Reactions Involving Coordinated Ligands

Nitric oxide insertion and other reactions involving coordinated ligands 115... 

Reactions Involving Coordinated Ligands 6.5.1 Metal-directed Reactions... 

In reactions involving coordination compounds, the metal acts as the Lewis acid (electron-pair acceptor), while the ligand acts as a Lewis base (electron-pair donor). In the reaction above, the ammonia ligand displaced the water ligand from the chromium complex because nitrogen is a better electron-pair donor (less electronegative) than oxygen. 

Redox reactions involving aromatic ligands may take unusual courses because of the ease with which electrons may be conducted through an aromatic system, though they need not do so. Thus the oxidation of free or coordinated 1,10-o-phenanthroline leads to 3,3 -dicarboxy-2,2 -bipyridine (69), and the oxidation of toluidine in sodium pentacyanotoluidine-ferrate(III) leads to phenazines (22) which can also be obtained by oxidation of the ligand itself (10). 

One of the earliest reported reactions of coordinated ligands involves the hydrolysis of acetonitrile coordinated to platinum(II) and the formation of acetamide.93 Such hydration is general... 

A different type of rearrangement involves cycloaddition of dimethyl acetylenedicarboxylate on to a copper complex which serves to activate the ligand (Scheme 73).229 The reaction is similar in some respects to that shown in equation (42), and indicates the difficulty in the categorization of many of the reactions of coordinated ligands according to mechanism. Consequently, the above account is of necessity somewhat subjective. 

On the other hand, Takacs and coworkers added organometallic reducing agents to the reaction mixture and promoted the formation of low-valent iron(O) bipyridine complexes. The mechanism of the low-valent iron-catalyzed Alder-ene reaction involves coordination of the two starting materials within the ligand sphere of the iron, which makes the Woodward-Hoffmann rules for such reactions obsolete [11]. Thereby, the scope of the reactions was broadened so that alkenes and 1,3-dienes could also be used as educts in a formal [4 + 4]-cycloisomerization (Scheme 9.3) [12]. Intriguingly, the diastereoselectivity of the cydopentane products can be influenced by either the application of the 2Z-isomer 3 or the 2 E-isomer 4. Especially the E-isomers 4 gave almost exclusive cis selectivity [13]. 

Bioinorganic chemistry, insofar as it involves catalytic transformations, is largely the chemistry of metalloenzymes (i.e., metalloproteins). Although the complex three-dimensional structure of the protein ligands obviously plays a vital role in determining the activity and stability of metalloenzymes, at the heart of the matter their reactions involve coordination catalysis by metal ions. Thus, at the molecular level we are dealing with homogeneous catalysis in aqueous media. 

Cu(I) triflate has been used as an olefin cycloaddition photoassistance agent as in reactions (42) and (43).127 Detailed study of reaction (43) shows that it is very likely that it is photoexcitation of a Cu(alkene)2 complex that results in the dimerization.127) This provides a second good example of the possible utility of transition metal photoassistance involving reactions between coordinated ligands. The nature of the excited state leading to reactions (40)-(43) is not known. 

It has been known for decades that a range of reactions occurs that involve chemistry of the ligands and in which metal-ligand bond cleavage is not involved. We can regard these as reactions of coordinated ligands. These early and deceptively simple studies provide fine examples of chemical detective work. One of the earliest studies probed the preparation of a H20—Co(III) species from a O2CO—Co(III) precursor, a reaction which was seen to... 

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. 

REACTIONS OF COORDINATED LIGANDS - dealing with the chemistry of molecules such as oxygen, nitric and nitrous oxide, carbon dioxide, oximes, and nitriles STEREOCHEMISTRY, STRUCTURE, AND CRYSTAL ENGINEERING structure and stereochemistry involving lone pair effects, outer sphere interactions, and hydrogen bonding. 

Other examples of 16-electron complexes come from the substitution reactions involving NHG ligands, carrying bulky R groups like Bu 13 and adamantyl 14, with Ni(GO)4. These substitution reactions afford three-coordinate complexes of the type Ni(GO)2(NHG), 13 and 14/ ... 

An important group of reactions of coordinated ligands which does involve the metal-ligand bond are the so-called insertion reactions. They are often of importance in the catalysis of organic reactions by transition metal complexes, although insertion reactions have a history dating back to the mid-19th century in the reaction... 

This chapter is concerned with reactions of coordinated ligands, and the enhanced reactivity resulting from coordination. It is divided into three main sections. The first deals with coordination complexes of cobalt(III), the second with complexes of other metal centers, and the last with ligand reactions of organometallic compounds. The emphasis is on stoichiometric reactions. Catalytic processes, particularly those involving organometallic species, are covered in Chapter 14. 

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