Axial ligand substitution reactions

Thusius has studied the kinetics of cobalamin-ligand substitution reactions trans to the pendant 5,6-dimethylbenzimidazole ligand (Eqn. 59) for a variety of inorganic ligands including thiocyanate, azide, iodide and bromide ions [103]. 

These reactions are, for the most part, extremely rapid, requiring stopped flow 

In contrast Brown and coworkers [105,106], who have intensively studied ligand substitution trans to the organic group in organo(aquo)cobaloximes by various nitrogen and sulfur donors (Eqn. 60) 

By far the most well studied ligand-substitution reaction trans to the organic ligand in organocobalamins is the simple displacement of the pendant axial 5,6-di-methylbenzimidazole in acid, the so-called base-on - base-off transition of organocobalamins (Eqn. 63). 

This ligand exchange causes a marked change in the electronic spectrum from the 

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Kinetic studies of aquation of dinuclear [ traras-PtCl(NH3)2 2 (p-NH2(CH2)6NH2)]2+ established rate constants for the loss of the first and second chloride ligands (7.9 x 10-5 and 10.6 x 10-4s-1), and for the reverse anations (1.2 and 1.5M-1s-1). Reactivities here are very similar to those in analogous mononuclear systems [Pt(amine)3Cl]+ (204). A kinetic and equilibrium study of axial ligand substitution reactions... 

C. Axial Ligand Substitution Reactions of Dinuclear Pt(3.0+)2 Compounds... 

Rate and Equiubriijm Constant Data for Axial Ligand Substitution Reactions in Aqueous Solutions... 

A. M. van Herk, Medium effects on axial ligand substitution reactions at vitamin B12 and model compounds, Ph.D. Thesis, Free University of Amsterdam (1986). 

The dirhodium(II) tetrakis(/u-carboxyIate) complexes have been the subject of numerous studies as a consequence of their catalytic and antitumor activities, quite apart from their own intrinsic interest. The four bridging carboxylate ligands are inert to substitution, but the two axial ligands at either end of the dirhodium axis are labile. Thus it has been shown that the axial ligand substitution reactions of the diaqua dirhodium(II) tetracacetate species proceeds in the two steps shown in Eqs. (71) and (72), where the rate-determining formation of... 

Table 9.13. Kinetic Parameters for the Axial Ligand Substitution Reactions of Dirhodium(II) Tetraacetate in Aqueous Solution at I = 0.10moldm (LiClOJ and...

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

The usual products obtained from metal insertion reactions are shown in Table 3. They are used as starting materials for any axial ligand substitution processes. 

Similar spectrophotometric studies were also carried out for the ligand substitution reactions with SO [54] [56] and Cl- [56] [59]. However, the reaction rates were so fast that only the spectra at the respective equilibrated states could be observed. The gradual displacement of N02 discussed above is possible only because most of the N02 exists in its protonated form in the strongly acidic solution. Hydrolysis of axial aqua ligand was also examined spectrophotometrically [56][57]. The stepwise formation constants... 

Evidence has been obtained for the occurrence of five-coordinate complexes in the bis-DMG (158), BAE (61), salen (60), and corrin complexes (70, 71) when one of the axial ligands is an alkyl group (see Section III, B). The detection of a five-coordinate complex under certain conditions is, of course, no proof that ligand substitution reactions occur by an aSjjI mechanism under other conditions, but is nevertheless suggestive. 

Very little work has been done on the kinetics of axial ligand substitution trans to the organic ligand in organocobalt corrins primarily due to the excessive speed of these reactions. However, two noteworthy attempts have been made to study the kinetics of benzimidazole displacement in methylcobalamin (Eqn. 63). Milton and... 

The mechanism of axial ligand substitution with a series of ruthenium(II) phthalocyanine adducts is dissociative D), the five-coordinate intermediate possessing little or no ability to discriminate between nucleophiles. The cis effect of tetraphenylporphyrin is considerably less in Ru(III) complexes than in their Fe(II) analogs. An estimate of the relative labilizing effects of the ligands L—L and Cl in the complexes [Pd(L—LiCb] (where L—L = l,2-bis(diphenylphosphino)ethane, o-phenylenebis(dimethylarsine), en, phen, bipy, or l,2-bis(phenylthio)-ethane) have been made through a study of the reaction of these complexes with en. 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

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