Coordination compounds reaction types

The majority of organometallic complexes contain 18 electrons see Bonding Energetics of Organometallic Compounds). These complexes undergo ligand substitution, reductive elimination, and electron-transfer reactions see Electron Transfer in Coordination Compounds). Each type of reaction will be covered in the following sections. 

A planar 3-coordinated compound of type (d) in Table 6.14 is in reaction [8],... 

Coordinate Indexing and Boolean Logic. Three methods of indexing have been prominent in the chemical Hterature in recent times. The first, articulated indexing, has been used in printed Chemicaly hstracts subject indexes from their earliest days until well into the 1990s. A number of important concepts are identified as permissible index entries, including specific compounds, material types, reactions, and processes. One or more modifying statements foUow each basic index entry. Thus, eg. 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

As in CljSn—P(CMe3)3 the metal-coordinated 39 may be interpreted as an ylide-type compound. Reactions and physicochemical measurements indicate that the phosphane acts as a simple donor130>. 

One of the techniques for producing coordination compounds is to simply combine the reactants. Some reactions may be carried out in solution, but others may involve adding a liquid or gaseous ligand directly to a metal compound. Some reactions of this type are the following ... 

The replacement of one ligand by another is the most common type of reaction of coordination compounds, and the number of reactions of this type is enormous. Some are carried out in aqueous solutions, some in nonaqueous media, and others can be carried out in the gas phase. One such reaction is... 

The enormous number of coordination compounds undergo many reactions, but a large number of reactions can be classified into a small number of reaction types. When one ligand replaces another, the reaction is called a substitution reaction. For example, when ammonia is added to an aqueous solution containing Cu2+, water molecules in the coordination sphere of the Cu2+ are replaced by molecules of NH3. Ligands are held to metal ions because they are electron pair donors (Lewis bases). Lewis bases are nucleophiles (see Chapter 9), so the substitution of one nucleophile for another is a nucleophilic substitution reaction. Such a reaction can be illustrated as... 

To this point, several types of rate processes that occur in solutions have been described. However, the study of reactions of solid coordination compounds has yielded a large amount of information on behavior in these materials. Several types of reactions of solid complexes are known, but the discussion here will be limited to four common types of processes. 

The most common reaction exhibited by coordination compounds is ligand substitution. Part of this chapter has been devoted to describing these reactions and the factors that affect their rates. In the solid state, the most common reaction of a coordination compound occurs when the compound is heated and a volatile ligand is driven off. When this occurs, another electron pair donor attaches at the vacant site. The donor may be an anion from outside the coordination sphere or it may be some other ligand that changes bonding mode. When the reaction involves an anion entering the coordination sphere of the metal, the reaction is known as anation. One type of anation reaction that has been extensively studied is illustrated by the equation... 

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

Although not all facets of the reactions in which complexes function as catalysts are fully understood, some of the processes are formulated in terms of a sequence of steps that represent well-known reactions. The actual process may not be identical with the collection of proposed steps, but the steps represent chemistry that is well understood. It is interesting to note that developing kinetic models for reactions of substances that are adsorbed on the surface of a solid catalyst leads to rate laws that have exactly the same form as those that describe reactions of substrates bound to enzymes. In a very general way, some of the catalytic processes involving coordination compounds require the reactant(s) to be bound to the metal by coordinate bonds, so there is some similarity in kinetic behavior of all of these processes. Before the catalytic processes are considered, we will describe some of the types of reactions that constitute the individual steps of the reaction sequences. 

When a coordination compound functions as a catalyst, there are usually several steps in the process. The entire collection of steps constitutes the mechanism of the reaction. Before describing several of the important catalytic processes, we will describe the types of reactions that often constitute the elementary steps. 

Oxidative addition is a process by which an atom is simultaneously oxidized and the number of bonds to it is increased as groups are added. The term oxad is used to denote this type of reaction. This type of process is not limited to coordination compounds, and it is easy to find numerous examples in other areas of chemistry. The following examples and many others can be found in earlier chapters of this book ... 

C-C couplings with aUcynes. An unprecedented coupling of this type was found in the reaction of the Ir(I) hydroxo-allenylidenes 71 with excess of HC=CR (R = Ph, C02Me) to afford, under remarkably mild conditions (r.t.), the novel five-coordinate compounds 72 (Scheme 24). The proposed mechanism involves an initial HO /R C=C ligand exchange followed by the oxidative addition of a... 

Three fundamental types of photochemical reactions are known for coordination compounds (A) substitution reactions, (B) rearrangement reactions, and (Q redox reactions. 

Chlorocarboxylato complexes were obtained from the reaction of Tads with RCOOH 2 dimeric [MCl4(RCOO)]2 compounds were isolated for R = H, Me, Et, Pr, Pr, Bu, CF3, while [TaCl3(Bu COO)2] was the only compound obtained with two carboxylato groups. Both type of compound were proposed to have sevenfold coordination. The reaction of NbCls witl dicarboxylic acids236 gave [NbCl3 X(COO)2 ], where X = (CH2) (n = 0-4), CH(OH)CH(OH ... 

With terpy, compounds of the type Ag(terpy)X have been prepared and cannot be tetrahedral for steric reasons, in that terpy cannot span three corners of a tetrahedron. It has been argued that more likely only two of the N atoms are coordinated and that a distorted linear structure is adapted.115 However, if this was the case the central N group would necessarily be within bonding distance. The other alternative was that they were three-coordinate. Addition reactions with neutral ligands such as H20, pyridine or phosphines would then yield distorted square planar structures. 

Most reactions of coordination compounds can be classified as either substitutions or oxidation-reductions. The classic book by Basolo and Pearson19 discusses both types in detail. The oxidation-reductions can occur either by simple electron transfer or by atom transfer. Taube s work on the reduction of cobalt(IJI) complexes by Cr11 is especially important in this regard. Among the many reactions which he has studied, the best known, perhaps, is20... 

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