Catalyst bridging ligand

Pathway (d) in Fig. 9.3 provides a possible explanation for the efficiency of a combination of a reductant and a complex former in promoting fast dissolution of Fe(III) (hydr)oxydes. In this pathway, Fe(II) is the reductant. In the absence of a complex former, however, Fe2+ does not transfer electrons to the surface Fe(III) of a Fe(III) (hydr)oxide to any measurable apparent extent. The electron transfer occurs only in the presence of a suitable bridging ligand (e.g., oxalate). As illustrated in Fig. 9.3d, a ternary surface complex is formed and an electron transfer, presumably inner-sphere, occurs between the adsorbed Fe(II) and the surface Fe(III). This is followed by the rate-limiting detachment of the reduced surface iron. In this pathway, the concentration of Fe(U)aq remains constant while the concentration of dissolved Fe(III) increases thus, Fe(II)aq acts as a catalyst to produce Fe(II)(aq) from the dissolution of Fe(III)(hydr)oxides. 

One of the most significant additions to the modern rhodium] ) catalyst ligand family was the development of the hybrid catalysts that combined the carboxamide bridging ligands with the enhanced reactivity of perfluoroalkyl substituents. In this series, rhodium] ) trifluoroacetamidate [Rh2]tfa)4] was the first described [30]. n addi-... 

Rasmussen, S.C., Richter, M.M., Yi, E., Place, H. and Brewer, KJ. (1990) Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorg. Chem., 29, 3926—3932. 

The dimerization of the copper complex takes place to form the bis-Cu(II) compound 3, where the phenolate anion is the bridging ligand, just as proposed by Karlin [75], in the dinuclear complexes that act as the actual catalyst in the active state. 

Patents have been reported for a series of bimetallic metallocene catalysts based on lanthanide and group IV metals. The bridging ligand is a substituted ethyl-linked fluorenyl indenyl bearing a substituent of varying length [26]. The complexes are reported to act as good olefin polymerization catalysts in the presence of MAO.52,53... 

In certain cases the surface of the support may be pre-modified to either increase or decrease its absorptive capacity. Techniques for the former have been thoroughly explored in the area of anchoring homogeneous catalyst complexes and metal clusters. Since this subject has been amply reviewed we will not discuss it further. These techniques primarily involve pre-treatment of the support surface with a compound that can serve as a bridging ligand. Techniques for decreasing the absorptive capacity are also of importance and these will be covered later in greater detail when we come to consider metal location on a catalyst support. 

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