Substitution Reactions of 18-Electron Complexes

General Features of the Kinetics of Dissociative Ligand Substitution 

Ligand substitutions at 18-electron, coordinatively saturated complexes typically occur by dissociative substitution mechanisms. Because this reaction mechanism begins with bond cleavage, these reactions are often slower than the associative substitutions of 16-electron complexes. For example, the 16-electron complex Rh(acac)(ethylene)2 reacts with ethylene (1 atm) by an associative mechanism with a rate constant of 10 s at 25 but the related 18-electron complex CpRh(ethylene)2 reacts by dissociative substitution with a rate constant that is about 10 times slower (about 4 X 10 ° s at 25 

Reaction coordinate diagram for a dissociative substitution reaction. 

Relationship between the enthalpy of activation and enthalpy of bond cleavage in a dissociative substitution reaction. 

As a result, activation enthalpies of many dissociative substitution reactions of 18-electron complexes are relatively high and close to the M-L BDE. The weak M-CO bond energy of Ni(CO) (25 2 kcal /mol) allows this complex to react rapidly at room temperature, but the higher M-L bond energies of the olefin in CpRhCethylene) and CO in Cr(CO)g (31 kcal/ mol and 37 2 kcal/mol, respectively) cause the reactions of these complexes to require temperatures of 100 °C and 80-140 °C, respectively, to occur at reasonable rates. 

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It has been known for many years that normally slow CO substitution in 27+ becomes rapid in the presence of a catalytic amount of reducing agent NEt3.145 This result and others suggest the possibility that many substitution reactions of 18-electron complexes thought to occur by conventional dissociative or associative pathways may in fact take place by an ETC catalyzed mechanism initiated by trace amounts of adventitious reductants in solution. 

Thermal substitution reactions of 18-electron complexes are often sluggish. By contrast, 17-electron species are highly reactive, e.g., V(CO)6 undergoes ligand substitution 1010 times faster than Cr(CO)6 via an associative mechanism. Shortlived 17-electron intermediates are known to participate in substitution reactions of Co2(CO)8 ... 

The dissociative mechanism is that usually encountered for the ligand-substitution reaction of 18-electron complexes. - - ... 

The two dominant characteristics for substitution reactions of 17-electron complexes are very rapid reactions and associative mechanisms. Each of these features is in contrast to reactions of 18-electron complexes. The reactivity has been attributed to the formation of a three-electron bond between the entering nucleophile and the 17-electron complex. Electron density analysis supports stabilization of the 19-electron transition state as the primary source for the labilization. ... 

Substitution reactions of 18-electron metal complexes may be associative if a pair of electrons can be delocalized from the metal to a ligand (NO, cyclopentadienyl, etc.), making available a vacant low-energy orbital for nucleophilic attack on the metal. 

Equations 1.6-1.9 provide examples of associative substitution reactions on 18-electron complexes. In all cases, modification of the electron contribution of one of the ligands occurs. In reaction 7.6 the nitrosyl group rearranges from its 3-electron, linear form (an 18 e complex) to a 1-electron, bent form (a 16-e complex).26 This allows the phosphine to attack to give the 18 c complex Co(CO)3(PR3)(NO). Finally, CO departs to give the product. The overall rate of the reaction is related to the nucleophilicity of the phosphine and follows the order PEt2Ph PPh3 > P(OPh)3 (see Section 6-3). 

Dissociative ligand substitution reactions of 18-electron Pd(0) complexes... 

Ligand substitutions of 18-electron complexes can also occur by radical-chain processes initiated by atom abstraction. These radical chains occur through 17-electron intermediates that imdei o facile associative substitutions. Substitutions of metal carbonyl hydrides, halides, and stannyl complexes by this mechanism are all known. These reactions are particularly prevalent in first-row metal hydrides because the M-H bond is weaker than the M-H bond in second- and third-row metal complexes, and hydrogen atom abstraction is one step of the radical chain. However, they have also been proposed to occiu with third-row metal-hydride complexes... 

Although additives to induce radical chemistry have allowed ligand substitutions of 18-electron complexes to be conducted under mild conditions, photochemical reactions provide a common and practical alternative. Photochemically induced dissociation of carbonyl ligands is most common, but photochemical dissociations of other dative ligands are known. Several examples are shown in Equations 5.36-5.40. These examples illustrate the dissociation of CO from homoleptic carbonyl compounds of iron - and chromium, the dissociation of CO from piano-stool carbonyl compounds, " ttie dissociation of N, and the dissociation of a carbodiimide to generate an intermediate that coordinates and cleaves the C-H bonds of alkanes. In some cases, like the formation of the two THE complexes, the products of the photochemical process are not isolated instead, they are treated in situ with a ligand, such as a phosphine, to form monosubstitution products selectively. 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

The central metal atom in complexes usually has a shell of the nearest noble gas, e.g., the shell of 18 electrons. Upon one-electron transfer, the electron number turns from an even into an odd one (e.g., 19 for anion radicals and 17 for cation radicals). The one-unit change of the electron amount leads to an increase in the complex reactivity. The ligand-substitution reactions are markedly facilitated. 

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. 

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