Binuclear metal clusters

The active site of methionine aminopeptidase contains a binuclear cobalt complex that is required for activity, although a number of divalent metal ions support turnover to varying degrees. X-ray crystallographic studies on the enzyme in complexes with transition state analogs suggests that the binuclear metal cluster serves to stabilize the tetrahedral intermediate in peptide hydrolysis. ... 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

Here, we should mention that there exists an extensive discussion in the literature on the capabilities of spin-DFT regarding, for instance, the question whether the Kohn-Sham spin density has to be equal to the spin density of the fully interacting system of electrons (and in the case of open-shell singlet broken-symmetry (BS) determinants (see below) for binuclear transition-metal clusters this is certainly not the case see Ref. (33) for a more detailed discussion). But the situation is much more subtle and one may basically set up the variational procedure in a Kohn-Sham framework such that the spin density of the Kohn-Sham system of noninteracting fermions represents the true spin density. However, the frame of this review is not sufficient to present all details on this matter (34,35). 

The question remains open whether or not there is a direct contact between flavin and metal in a le -accepting binuclear iron cluster as, for example, in plant ferredoxin. Outer sphere le -transfer through flavin position 8 remains the alternative. On the other hand, there can no longer be a question about direct and reversible electron transfer between dihydronicotinamide and metal proteins. Any such claims (49) must be taken with greatest reservation and require careful independent confirmation. 

Figure 3 (a) The linking of phosphomolybdate clusters through binuclear metal organic... 

The activation of simple organic molecules by more than one transition metal constitutes an area of research which has grown in interest since the mid 1970s. Transition metal clusters, homonuclear and later heteronu-clear compounds, have been widely studied because of the potential for application in catalysis. The chemistry of such complexes has been the topic of many review articles, some of which appeared in this series (1-5). The concept of utilizing binuclear complexes with metal-metal bonds for ligand activation has been recognized (6-8), and, as stated by Casey and Audett, there is some hope that a smooth transition from the chemistry of mononuclear compounds to dinuclear compounds to metal clusters to metal surfaces may be found (9). 

Chapter 9 covers binuclear metal-metal bonded systems and acts as a link between the mononuclear species and the cluster chemistries of Chapters 10-12. The rapid evolution of the latter aspects is another highlight of development of the chemistry of Groups 4-6. 

Mn(II) centre (c, d) Following a proton transfer to the leaving amino group mediated by Asp 128, the tetrahedral intermediate collapses to yield the products L-ornithine and urea, (e) A water molecule enters to bridge the binuclear Mn cluster, causing the urea product to move to a terminal coordination site on Mn. Product dissociation facilitates ionisation of the metal-bridging water molecule to yield the catalytically active hydroxide ion. Proton transfer from the metal-bridging water to the bulk solvent is mediated by His-141, followed by the release of the two products. 

Very few investigations of the electronic spectra of clusters have been made, probably because satisfactory assignment of the spectra of simple binuclear metal-metal-bonded carbonyls has yet to be made. Gray and co-workers 72) have had some success in measuring and assigning polarized electron absorption bands of Mn2(CO)4o and Re2(CO)io by orienting the molecules in a nematic liquid crystal, but this technique would not appear to be capable of application to clusters. 

---