Bis fulvalene

As happens for ferrocenes, almost all metallocenes can be incorporated in multimetallocenic assemblies. We thus conclude the discussion by considering the redox behaviour of bis(fulvalene)dinickel(II,II), [Ni2(>/5 75-CioH8)2]. Its X-ray structure is shown in Figure 50.97 The inequivalence of the Ni-C and C-C distances is attributed to the structural constraints imposed by the two bridges which join the two nickelocene units. 

A number of molecular mechanics studies of metal-cyclopentadienyl complexes have been reported recently. The systems studied include linear metallocenes (in particular ferrocene), ferrocene derivatives (such as complexes with substituted cy-clopentadienyl ligands, bis(fulvalene)diiron complexes, ferrocenophanes and mixed-ligand complexes with carbonyls and phosphines), and nonlinear cyclopentadienyl complexes 8,153,221 231]. 

Other derivatives such as CpCo(indenyl) and Co(indenyl)2 have been prepared. These compounds exhibit properties similar to that of cobaltocene but are of lower thermal stability. There have been numerous studies of linked metallocenes with a focus on the nature of metal-metal interactions. The bis(fulvalene)dicobalt complex was found to be diamagnetic, either because of direct metal-metal coupling or by electron coupling through the fluked five-membered ring (Scheme 30). 

Multimetallic sandwiches, including polymetallocenes, bis(fulvalene)-dimetal compounds, multi-decker sandwiches, and compounds in which two metals are bonded to a common unsaturated cyclic hydrocarbon are discussed in Section II. Sections III and IV are subdivided according to the metal group, and describe ligand-bridged bimetallics and metal clusters, respectively. Where appropriate, redox potentials have been included. Unless otherwise stated they are referenced versus the aqueous saturated calomel electrode (see). 

Biferrocene 1 (M = Fe) oxidizes in two separate one-electron steps (6, 7) (Ei/2 = 0.31 and 0.64 V), of which only the first is chemically reversible (i.e., the monocation is stable but the dication is not) mild oxidation yields the monocation [l]" (M = Fe) as a mixed-valence Fe(II)Fe(III) species (8-10). Bis(fulvalene)diiron 3 (M = Fe) similarly undergoes two successive one-electron oxidations (the dication is also stable), but in this case the Mdssbauer spectrum of the monocation shows the iron atoms to be equivalent, implying a delocalized electronic structure and an oxidation state of 2.5 for each metal (9,11). The separation of... 

Cobalt analogues of biferrocene and bis(fulvalene)diiron have not been investigated as extensively as the iron compounds, despite the information on electronic interactions which could be gained from observing cobalt hyperfine splittings by ESR spectroscopy. The bis(fulvalene)dicobalt dication (M = Co) can be reduced in two successive one-electron steps at mild potentials ( = -0.07 V and -0.95 V), and [Co2(tj, t7 -CjoHg)Cp2], [1] " (M = Co), is reduced at -0.53 V and -0.88 V (3i) the preparation of the monocation [3]" (M = Co) has been briefly reported (32, 33). 

Bis(fulvalene)divanadium complexes in three different overall oxidation states have been prepared (34). The dication [V2(i7 ,t -CioH8)2], [3] (M = V) is reversibly reduced to the monocation ( = 0.14 V), but the reduction to the neutral complex ( pk = -0.09 V) is irreversible in acetonitrile. Since 3 (M = V) can be synthesized separately by the reaction of the fulvalene dianion with VCl2 2THF in THF, the irreversibility of the one-electron reduction of [3]" (M = V) in acetonitrile is probably due to the reaction of the neutral complex with the solvent. 

Theoretical studies have shown that the monocations of both biferrocene and bis(fulvalene)diiron show some delocalization the latter is much more delocalized than the former (202). By contrast, [Feairj, -CsH4CH(Me)C5H4 2] appears to be a trapped valence cation X-ray structural studies have revealed different environments about the two metal atoms (203) (Section II,A). 

To achieve electrical conductivity the metal atoms have to be in different oxidations states [90]. Since the discovery of the Cruetz-Taube complex 181 [164] the delocalization of electrons in various sandwich complexes has been examined [165, 166]. As was shown, the unpaired electron in the bis(fulvalene)bis(iron) ion 182 is completely delocalized and regarded as an ion of class III the two iron atoms exist in a mixed valence state (Fig 47) [166,167]. The ion 183 belongs to class II, because a considerable energy barrier in the interaction of the two metal centers was established (Fig. 47) [166,167], The ion 184 is not suitable for the design of a polymer, because the unpaired electron is completely localized at one of the chromium atoms, so that one of them carries a positive charge, while the other remains uncharged (Fig. 47) [166, 168]. 

Fulvalene dimetal complexes have been profitably employed in studies of metal-metal interactions across a spacer ligand with two nominally aromatic components. Work on the oxidation of biferrocene, 1, bis(fulvalene)diiron, 2, and their analogues occupy a central focus in these investigations.[1] The ferrocenyl moiety... 

C2 6H2 2C02Mn04P, Dicarbonyl-cyclopentadienyl-manganese-M3 (benzyl-phosphido)-bis(carbonyl-cyclopentadienyl-cobalt), 45B, 980 C26H25F12N3P2V2, Bis(fulvalene)bis(acetonitrile)divanadiumUII)(V-V) bis(hexafluorophosphate) acetonitrile, 45B, 980 C2 6H2 sN2Ti, 4-Methylene-3,7,8-trimethyl-1,10-phenanthrolinebis( t -cyclopentadienyl)titanium, 46B, 902 C2 6H2 sNijTi 2, M-Pyrazole-bis(Ty -cyclopentadienyl)titanium(III), 44B, 770... 

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