4-2 oxidation state cyclopentadienyls

Intermediate and Higher Oxidation State Cyclopentadienyl Complexes 392... 

Cu ( j -C5H5)2] is not. Likewise, Fe and Ni carborane derivatives are extremely stable. Conversely, metallocarboranes tend to stabilize lower oxidation states of early transition elements and complexes are well established for Ti", Zr , Hf , V , Cr and Mn" these do not react with H2, N2, CO or PPh3 as do cyclopentadienyl derivatives of these elements. 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Indeed, it is the cyclopentadienyls which provide the major part of the organometallic chemistry of this group and they are known for metal oxidation states of IV, III and II though III... 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

An alternative method is to use electrochemical mediators that are at a higher concentration that O2 and can therefore be shuttled back and forth between the protein and the electrode faster than the enzyme is reduced, so that the arrival of the glucose is always rate-limiting. A typical chemical that works in this way is ferrocene, which is an iron cation between two cyclopentadienyl anions, as shown in Figure 6.47. It exists in neutral and - -1 oxidation state that are readily interconvertible at metal or carbon electrodes. 

The compounds in question are classified by oxidation state of the central metal atom. Assignment of oxidation state is normally straightforward when the formula is known, but there are cases where the choice is somewhat arbitrary. Thus bis(cyclopentadienyl)nickel, (CcH Ni, may be considered as the nickel (II) cation complexed by two cyclopentadienyl anions, or as a combination of an uncharged nickel atom with two cyclopentadienyl... 

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