Reactions other coordinated ligands

Reactions such as this are electrophilic substitutions, and other electrophiles such as N02 , Cl l3CO, or Cl IO can be substituted on the rings without destroying the complex. Such chemical behavior illustrates the extreme stability of complexes of this type. It is apparent that the nature of the bonding in the chelate rings is as important as is their size. Other reactions of coordinated ligands will be shown in later chapters. 

A number of metal carbonyls undergo disproportionation reactions in the presence of other coordinating ligands. For example, in the presence of amines, Fe(CO)5 reacts as follows ... 

CH3Mn(CO)4THF has been identified, so the conjecture that THF and DMF behave in a manner that is different from the other solvents is not without some justification. Additional work on the influence of the solvent could yield additional insight regarding the mechanisms of this and other reactions of coordinated ligands. 

A detailed stopped-flow kinetics investigation (85) of this reaction (Por = TmTP and OEP) demonstrated that the mechanism occurs in two stages. The first was quite fast and was suppressed by the presence of excess CO and other coordinating ligands. Time resolved infrared... 

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

The synthetic utility of reactions of coordinated ligands is an important and varied subject. It is based on the enhancement in reactivity of organic ligands as a consequence of metal coordination. For example, the metal can act as a super add and cause enhanced nucleophilic attack on coordinated carbonyl and imine ligands. The metal ion can also enable the ligand itself to act as a nucleophile, sometimes by direct activation, sometimes by protecting other parts of the ligand and sometimes by a combination of both. 

In LCu(I) /02 reactivity studies, a number of other examples of oxidative N dealkylation [9] or 0 dealkylation [174,177] are known. As such, these represent possible model systems for PHM (discussed earlier). Other recent examples [134,178] of LCu(I)n/02-mediated hydrocarbon oxygenation reactions on coordinated ligands are also noteworthy. 

By taking advantage of the basicity of NH2- in liquid NH3, it has been possible to carry out reactions on coordinated ligands to prepare complexes that are not obtainable by other means in aqueous solutions. 

The material presented in this chapter provides an introduction to the vast area of reactions of coordination compounds. In addition to the types of reactions described, there is an extensive chemistry of reactions of coordinated ligands. Because many ligands are organic molecules, it is possible to carry out reactions on the ligands without disruption of the complexes, and some such reactions have been mentioned in this chapter. Several others will be shown in Chapter 21. Many reactions of coordination compounds have been studied in detail, and much is known about processes taking place in both solids and solutions. However, it is not possible in a book such as this to do more than introduce the field, but the listed references provide a basis for further study of this area. 

New Species from Reactions of Halocarbonylmetallate Anions. The basic idea for this aspect of the work was that reactive metal halide fragments might be generated from the halocarbonylmetallate anions under conditions where loss of the CO ligands is promoted in the absence of other coordinating ligands. For example, dinuclear or cluster species might be obtained via thermal decomposition of the M(CO)4X anions as in the scheme ... 

The specific acceleration of departure of anions also requires that attack of anions will be accelerated by complexation of the nucleophile with a metal ion. Metal-ion-catalysed enolization reactions of 3-ketophosphonates has been examined in order to investigate the interactions of metals and phosphates. In these studies (Kluger et ai, 1984b) it was found that the metal ion serves to provide the catalytic functions described previously. In addition, from the observed change in steric effects on the reaction in the presence of metals, it appears that the coordinated base can react through a mechanism whereby it serves as a relay for reaction by other coordinated ligands. 

The typically faster rate for associative substitution does not preclude dissociation of ligand occurring as the first step of reactions other than ligand substitutions (Equation 5.10). For example, dissociation of ligands from square-planar, d complexes occurs as the first step of reductive eliminations (Chapter 8). Thus, dissociation of ligands from this class of compound can occur, but association to form a five-coordinate intermediate is typically faster. 

Commonly utilized starting transition metal materials are oxides, halides, salts of fatty acids, complexes containing sulfur, etc. The role of the reducing agents is to lower the oxidation state of a metal and not to reduce the compound to an active metallic phase because this would, in most cases, lead to the inhibition of the reaction as shown by thermodynamic calculations. This point is also indicated by the formation of carbonyl derivatives possessing CO and other coordinating ligands as intermediates. 

Template processes and reactions of coordinated ligands, in situ, direct complex-ation, demetallation, transmetallation and catalytic transformations all exhibit similar features. This sometimes leads to unjustifiable classification of such reactions as template processes and vice versa. To define the relationships between template reactions and other coordination chemistry processes. Scheme 1-5 is proposed, constructed on the principle of Venn logic diagrams [78]. 

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 ... 

When alkyl-substituted carbonyl complexes are photolyzed, reactions other than ligand substitution can occur. One such reaction is the migratory insertion reaction that involves carbonylation of an alkyl complex to give an acyl product. Thus when MnCH3(CO)5 is photolyzed in an argon matrix at 17 K, the 5-coordinate acetyl complex Mn(COCH3)(CO)4 formed by migratory insertion is observed ... 

Generally, a pair of co-ligand isomers results due to an intramolecular nucleophilic addition reaction in coordinated ligands. One of the isomers is an ordinary complex, while the other isomer formed upon the above-mentioned reaction is a chelate. 

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. 

---