Ligand-bridged processes

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

If ligands are involved in the formation of discrete intermediates or if metal ions become ligand-bridged, the process is designated as inner-sphere (IS) electron transfer [52]. In these cases, the electronic interaction between the redox centers is increased substantially, and leads to a lowering of the activation barrier (and hence to increased rates) for the ET reaction [13, 15, 53],... 

Three types of stereochemical lability have been observed (1) rotations about M-M bonds, (2) cis—r- trans isomerizations at each metal center and (3) bridge = terminal ligand exchange processes. 

Dimetallocycles have been discovered which exhibit high reactivity with respect to carbon-carbon bond-making and -breaking processes. They allow the synthesis of a variety of simple but important hydrocarbon ligands bridging a dinuclear metal centre. y-Carbene complexes are readily available by several routes and their reactions have implications for both alkyne polymerisation and alkene metathesis. A substantial chemistry of organic species co-ordinated at dinuclear metal centres is in prospect, with significance for metal surface chemistry and catalysis. 

Equation (65) is an example of a process that may be readily promoted by a cluster but not necessarily with a mononuclear complex. Scheme 8 shows how the multiple-coordination capabilities of a trinuclear cluster could reduce the N—C bond and eventually cleave it. To date, examples of the NCO ligand bridging more than two metals in either a mono- or polyhapto fashion are unknown. The reverse process represents a unique method for forming carbon-nitrogen bonds. Although the above chemis-... 

At this point, the ligand is a cyclidene. Several crystal structures have shown that the conformations of the 16-membered cyclidene rings, in their complexes, are saddle shaped. This facilitates the fifth reaction, the bridging process. Since the presence of the metal ion is required to produce this saddle conformation, the process is indeed a template reaction. The lacunar ligand is then removed from the nickel(II) ion (sixth reaction) and used to form a cobalt or iron complex (last reaction). The procedures for forming the iron and cobalt complexes are different the scheme exemplifies the reactions used to synthesize the iron(II) complex. 

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