed sandwich species, [EeLL ], Eq. 50.7 should be applied [31]. For the Cp ligand itself, an El value of 0.33 V was thus proposed [31]. 

The pentafulvalenes have a rich organometallic chemistry, which is the result of the redox relation between pentafulvalene (101) and its reduction product, the bi(cyclopentadienyl) dianion (102). The latter can also be regarded as the product of a double deprotonation of a dihydrofulvalene. In many aspects, this chemistry resembles that of cyclopentadienyl complexes. However, there are a number of features unique to the system of two coupled cyclopentadienyl systems, in particular the chemistry of bimetallic complexes with and without intermetallic bonding and their redox processes [90]. A number of review articles have appeared [91-97] here, only the key aspects and actual developments are summarized. 

Distinction should be made between the value of the geometrical and the electrochemical (active) areas of an electrode. The meaning of geometric area is obvious. The electrochemical area should be computed on the basis of the response to a benchmark species in one of the techniques discussed in the following. Once the diffusion coefficient of the species chosen, typically one partner of a reversible redox couple, such as the hexacyanoferrate anions in water or bis(cyclopentadienyl)iron (II)—ferrocene—in organic solvent, is known, the ratio between the measured current and the expected current density constitutes a reliable estimate of the electrochemical area. The dependence of this area value on the exact nature of the electroactive species may be discarded as a first approximation, once poisoning of the electrode and the occurrence of unknown complex electrode mechanisms can be excluded. 

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