Attack on a Coordinated Ligand

On coordination to a metal center, the electronic environment of the ligand changes. Depending on the extent and nature of this change, the ligand may become susceptible to electrophilic or nucleophiUc attack. It is the enhanced reactivity of the coordinated Ugand that is often encountered in homogeneous catalytic processes. 

A nucleophilic attack by a hydroxide on a coordinated ethylene, as shown by reaction 2.3.4.1, is the key step in the manufacture of acetaldehyde by the Wacker process (see Section 8.2). In reaction 2.3.4.2, the high oxidation state of titanium makes the coordinated oxygen atom of an organic hydroperoxo group sufficiently electrophilic for it to be 

Reaction 2.3.4.3 has relevance as a model in the base-promoted water gas shift reaction and is similar to reaction 2.3.4.1. Instead of palladium-coordinated ethylene, it is iron-coordinated carbon monoxide that undergoes nucleophilic attack by hydroxide. The extent to which the reactivity of the ligand may be affected on coordination is often reflected in the rate constants. The ratio of the rate constants of nucleophilic attack by hydroxide on coordinated and free CO may be as high as 10 . 

Finally, insertion of an alkene into a metal carbon bond may also be formally looked at as a nucleophilic attack on the alkene by the alkyl group. This is illustrated by reaction 2.3.4.4, where propylene is attacked by RCH to give the anti-Markovnikov product. Note that in this case both the ligands are coordinated to the metal. 

---

Identify reaction steps in Fig. 8.2 that illustrate the concepts of (a) the trans effect, (b) nucleophilic attack on a coordinated ligand, (c) insertion of alkene into an M-H bond, and (d) intramolecular electron transfer. 

In contrast, spectroscopic and crystal structure analysis indicates that nucleophilic attack of hydride on 72 occurs on the face of the ligand which is coordinated to the metal (Scheme 17). No intermediate species could be detected for this latter reaction. Monitoring of the reduction of the rhenium analog 74 with sodium borohydride indicated the intermediacy of a rhenium formyl complex 75, presumably formed by attack on a coordinated carbon monoxide. Signals for 75 eventually disappear and are replaced by those of the (diene)rhenium product 76 (Scheme 18)95. 

Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

Mechanistically, the formation of the double carbonylation products results from CO insertion into the arylmetal species, followed by amine or alkoxide attack on a coordinated CO ligand to form 46, which then undergoes reductive elimination to form the observed products (Scheme 6)366 370 371. 

The oxidative reactions of metal complexes with halogens are too numerous to mention, and the formation of complexes with less common oxidation states such as PdIV by this route has already been pointed out. While in reactions of L M with X2 usually both X atoms are bound to the metal, salt formation or attack of X on a coordinated ligand has also been observed, for example,46... 

W(phen)(BuSH)(CO)3 serves to lower the S-H bond strength for the bound thiol versus free thiol. The mechanism proposed for this rapid reaction is shown in Scheme 10.4. This involves attack of a metal radical on a coordinated ligand at a nonradical metal complex. The coordination of the thiol reduces the S-H bond strength so that the first-order attack by a metalloradical is feasible. 

These reactions are steps in the synthesis of organometallic complexes, in transformations for organic synthesis, and in the catalytic chemistry of CO. For example, the first synthesis of a carbene complex was achieved by a two-step sequence initiated by nucleophilic attack on a coordinated CO ligand. In addition, the production of CO and fi om... 

Although many more examples of attack on coordinated CO have been published than examples of attack on isonitriles, the attack on isonitrile ligands is well established. The reaction in Equation 11.7 illustrates one example of such a nucleophilic attack on a coordinated isonitrile. The kinetics of this reaction are consistent with the direct nucleophilic attack of the amine on the coordinated isonitrile to form a stable intermediate, which undergoes a subsequent proton transfer to form the observed carbene complex. The resulting electron-rich carbene complex resists further nucleophilic attack. 

Examples of palladium- and rhodium-catalyzed hydroaminations of alkynes are shown in Equations 16.90-16.92 and Table 16.9. The reaction in Equation 16.90 is one of many examples of intramolecular hydroaminations to form indoles that are catalyzed by palladium complexes. The reaction in Equation 16.91 shows earlier versions of this transformation to form pyrroles by the intramolecular hydroamination of amino-substituted propargyl alcohols. More recently, intramolecular hydroaminations of alkynes catalyzed by complexes of rhodium and iridium containing nitrogen donor ligands have been reported, and intermolecular hydroaminations of terminal alkynes at room temperature catalyzed by the combination of a cationic rhodium precursor and tricyclohexylphosphine are known. The latter reaction forms the Markovnikov addition product, as shown in Equation 16.92 and Table 16.9. These reactions catalyzed by rhodium and iridium complexes are presumed to occur by nucleophilic attack on a coordinated alkyne. 

Lastly, the decompositions of the Au(I) complexes (VII) and (VIII) yield a variety of interesting products, some of which probably arise from halide attack on the coordinated ligand (Hiittel and Forkl, 1972). In chloroform solution, complex (VII) gave a mixture of traws-dichlorobutene, substituted butadienes, and cyclobutenes in modest overall yield. The neutral butyne... 

---