Associative ligand substitution

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

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... 

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... 

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. 

Normal associative ligand substitutions at these sterically hindered compounds are dominated by the solvento (Ai) route, and scope for forming the CB of aquo intermediates at high pH [(Eq. (16)] was recognized as a potential mechanistic complication (86a). 

Equation 7.23s4 demonstrates another example of an associative ligand substitution involving a 17-e substrate the resulting 17-e substituted product then rapidly dimerizes. 

Reaction 10.5 initiates the chain reaction via H-atom transfer to generate Re(CO)5. Rapid associative ligand substitution occurs in reaction 10.6. H-atom transfer between the substituted metal radical and the starting hydride completes the chain, as shown in Equation 10.7. All of these reactions occur much faster in radical systems than their saturated counterparts. The result is that ligand substitution in the net reaction 10.8 (reactions 10.6 and 10.7) occurs most rapidly by a pathway involving largely unseen radical intermediates. The paper by Byers and Brown also contains two statements of relevance to those new to the area of organometallic radical chemistry ... 

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 ... 

On the other hand, using a stronger base should lead to a more stable Al-amine adduct. Secondary amines are stronger, but due to steric hindrance, the reaction of secondary amines with the (Al(OBu )3)2 dimer is very slow. This problem can be overcome by the associative ligand substitution approach shown by Reaction R2.4. Addition of piperidine to a solution containing monomeric Al(OBu )3(Pr"NH2) results in the formation of Al(OBu03(Pipy), even at room temperature. The complex is stable at room temperature and provides the desired protective function to the Lewis acid site of alumina during hydrolysis. 

The rate behavior of associative ligand substitutions of square-planar complexes reflects the two pathways that often occur in parallel with and without solvent assistance. Thus, the corresponding rate law provided in Equation 5.8 contains two terms, one of which tjrpically corresponds to the rate constant for associative substitution with solvent assistance and one of which typically corresponds to the rate constant for associative substitution without solvent assistance. Often a plot of versus [Y] appears like the plot... 

These relative rates fit the general trend that reactions of first-row metal complexes tend to be faster than reactions of their second-row analogs, which tend to be faster than reactions of their third-row congeners, but the precise origin of this effect in associative ligand substitutions has been rationalized in more than one way. One text has attributed this trend for associative ligand substitution as reflecting the relative propensities of the metals to form five-coordinate, 18-electron complexes. In a few cases, five-coordinate... 

Considering the 18-electron rule, it was initially thought - " ttiat 17-electron species, such as V(CO)5 or the transient MufCOl, would undergo ligand substitution by the same dissociative pathways of coordinatively saturated complexes. In this case, the complexes would dissociate CO to form the 15-electron intermediates V(CO)5 and Mn(CO). In contrast to this initial expectation, these complexes undergo associative ligand substitutions. 

The complexes [Rh(diolefin)(PN3)]BF4 feature P,N-coordinated chelated hg-ands with two pendant amino groups. The corresponding metallacycle has a boat like conformation (racemic chiral), which defines concave and convex sites for the attack of the pendant ligands in an associative ligand substitution process of the coordinated by the pendant amino groups (Figure 4.16) [40]. 

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