The Bridging Ligand

Since it is known fliat the identity of the 3rd bridging iigand is changed upon activation of the enzyme and aiso in the cataiytie cycie, its eiear identification in the different states is of utmost importanee. This ean be achieved by EPR/ENDOR techniques. 

The presence of a sulfur species (S , SH , H2S) in the bridge between Ni and Fe has been discussed by various authors [6,7,9,21,88]. For the Ni-A and Ni-B states (Fig. 5) this can, however, be excluded based on the data. It is expected that a sulfur-based ligand would lead to an EPR spectrum with different g values. Such minority species with different g values have recently been reported for the oxidized states of D. vulgaris Miyazaki F (see, e.g.. Fig. 5 and [9,88]). However, up to now the identity of these species remains unclear. 

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Using a 2-diphenylphosphinopyridine as the bridging ligand (with a similar bite to dppm) leads to a similar dimer (Figure 3.20). 

Role of the bridging ligand in inner-sphere electron transfer reactions. A. Haim, Acc. Chem. Res., 1975, 8, 264-272 (80). 

The scheme in Fig. 9-5 above illustrates the case in which the bridging ligand, X, is transferred from metal center Mi to M2 in the course of the reaction. Although this is not a necessary consequence of an inner-sphere pathway, it is often observed, and provides one method for establishing the mechanism. 

If this complex now collapses, it will be the labile Co-Cl bond which is broken, as opposed to the inert Cr-Cl bond. The labile cobalt(ii) complex reacts further with bulk water to generate [Co(H20)6] (Eq. 9.37). The key feature is that a necessary consequence of this inner-sphere reaction is the transfer of the bridging ligand from one center to the other. This is not a necessary consequence of all such reactions, but is a result of our choosing a pair of reactants which each change between inert and labile configurations. In the reaction described above, the chloride... 

However, it is difficult to separate direct Cu-Cu interactions from Cu-Cu interactions taking place through the bridging ligand atoms. 

CNs, the Fe ion is postulated to be coordinated by one SO and a mixture of CO and CNs. The interpretation is based on the temperature factor refinement and pyrolytic analysis of oxidized sulfur species (33). In addition, the bridging ligand is postulated to be an inorganic sulfur ion (instead of an oxo ligand, as proposed for the D. gigas). This... 

Stang and coworkers also reported the synthesis of platinum-acetylide dendrimers using 1,3,5-triethynylbenzene as the bridging ligand [27]. Their strategy was very similar to the above, and they prepared dendrimers up to the second generation. 

The binuclear Ir complex (93) is produced from the direct coupling reaction of the Ir-bound 5-chloro-phen ligand as shown in reaction Scheme 12.153 The nonplanar structure of the bridging ligand results in the electronic isolation of the two Ir centers. Hence both metal centers and ligands are oxidized and reduced, respectively, in two-electron processes. 

Lippert and co-workers have reported a similar series of A-,O-bridged platinum(III) dimers in which the bridging ligands are the pyrimidines, 1-methyluracil, 1-methylthymine, or 1-ethylthy-mine. Chemical oxidation of dimeric platinum(II) complexes gave [Pt2XY(L)2(NH3)4]2+, X,Y = N03", N02, H20, Cl", or Br", L=l-MeU, 1-MeT, 1-EtT. 27 t30 They too found that the HT dimers are more stable than the HH.430... 

In this regard it would be interesting to consider the electronic structure of some binuclear complexes of other elements. For example, an analogous effect of the bridging ligands on the reverse energy order of <5M-M and <5g M MO s was... 

Sickle erythrocytes have increased surface expression of CD36. Microvascular endothelial cells do express CD36. Adhesion between these CD36 molecules of the two cells types can be mediated via the bridging ligand thrombospondin". Indeed plasma thrombospondin levels are increased in SCA patients with pain crisis [15]. 

Layer-by-layer Ru3 cluster-based multilayers were fabricated onto preorganized self-assembled monolayer gold electrode surfaces by Abe et al. [15], in which [Ru3(q3-0)( i-0Ac)6(4,4/-bpy)2(C0)] was utilized as the synthetic precursor. The stepwise connection of oxo-centered triruthenium cluster units onto the gold electrode surface is a feasible approach for construction of Ru3 cluster-based oligomers on a solid surface, in which the bridging ligand 4,4 -bipyridine appears to mediate weak cluster-cluster electronic interaction between the Ru3 cluster centers. 

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