Ligand assodation

The example in Scheme 6.16.4 shows the association of the neutral ligand ethylene to the Ni-hydride catalyst applied in the SHOP process, which carries an o-diphenylphosphinebenzoic add ligand (Vogt, 2002). In addition, the first step in the Cr-metallacyde mechanism, the addition of two neutral ethylene molecules to the Cr prior to the oxidative coupling step, is a ligand assodation step. 

Ligand dissociation is the reverse reaction of ligand assodation. Consequently, the CN of the complex reduces by one, the VE by two, and the ON stays constant. As ligand assodation is usually a reversible step, all reverse readions of the examples given above for the assodation step represent suitable examples for ligand dissociation. In ethylene oligomerization, the dissociation step plays a role in the release of the finally formed 1-alkene from the catalyst (after f-H-elimination). An example for the case of 1-octene liberation from a Cr(I) complex is given below in Scheme 6.16.8. 

Scheme 3.4-3. Typical selectivity curves of the titration of a catalytical process for the stepwise twofold assodation of ligand L to the intermediate M (Fig. 1), the association of L to two intermediates M and N at equilibrium (Fig. 2) and the assodation of L to two intermediates M and N being not at equilibrium (Fig. 3) (kinetic separation)...

Binding of the ligand of the Fas receptor triggers clustering of the receptor and association of the cofactor FADD (fas-assodated protein with death domain) which interacts with the receptor via its death domain (DD). Procaspase 8 binds to FADD via a common DED (death effector domain) motif and is thereby also recruited into the Fas-receptor associated complex. Due to the clustering of the proteins, proximity-induced cleavage of procaspase 8 to the mature initiator caspase 8 takes place. This activates the effector caspases and triggers cell death. 

Molybdenum, in particular, is known to form a very large number of structurally diverse polyanions in assodation with other oxo ligands such as squarate (C404 ), methoxide, malate, phosphate, and arsenate anions.7 Space does not permit discussion of this plethora of interesting compounds, nor of still others. We shall focus mainly on the so-called classic polymolybdates and polytungstates. 

The enantioselective hydrogenation of some a-amino add precursors 1 [Eq. (1)) in water or in an aqueous/organic two-phase system has been thoroughly investigated using rhodium or mthenium complexes assodated with chiral water-soluble ligands 3-13. Some of the most interesting results are summarized in Table 1. 

A nitric oxide (NO) QD-based sensor was developed via the NO-stimulated ligand substitution of a transition metal complex assodated with CdSe/ZnS QDs [142]. The red-colored tris-(N-(dithiocarboxy) carcosine) iron(III)[Fe(DTCS)3] was linked to ammonium-capped QDs by ionic interaction. As a consequence, the functionalized QDs readed with NO by an ET process, followed by ligand substitution to yield the colorless paramagnetic bis(dithiocarbamato) nitrosyl iron(I) complex as a capping layer. This process triggered the luminescence of the QDs that enabled the detection of NO (Figure 6.11). 

Suppose is a hard acid. It is already assodated with hard H2O ligands, i.e. there is a favourable hard hard interaction. If L is a soft base, ligand substitution will not be favourable. If L is a hard base, there are several competing interactions to consider ... 

Ligand substitutions of bimetallic complexes and higher-nucleaiity dusters occur by the dissociative, assodative, and catalyzed pathways tiwt have already been presented. However, they also react by pathways ttiat are available because of the presence of a metal-metal bond. This section discusses pathways that involve the metal-metal bond. Reviews of ligand substitutions in metal-metal bonded systems are available. ... 

Reversible homolysis of relatively weak first-row M-M bonds generates 17-electron metal fragments. As stated previously in this chapter, 17-electron metal-centered radicals undergo fast assodative ligand substitution. Thus, substitutions of complexes containing metal-metal bonds can occur by initial dissodation into two 17-electron fragments. 

Potential oxidizing agents can also enter the coordination sphere of the duster and be assodated in a stable or a labile fashion. An impressive number of products derived from the simple addition of one or more CuL, AgL, AuL, or HgR moieties (where L is a phosphine or acetonitrile ligand and R is either an alkyl group or a halogen atom) at the surface of a cluster are examples of stable adducts. [190] The few compounds collected in Ihble 3-6 illustrate the enormous potential variety of these adducts, and some of their very interesting properties... 

The differential response to gastrin in terms of secretion and DNA synthesis may reflect different intracellular coupling or transduction mechanisms that follow ligand binding. Genistein, a nonselective protein tyrosine kinase inhibitor, inhibits both cytoplasmic and membrane-assodated tyrosine kinases. At 10- M, genistein failed to inhibit histamine secretion but blocked gastrin-stimulated DNA synthesis. 

Thus, it is clear that the metal-assodated ligands can profoundly aflect their solubility, absorption, bioavailability and the effects of exposure, and then the subsequent toxicity in vivo. 

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