Dissociative mechanism, lead-ligand

Dissociative mechanisms lead to products where the stereochemistry may be the same or different than the starting complex. Table 12.9 shows that cw-[Co(en)2L(H20)] is a hydrolysis product of both ct5 -[Co(en)2LX] and trfl 5-[Co(en)2LX] in acid solution. While these aquation reactions with pure ci5-[Co(en)2LX] lead exclusively to cis products, retention of the trans ligand orientation in fra 5-[Co(en)2LX] depends on both L andX. The conjugate base mechanism is unlikely in these reactions they are carried out in acidic solution. 

Barrios-Landeros and Hartwig [80a] have reported the mechanism of the oxidative addition of aryl bromides with Pd(0) complexes ligated by bulky electron-rich phosphines such as Pd°(Fc -P-t-Bu2)2 (Fc = aryl-substituted ferrocenyl). All Pd(0) complexes are found to react via a monophosphine complex Pd L in a dissociative mechanism, leading to the monophosphine T-shaped complex ArPdXL which may be stabilized by a weak agostic Pd-H bond (H from the ligand) [80b,c] (Scheme 1.52). The mechanism of the oxidative addition of PhBr to Pd°(P-r-Bu3)2 (P-t-Bus cone angle 182°, pKa = 11.4) is not reported. 

The ratios k k are observed to be 2.2 0.39 1. Thus, if At2 is taken to be normal , the rate constant for the reaction of ser with [Cu(en)] + is also consistent with normal substitution whereas the presence of the bulky histamine molecule leads to a relatively low rate constant. Furthermore, k- the rate constant for the dissociation of ser from [Cu(hm)(ser)]+ is also substantially lower than the rate constant for the dissociation of this ion from the other serinato-complexes (Ar i, A 2, At-ij), which all have the same value to within a factor of two. This rate reduction may indicate steric hindrance between the co-ordinated histamine molecule and an entering water molecule. In a dissociative mechanism for ligand substitution bulky non-leaving ligands appear to enhance the rate. The opposite effect shown here by histamine suggests that an associative mechanism holds for Cu substitution. Further evidence for this conclusion has been obtained from the copper(n)-ethylenediamine-histamine system. 

Mechanisms leading to geometrical isomerization in complexes of this type resemble those already discussed with M(AA,)3 and M(Li)(L2), Secs. 7.6.2 and 7.6.3. As well as twist mechanisms, dissociation of either the unidentate ligand or one-ended dissociation of the... 

A heterolytic rupture will, in the case of an ordinary covalent bond, lead to formation of a cation and an anion in the case of a coordinate bond, the ligand simply departs along with the electron pair it contributed to form the bond. A dissociative mechanism exhibits first-order kinetics its rate is independent of the concentration of the incoming group Z. The intermediate EX in the overall reaction ... 

In both Re(CO)5X and M(CO)s(amine) higher energy photolysis leads to population of LF states which feature population of the dxzy2 bi )° y orbital which labilizes the equatorial CO s and leads to larger CO substitution quantum yields. In all of these C4v complexes the ligand photosubstitution most likely occurs by strictly a dissociative mechanism to yield coordinatively unsaturated intermediates. For the Re(CO)sX, photolysis in the absence of added nucleophiles yields the dimeric species [Re(CO)4X]2 reaction (12), which likely form via coupling of two coordinatively unsaturated Re(CO)4X intermediates.68 ... 

Fig. 6. A multistate model of receptor function with three states. The receptor population consists of an inactive receptor conformation (R) in equilibrium with two (or more) active receptor conformations (R and R ). Each active conformation can differentially activate effector mechanisms, leading to response 1 or response2 in the absence of an agonist. Two isomerization constants (L and M) define the propensity of the receptor to adopt an active conformation in the absence of a ligand. Agonists can differentially stabilize R vs R depending on the value of the equilibrium dissociation constants KA and KA relative to KA. Inverse agonists can also have differential effects on response 1 vs. response2 depending upon the relative values of L and M and of the affinity constants. Additional active states with additional isomerization and affinity constants can be added. Adapted from Leff et al. (86) and Berg et al. (22).

The key steps of a concerted three-center reductive elimination mechanism (Fig. 4, path b) are dissociation of the ligand L trans- to the hydrocarbyl R (step b-i), a concerted M-C bond cleavage and C-X bond formation (step h 2). and a displacement of the organic product R-Z by the ligand L (step b i). The reaction leads to the product of cis-elimination of R-Z with the retention of the configuration of the metal-bound carbon atom. 

Introduction of a polyaminecarboxylate ligand leads to a more dissociative mechanism compared to the fully aquated hexa-aqua complexes. For Fe and a complete changeover from an la... 

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