Associative ligand substitution reactions

The neutral Ni l or Ni-R complexes undergo associative ligand substitution reactions (Scheme 5, see Associative Substitution Mechanisms of Reaction of Organometallic Complexes), and react with methylaluminoxane (MAO) to generate intermediates that polymerize ethylene to high MW poly(ethylene), and alkynes to cis, tran5 otW-poly(alkynes) (Scheme 6). ... 

There are exceptions to Tolman s rule, however [24,25]. For example, if the ligands are very bulky, the 16-electron complex may be sterically hindered, making a 14-electron species the more stable one. The complex Pd[P(ferf-Bu)3]2 is a case in point [26]. Also, a solvent such as benzene can act as electron donor and thereby stabilize a nominally 14-electron complex as a 16-electron solvate [27]. A few reactions appear to proceed through paramagnetic, 17- or 19-electron complexes as intermediates [28,29]. 20-electron species are believed to be formed as intermediates in some associative ligand substitution reactions [30,31]. All such species are much less stable than the corresponding 16- or 18-electron complexes. 

Associative ligand substitution reactions such as that shown in Equation 10.46 were also investigated.68... 

Associative Ligand Substitution Reactions and the Berry Rearrangement... 

The neutral Ni-Cl or Ni-R complexes undergo associative ligand substitution reactions (Scheme 5, see Associative Substitution, Mechanisms of Reaction of OrganometaUic... 

Associative ligand substitution reactions of 16-electron d Pd(II) complexes... 

Provide a rationale for how the 18-electron compound [CpRe(CO)(NO)Me] can undergo the following associative ligand substitution reaction ... 

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. 

Non-Marcusian linear free energy relationships (if I may again be permitted that barbarism) provide direct evidence for this type of potential surface in octahedral ligand substitution reactions. Both dissociative (e.g., the chloropentaamine of cobalt(III)) and associative systems (e.g., chloropentaaquo chromium(III)) may have values of slopes for the linear free energy relationships indicating non-Marcusian behavior. 

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... 

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

Figure 5-38. The prototypical ligand substitution reaction in octahedral complexes. In principle, the reaction could proceed by associative or dissociative mechanisms.

N-donor induced disproportionation of [Fe(CO)3(PR3)2]+ (R = Me, Bu, Cy, Ph) as well as halide induced disproportionation of [M(CO)3(PCy3)2]+ (M = Fe, Ru, Os) has been interpreted in terms of nucleophilic attack being rate determining.103 104 The rate data led to the conclusion that the reactivity of these 17-electron complexes is only weakly dependent on the metal, and the suggestion was made that periodic trends in 17-electron systems are generally attenuated in comparison to those for 18-electron analogues. However, it was noted previously that W > Cr by ca. 106 1 for substitution in [CpM(CO)3]. A direct comparison of the rate of associative ligand substitution at a 17-electron center as a function of the metal for a complete triad (Cr, Mo, W) was reported for the reaction in Eq. (20).14... 

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. 

While certain reactions of complexes which contain the M2L8 skeleton, and M-M quadruple or triple bonds (i.e. molecules of classes (a) and (B), vide supra), resemble those of dimers of classes (C) and (D), other aspects of their chemical reactivity are noticeably different. They resemble dimers (c) and (D) in undergoing ligand substitution reactions(2.), Lewis base associations (particularly in the case of carboxylate-bridged dimers of the type M2(02CR)u)( ) and dimer to cluster transformations, recent examples of the latter being M2 -----M3 (triangular)(15., 16),... 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Ligand substitution reactions of square-planar complexes most often occur by associative reaction sequences. An example of square-planar organometallic complexes that will illustrate this reactivity is trani -Ir(Cl)(CO)(PPh3)2 (frequently referred to as Vaska s Complex), see Vaska s Complex). This complex undergoes rapid ligand substitution with CO, PR3, and... 

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