Ligand displacement

The chemistry of Cr(III) in aqueous solution is coordination chemistry (see Coordination compounds). It is dominated by the formation of kineticaHy inert, octahedral complexes. The bonding can be described by Ss]] hybridization, and HteraHy thousands of complexes have been prepared. The kinetic inertness results from the electronic configuration of the Cr ion (41). This type of orbital charge distribution makes ligand displacement and... 

The vast majority of phosphine/phosphite-substituted products involve F-ligand ligation at late transition metals. In contrast, phosphite ligands displaced rhenium-coordinated CO or acetylene in [RePt3(/i-dppm)3(CO)3(L)l", - which are the... 

CHjLi gave only Pt(diphos)R2 with [Pt(PPhj)2(CNCH3)2] and CsFjLi ligand displacement also occurred to give a mixture of Pt(PPh3)(CNCH3)-(C,F,)2 andPt(PPh3)2(C,F3)2 (747). 

A combination of neutral and anionic ligand displacements is shown in Eq. (55) 115) it was suggested that the high trans influence of Me3Si leads to Pt—Cl bond weakening and hence the ionic product rather than cis-Me3SiPtCl(diphos) [which is a stable compound 69, 71)]. 

A number of trimethylsilylmethyl and related complexes have been converted into others by standard ligand displacement procedures from... 

The species (MS03)" represents an inner-sphere complex between sulphite and the oxidant formed by ligand-displacement. (MS03)" is formed by abstracting an electron from... 

These share tendencies to behave as one-equivalent oxidants with redox potentials in the range 1.0 to 2.0 V and to undergo hydrolysis in aqueous solution above pH 2. They form complexes with anions, e.g. sulphate, which make their reactivity dependent on the nature of the medium both from the point of redox potential and of ligand displacement (Table 11). 

Wiberg et have performed the reaction in the presence of C-labelled cyanide ion and find no incorporation of activity into product ferrocyanide. Evidently the reversible ligand displacement proposed by the Czech workers does not take place and the electron-transfer scheme of Swinehart is preferable. Recent spectroscopic studies indicate that a complex [Fe(CN)5(CNS03)] functions as an intermediate in this reaction. 

These present an interesting dichotomy in their reductions by tm(l,10-phen-anthroline)iron(ri) (ferroin) °. That of CIO2 to CIOJ is rapid, is first-order in each component ki = 1.86 0.13 l.mole sec at 35 °C) and is independent of acidity. Ferriin is the immediate product and an outer sphere electron-transfer is proposed. The reduction of CIO2 is much slower, proceeding at the same rate as dissociation of ferroin at high chlorite concentrations and a major product is feriin dimer, possibly [(phen)2Fe-0-Fe(phen)2] . Clearly the reaction depends on ligand-displacement followed by an inner-sphere electron transfer. 

Gold carbene complexes preparation, oxidation, and ligand displacement. Journal of Organometallic Chemistry,... 

Mixed gold] III) and molybdenum or tungsten derivatives can be obtained by ligand displacement or bridge-cleavage reactions. Thus, treatment of [M]CO)4]tdppme)]... 

Screening of chemokine receptors has focused mostly on ligand displacement assays. These assays use a radiolabeled ligand and cells or membranes... 

Tetraphenylarsacymantrene (74) undergoes several CO ligand displacements without disruption of the arsolyl ligand (Scheme 16).24 It is reported that these displacements take place more readily and in higher yield than those for the corresponding pyrolyl derivatives. However, 108 and 71 are found to be much less reactive than 107 toward nonphotochemical CO displacement by phosphines.37... 

Three approaches have been tested, as already described above for inorganic supports. The first attempts concern the direct reaction of transition metal carbonyls with unmodified organic polymers like poly-2-vinyl-pyridine.61 62 However, this kind of anchoring is restricted to only a few complexes. Various polymers have been functionalized with donor groups 63-72 ligand displacement reactions using these afforded the corresponding immobilized complexes. Finally, tests with modified complexes and unmodified polymers are scarce because of the low stability of these complexes under the conditions of reactions. 

For purposes of pyridine N-oxide reduction, it suffices to say that many examples of Eq. (14) have been explored, including those in which PR3 is displaced contra-thermodynamically (p/TM may be ca. 2) by PyO. Invoking such a step in the catalytic cycle is therefore perfectly acceptable. Actually, the ligand displacement chemistry is a rich and fascinating area in its own right, and for that reason it will be explored independently in Section V. 

The first step in oxygen transfer is ligand substitution at an oxorhenium(V) center, Eq. (14). The final step (see Scheme 2, step 2) very likely is also ligand substitution. We have therefore examined the kinetics and mechanism of several reactions in which one monodentate ligand displaces another, represented in general as follows ... 

The low reactivity of both Cyt111 and Cyt11 toward NO can be attributed to occupation of the heme iron axial coordination sites by an imidazole nitrogen and by a methionine sulfur of the protein (28). Thus, unlike other heme proteins where one axial site is empty or occupied by H20, formation of the nitrosyl complex not only involves ligand displacement but also significant protein conformational changes which inhibit the reaction with NO. However, the protein does not always inhibit reactivity given that Cat and nNOS are more reactive toward NO than is the model complex Fem(TPPS)(H20)2 (Table II). Conversely, the koS values... 

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