Associative ligand substitution Kinetics

Kinetic and mechanistic studies of nucleophilic substitution at metal(IV) centers are fairly rare (263). Platinum(IV) has the substitution-inert low-spin d configuration, and presumably undergoes nucleophilic substitution by an associative mechanism thanks to its high charge and large size. However there are actually very few data, probably thanks to the tendency for platinum(IV) to oxidize ligands. Substitution kinetics at metal(IV) centers may be more conveniently studied for complexes of the type ML2X2, where M — e.g., Sn, Ti, V, or... 

The Associative Mechanism The A mechanism can be expected to predominate in complexes that can expand their coordination number relatively easily. The obvious candidates are the square planar complexes of Pt11, Pdn, and Aum, for which many comparable five-coordinate complexes are also known. The salient feature of ligand substitution kinetics in these square planar complexes, however, is not the detectability of five-coordinate intermediates—the reaction rates are usually accurately second order overall—but rather the wide-ranging dependence of the second-order rate constants on the nature of the incoming ligand. This implies that in the scheme... 

The transmetallation of oxidative addition products of alkenyl halides and zerovalent palladium species by organotin reagents is an important step in Stille type reactions and its mechanism depends on many variables. A recent review on this topic accounts for transmetallation of alkenylpalladium(ii) species.The involvement of 14-electron T-shaped species [PdX(R)(L)] in Stille reactions involving alkenyl and other substrates has been refuted as unrealistic. Instead, associative ligand substitution at palladium is advocated, which has been corroborated by full and quantitative evaluation of kinetic data as well as MO considerations. ... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

Rossetti and co-workers (133) investigated the kinetics and mechanism of the substitution of FeCo2(CO)9S and its derivatives. Both associative and dissociative paths were observed in each of the three substitution steps. However, the first and second phosphine ligands substituted predominantly by an associative pathway, although the first substitution was... 

Kinetically inert square-planar complexes are found by d8 low-spin ions, particularly Pt2+. Ligand substitution is associative and correlated with the ease of forming a five coordinate transition state (or intermediate). Substitution is much faster with Ni2+ where five-coordinate complexes such as [Ni(CN)5]3- are more stable than for Pt. For a given metal, the rate of substitution is controlled by ... 

In addition to carbenes exerting a strong trans influence (thermodynamic), which may be manifest as a trans effect (kinetic) in ligand substitution reactions, heteroatom-functionalized carbene ligands (with their associated dipolar resonance contributor, Figure 1.16) may assist in the stabilization of transition states of reduced coordination number by electron donation to the metal. 

On the coordination chemistry side, ligand substitution on metal complexes in ILs has attracted quite some interest. This is mainly due to the fact that both spectroscopic and catalytic properties are strongly governed by the nature of the ligands and the stability of their bond to a metal center. Begel et al. have studied the role of different ILs on ligand substitution reactions on [Pt(terpy)Cl]+ (terpy = 2,2 6, 2"-terpyridine) with thiourea with stopped-flow techniques. The substitution kinetics show similar trends if compared to conventional solvents with similar polarities. Moreover, much like in conventional solvents, the authors find an associative character of the substitution reaction [205], These results are essentially supporting an earlier study by Weber et al., who found the same behavior [206],... 

Kinetically inert square-planar complexes are formed by cP low-spin ions, especially Pt2+. Ligand substitution is associative and correlated with the ease of forming a five-coordinate transition state... 

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