Reactions at Coordinated Ligands

Nucleophilic attack on coordinated ligands is a widely encountered type of reaction. For example, carbonyl complexes are readily attacked by various nucleophiles, including OH , OR , NR3, NR, and CH. A well-known example is the base reaction of carbonyl complexes (Eq. 2-74). 

The carbonyl carbon atom of carbonyl complexes is an electrophilic center that according to the HSAB concept can be regarded as a hard acid (similar to H ). The attack of the hard base OH initially gives a hydroxycarbonyl species, which, however, is unstable and loses CO2, forming a cabonyl metallate anion. The effectiveness of nucleophiles with respect to the carbonyl carbon atom decreases in the following sequence [T12]  

Thus the hard oxygen bases react more readily with metal carbonyls than the softer bases. Alkoxide ions attack coordinated carbon monoxide to form alkoxy carbonyl complexes. This reaction (Eq. 2-75) has been observed for many complexes of the metals Mn, Re, Ee, Ru, Os, Co, Rh, Ir, Pd, Pt, and Hg. 

As a final example of ligand reactions of carbonyls, the rhodium-catalyzed CO conversion reaction will be mentioned. Anionic rhodium complexes such as [Rh(CO)2l2] undergo nucleophilic attack by water with formation of CO2 (Eq. 2-76). 

The resulting rhodium carbonyl complex can be oxidized back to rhodium by protons (Eq. 2-77) the final products are CO2 and H2. 

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For mononuclear complexes, reactions at coordinated ligands is a topic of major study, and numerous reviews and even books have been written on this topic. At the time of writing this review, the authors could find no major review articles in this area for dinuclear compoimds. The best examples are probably those on the reactions of coordinated molybdenum sulfides discussed earlier. A related observation is the reaction of metal carbonyls with Lewis acids. It has been observed by Shriver and coworkers that carbonyl ligands bridging metal-metal bond atoms show increased tendency to coordinate Lewis acids (equation 96). 

Coordination to the metal changes ligand properties sufficiently to make possible reactions at the ligands that either (a) could not happen with the unbound ligand or (b) could occur without the metal but much more slowly. Reactions at coordinated ligands are a vital aspect of organometallic chemistry (Chapter 14). We will describe a few examples of these reactions within coordination chemistry. 

From the properties outlined in the previous section it might well be expected that synthesis of an amino acid via reaction at a ligand coordinated to a chiral metal centre could lead to a chiral... 

Organometallic compounds also undergo reactions of coordinated ligands readily. A simple example involves the susceptibility of coordinated carbon monoxide towards nucleophilic attack. The [Mo(CO)6] complex reacts with methyllithium (6.48), with the new ligand produced at one site also able to undergo additional reactions, not described here. 

The chemistry in metalloenzymes usually occurs at the metal centre(s), and involves effectively reactions of coordinated ligands. 

The immobilization of the metal complex can be achieved by adding the metallic derivative to the already formed polymer, via reactions at the ligand functional groups in the polymer chains. When the metal complexes to be anchored are soluble, reaction with the coordinating polymer (soluble or insoluble) may be carried out by conventional synthetic methods. It is highly desirable that the metallic precursor is soluble. Otherwise the metal-polymer reaction between two solids suspended in a solvent leads to conditions not favorable to a high yield of product. 

Template reactions and reactions of coordinated ligands occupy similar positions in the Scheme. Almost all template transformations fall into the category of coordinated ligand reactions. At the same time, it needs to be noted that not every chemical transformation of ligands may be classified as a template process. In the case of reactions of coordinated ligands, the metal ion plays an electronic role in... 

The reactions of coordinated ligands have been thoroughly investigated by Busch and many others. My first encounter with a ligand reaction (which had not yet been so named) was as a graduate student in collaboration with Bailar. At that time we discovered the ability of copper(II) ions to sever the double bonds of the Schiff base formed by reaction of 2-thiophenaldehyde with ethylenediamine. The reaction was the following ... 

The most reasonable interpretation (there have been many) is to consider the hydroxide or cyanide as forming first an sp -hybridized carbon atom (a pseudobase or Reissert-type adduct, respectively) and then being transmitted from carbon to metal ion. In other words, the change in reactivity of an N-heterocycle on coordination to a metal ion is akin to that of the same N-heterocycle on classical quaternization by an organic agent such as methyl iodide. The unusual rate equation [Eq. (67) or (68)] involving the nucleophile s concentration in first- and second-order terms arises because the rates of these reactions (apparently hydrolysis or substitution by cyanide at the metal ion) are actually controlled by rates of reaction at the ligand (27 28). 

Ligand substitution reactions at low-valent four-, five- and six-coordinate transition metal centres. J. A. S. Howell and P. M, Burkinshaw, Chem. Rev., 1983, 83, 557-599 (468). 

Bolm et al. [108] prepared a C2-symmetric bis (sulfoximine) as ligand for the copper-catalyzed hetero-Diels-Alder reaction. The stereogenic sulfur atom being located near the AT-coordinating atom, these structures were assumed to be promising for asymmetric catalysis. Their Hgand (79 in Scheme 43) was synthesized by palladium-catalyzed N-aryl imination from 1,2-dibromobenzene and (S)-S-methyl-S-phenylsulfoximine with Pd2dba3 in 70% yield. 

When the reaction is carried out in heptanol [61], the particles are monodisperse in size (3 nm), well dispersed in the solvent, and adopt the hep structure of bulk rutheniiun. They can be isolated and re-dissolved in various solvents, including d -THF for NMR analysis. In this case, it is clear that coordinated heptanol is present at the surface of the particles and acts as a weakly coordinating ligand. In this case, the presence of surface hydrides was demonstrated by NMR techniques. 

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