Substitution associative

Associative substitution differs from dissociative in that the incoming ligand binds to the complex before the departing ligand leaves. This is typical of 16e complexes because the intermediate is then 18e and is analogous to the associative Sn2 organic reaction. 

The slow step in associative substitution is the attack of the incoming ligand V on the complex to form an intermediate that only subsequently expels one of the original ligands L. The rate of the overall process is now controlled by the rate at which the incoming ligand can attack the metal in the slow step, and so [L ] appears in the rate equation (Eq. 4.31) and the rate also depends on the nature of L.  

For 16e complexes, the 18e intermediate of an A mechanism usually provides a lower energy route than the 14e intermediate of a D substitution. The entropy of activation is negative (A5 = —10 to -15 eu), as expected for a more ordered transition state. 

Classic examples of the A mechanism are seen for 16e, (f square planar complexes of Pt(II), Pd(II), and Rh(I). The 18e intermediate is a standard trigonal bipyramid with L in the equatorial plane (4.24). By 

The solvent, present in high molarity, can act as an incoming ligand and expel L to give a solvated four-coordinate intermediate. A subsequent associative substitution with L then gives the final product. Substitutions of one halide by another on Pd(II) and Pt(II) can follow this route (Eq. 4.34). 

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For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Considerable investigation of the octahedral carbonyl complexes has been carried out. To a certain degree this is because definitive evidence for associative substitution in the case of type A complexes has been conspicuously lacking whereas for the type B compounds there seem to be several well-substantiated examples. A general summary of the main types of octahedral substitutions which have been kinetically examined is given in Table 15. 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

Associative substitution (A) in organometallic octahedral complexes involving 7i-type ligands is well established but not common. F. Basolo, Inorg. Chim. Acta 100, 33 (1985). See Also Ref. 19. 

One obvious area in which anchimeric effects might materialize is in chelation reactions this is the underlying reason that chelation is dominated by the first step (Sec. 4.4.1). We have seen it operate in the formation of Pt(II) chelates (Sec. 4.7.6). The accompanying values of AS are less negative than is usual for the associative substitution in Pt(II), consistent with the intramolecular mechanism. The rate constant for the ring-closure reaction (6.1)... 

This is expected to be favoured for metal-centred excited states for example, in d-d states of d or d complexes, where excitation often involves promotion of an electron from an essentially non-bonding orbital to one with appreciable sigma antibonding M-L character (e.g. in CrfNHalsCl Eq. 3). The net effect is lengthening of the M-L bond, which predisposes the complex to dissociation or associative substitution. The incoming ligand is often the solvent (e.g. as in Eq. 3) or counterion of an ion pair (Eq. 4). 

On the basis of the PM-RAIRS evidence, Drent has proposed a catalytic cycle (Scheme 7.8) where ethene insertion in the propagation step is CO-assisted and the substitution of the chelating ketone in c by ethene would proceed in two consecutive steps associative substitution of the chelating ketone by CO (c —> d —> e), followed by associative substitution of CO by ethene (e —> f —> i). The disruption of the chelate structure of c would be more facile for CO than for ethene for steric reasons (end-on vs. side-on approach). 

It has always been of some interest to examine the extent to which associative activation dominates the mechanism of substitution of four-coordinate planar cP metal complexes. The coordination unsaturation of these formally 16-electron valence shell complexes predicts that a substitution pathway with increased coordination number (18-electron valence shell) will be favoured over one with a reduced coordination number (14 electrons). This was well understood by workers in the field438 long before Tolman94 published his rules. The first attempt to force a dissociative mechanism was made by Basolo and Baddley513>514 who reasoned that since the steric requirements of associative substitution (rates reduced by steric hindrance from the cis position) were opposite to those of a dissociative mechanism (rates increased or unchanged by increased steric hindrance), sufficient congestion in the substrate should reduce the rate of the associative process to the point where dissociative activation took over. If this did not produce a change in mechanism it could at least indicate a lower limit to the difference of the two modes of activation. 

Another polyhalophenyl gold(I) complex, [Au(3,5-C6Cl2F3)(tht)], has been found to be a very efficient catalyst for the isomerization of fra/w-[Pd(3,5-CftCI2F3)2(tht)21 to c/s-[Pd(3,5-C6Cl2F3)2(tht)2].40 The reaction takes place through a novel reversible aryl exchange between Pd(II) and Au(I). The mechanism involves associative substitution of... 

The mechanism of oxycyclobutenyl (CO)(NO)(PBu3)Fe(C3Ph3CO) formation by the associative substitution reaction is suggested to involve cyclopropenyl ring slippage... 

The activation parameters and dependence on L are shown in Table 13. These data are fully consistent with an associative reaction. The 17-electron complex V(CO)6 has an associative substitution reaction rate that is > 10 ° more facile than for the 18-electron Cr(CO)6 complex. The vanadium complexes are among the most inert of the 17-electron complexes. Table 14 shows the rate constants for substitution of several complexes. As expected from size considerations, substituting a phosphine ligand for a CO decreases the rate for an associative reaction. 

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). ... 

Kinetic studies ou reactions of Pt(ll) complexes with biologically relevant nucleophiles have been reported. The substitution of both coordinated water molecules by a series of nucleophiles (namely, thiourea (tu), L-methionine (L-Met), and gnanosine-5 -monophosphate (5 -GMP)) was investigated as a fimction of concentration, temperature, and pressure using stopped-flow techniques and was found to occur in two subsequent reaction steps. The activation parameters for all reactions suggest an associative substitution mechanism. 

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