Ferrocene and other metallocenes

The best-known cyclopentadienyl complex is the sandwich compound ferrocene, (r -Cp)2Fe it is a diamagnetic, orange solid (mp 393 K) which obeys the 18-electron rule (structure 23.25). In the gas phase, the two cyclopentadienyl rings are eclipsed (23.57) but the solid exists in several phases in which the rings are co-parallel but in different orientations. 

Metallocenes of the first row metals are known for V(II), Cr(II), Mn(II), Fe(II), Co(II) and Ni(II) reaction 23.96 is a general synthesis for aU except (r -Cp)2V where the starting chloride is VCI3. Reaction 23.97 gives an alternative synthesis for ferrocene and nickelocene. 

Blood glucose being tested using an ExacTech. A drop of blood is placed in a chemically coated strip which activates the digital display of the sensor. 

The exchange of an r -Cp ring for an rj -arene hgand (equation 24.116) is accompanied by a change in overall charge so that the Fe atom remains an 18-electron centre. 

FeCl3) to the paramagnetic, blue ferrocenium ion, [(ri -Cp)2Fe]. Equation 24.109 gives relative to the standard hydrogen electrode, but the Fc /Fc couple is commonly used as a convenient internal, secondary reference electrode (i.e. If is defined as 0 V for reference purposes, see Box 8.2). 

Forrow and S.J. Walters (2004) Biosens. Bioelectron., vol. 19, p. 763 - Transition metal half-sandwich complexes as redox mediators to glucose oxidase . 

Sterically demanding substituents are needed to stabilize the system, e.g. (ri -C5H4SiMe3)2Ti (scheme 24.113). 

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Many of the properties of the group 15 element diheteroferrocences are very similar to ferrocenes and other metallocenes. It seems justified to regard the diheteroferrocenes as perturbed ferrocenes just as we regard the group 15 heterobenzenes as perturbed benzenes. Thus, it is very clear that the elements phosphorus, arsenic, antimony, and bismuth can take part in 7r-bonding in a manner similar to carbon. 

A comprehensive chemistry of ferrocene and other metallocenes has been developed. The introduction presented here is meant to acquaint the reader with the general scope and nature of the field, and more details will be found in the references at the end of this chapter. 

Sodium and thallium cyclopentadienide provide more versatile syntheses of ferrocene and other metallocenes. 

Owing to its stability, solubility, and highly reproducible oxidation behavior, ferrocene has long been used as an electrochemical standard in nonaqueous solvents. Not surprisingly, the electron-donor or -acceptor properties of ring substituents in ferrocenes and other metallocenes have been repeatedly evaluated with electrochemical techniques. Measurements have been obtained using polarography,150 cyclic voltammetry (CV),151 chronopotentiometry,152 photoelectron spectroscopy, 53 and Fourier transform ion cyclotron resonance mass spectrometry.154 Extensive compilations of such data are available.155 156 Historically, variations of oxidation potentials have been discussed almost solely in terms of the... 

For a series of more modem articles by several authors on the history of the early research on ferrocene and other metallocenes, see /. Organomet. Chem., 2001, 637-639, 3-26. See also P. Laszlo and R. Hoffmann, Ferrocene Ironclad History or Rashomon Tale, Angew. Chem. Int. Ed., 2000,39,123, for a short, interesting article on the discovery of the stmcture of ferrocene. 

Fig. 7. A qualitative MO scheme for ferrocene and other first-row transition-metal metallocenes. The inset highlights the ordering of the (/-orbitals in the frontier orbital region and provides the definition for A, and A (see the text).

It is well known that ferrocene and related metallocene compounds undergo reversible oxidation by a variety of chemical oxidants. The particular ease with which ferrocene undergoes oxidation to its ferricinium cation allows one to determine the effect of a series of substituents on the oxidation potential of the ferrocene system. Thus a sensitive probe is available for determining the electronic effect a substituent has on the iron heteroatom and a device is made available by which one might elucidate the mechanism by which such an effect is transmitted from the substituent to the iron atom in the ferrocene system. It is unfortunate that lack of suitable derivatiyes has precluded similar studies in systems containing other transition metal atoms. 

Uden et al have discussed the gas chromatography of ferrocene and other iron group metallocenes. The microwave emission... 

We therefore open our electrochemical examination of metallocenes by starting with ferrocene derivatives and then the other metallocenes. It has to be anticipated that the chemistry of these latter compounds is, however, considerably less developed. 

A unique type of R2Pb compound is the cyclopentadienyl derivative of divalent lead. Only two such compounds are known, bis(cyclopenta-dienyl)lead and bis(methylcyclopentadienyl)lead. From dipole moments and infrared and NMR spectra, these compounds are postulated to possess a sandwich-type configuration, similar to that of ferrocene and the other metallocenes, but one in which the cyclopentadienyl rings are located angularly about the lead atom. This type of structure is the first... 

In addition to the many functionalized cyclopentadienyl ligands shown in Figures 7.23 and 7.30, a vast array of possible variants are accessible via modification of free cyclopentadienes. This is fortunate, in that ferrocene is unique in its versatility towards functionalization. The majority of other metallocenes, once constructed, are not as tolerant of such subsequent transformations. 

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