Substitution mechanisms square planar complexes

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

Mechanism of Nucleophilic Substitution in Square Planar Complexes... 

The detection of a reaction intermediate is usually not possible in coordination chemistry because lifetimes of intermediates are commonly extremely short. The simple mechanisms of reaction are commonly designated as an associative mechanism (A, with an intermediate of expanded coordination number formed) or a dissociative mechanism (D, with an intermediate of reduced coordination number formed). Intermediates of expanded coordination number are important in ligand substitution in square-planar complexes and in a few cases can actually be detected. For example, NifCNls " is known from exchange reaction of Ni(CN)4 with CN (288). Even in octahedral complexes, some evidence for associative processes exists indirectly. The [RulNHsle] " ion reacts with NO in acid to form [RuINHslsNO] and NH4 much more rapidly than can be explained by aquation of the hexaamine as the initial step, and a bimolecular mechanism with a 7-coordinate intermediate has been proposed (11, 226). 

Figure 6.3 General mechanism of substitution in square planar complexes highlighting the 5 coordinate intermediate.

Although the combination of [Ir(COD)Cl]2 and LI was shown to catalyze the alkylation, amination, and etherification of allyiic esters to form the branched substitution product in high yield and enantioselectivity, the identity of the active catalyst in these reactions had not been identified. The combination of [Ir(COD) Cl]2 and LI forms the square-planar [Ir(COD)(Cl)Ll] (4) (Scheme 11) [45]. However, this complex does not react with allyiic carbonates to form an appreciable amount of an aUyl complex, and the absence of this reactivity suggested that the mechanism or identity of the active catalyst was more complex than that from simple addition of the allyiic ester to the square-planar complex containing a k -phosphoramidite ligand. 

Cquare planar complexes are generally of the low-spin d8 type. This includes the four-coordinated complexes of Ni (II), Pd(II), Pt(II), Au(III), Rh(I) and Ir(I). The best known and most extensively studied are the compounds of Pt(II). The kinetics and mechanisms of substitution reactions of these systems have been investigated in considerable detail. Studies on complexes of the other metal ions are rather limited, but the results obtained suggest that their reaction mechanism is similar to that of the Pt(II) systems. This paper briefly surveys some of the available information, and presents the current view on the mechanism of substitution reactions of square planar complexes. 

Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.

Ideally, chemists hope to understand a number of reaction mechanisms well enough that predictions about a diverse assortment of complexes involving different metals, ligands, and reaction conditions can be made. A good example of a type of reaction for which this level of understanding has been achieved is substitution in four-coordinate square planar complexes. 

Fig. 13.2 Mechanism for nucleophilic substitution in square planar ML L2XT complexes.

Although in the previous section the basic concepts related to substitution reactions were explained with reference to octahedral complexes, substitution reactions are also common in square planar complexes. Studies on these complexes have resulted in a great deal of knowledge of the mechanisms of these reactions, so a brief description of the topic is presented next. 

Substitution Reactions in Square Planar Complexes 538 Thermodynamic and Kinetic Stability 547 Kinetics of Octahedral Substitution 548 Mechanisms of Redox Reactions 557... 

Much of what is currently known about substitution reactions of square planar complexes came from a lar e number of careful studies executed in the I960s and I970S.3 You should not conclude, however, that details of the mechanisms of these I eactions are of historical intei est only. Work in this area continues unabated as studies focus on chelation, steric effects, biological i eactions. and homogeneous catalysts. For example, the mechanism for the Wacker process (Chapter 15), which utilizes squai e planar [PdCl ] as a homogeneous catalyst for the industrial conversion of ethylene to acetaldehyde, is still a subject of investigation. The overall reaction for the process is ... 

For substitution of monodentate 77-hydrocarbon ligands (ethylene, acetylene) a priori both mechanisms are possible. In this case an ability to change the coordination number in the transition state will be decisive. It is probable that square-planar complexes react by an associative mechanism with an increase in coordination number in the transition state. For the octahedral complexes, intermediates with lower coordination number are preferable (D-type mechanism). There is as yet no evidence for a transition state involving a-bonded ethylene or acetylene. However, both molecules are capable of inserting into transition metal-carbon u-bonds 10). It is quite probable that such an insertion mechanism operates in the Ziegler-Natta ethylene polymerization 11). 

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