Square-planar complexes substitution reactions

Squai e-planar rhodium I) complexes, phosphorus-nitrogen donor ligands, 44 295 Square-planar substitution reactions, 34 219-221... 

Fig. 13.3 Reaction coordinate/energy profile for a square planar substitution reaction having (a) a trigonal bipyramidal activated complex and (b) a trigonal bipyramidal intermediate. [From Burdetl, 1. K. Inorg. Chem. 1977, 16, 3013-3025. Used with permission.)...

The kinetics of square planar substitution reactions are complex, as shown in Figure 17.6. The experimentally determined rate law has two terms, as shown in Equation (17.39). 

The substitution reactions (23), where the chelate ligands are cyclo-octa-1,5-diene (cod), ethylenediamine (en), or iV,iV -dimethylethylenediamine, offer two further illustrative examples of square-planar substitution reactions, where reversible solvolysis of the substrate complex has to be considered. In these cases also, the observed rate constants depend on the concentration of the leaving ligand (X=C1 ), so the more complete rate law (2) applies. 

Transition metal square-planar complexes generally contain eight d electrons and are almost always diamagnetic. This includes complexes of Pt, Pd % Au, Rh, and Ir. While such complexes can imdergo other reactions such as redox processes, we shall focus on substitution reactions. Good reviews of square-planar substitution reactions are available. The following is a summary of some of these substitution processes, wifli emphasis on those involved with polymer formation. These substitution reactions are the most widely studied of the transition metal square-planar complex reactions. 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

The general rate law for substitution in square-planar Pt(II) complexes is valid for the reaction... 

Associative substitution is usually found with square planar cP metal complexes such as those of Ni(II), Pd(II), Pt(II), Ir(I), and Au(III). Substitution reactions of these complexes have been thoroughly investigated.19 As in the... 

The Russian School, particularly Chernyaev, did much of the early research on platinum(ii) chemistry. This arose from the large platinum resources in some of their natural minerals. So important was the availability of platinum to them that they had an Institute devoted in large part to the chemistry of platinum. Most of their research dealt with the syntheses and reactions of platinum complexes. Their primary goal seemed to have been to enhance the extraction of platinum from its mineral source. As early as 1926, Chernyaev" reported that certain ligands in the position trans to the leaving group of square-planar platinum(ii) complexes have a marked effect on its replacement substitution. He used this with considerable success in the preparation of desired platinum(ii) complexes. For example, he was able to prepare cis- [PtCl2(N02)(NH3)]"bythe reactions shown in (1). 

We have already touched on some aspects of inorganic reaction mechanisms kinetically inert metal centres such as Co(III) (Section 21.10) and organometallic reaction types (Section 23.7). Now, we discuss in more detail the mechanisms of ligand substitution and electron-transfer reactions in coordination complexes for the substitution reactions, we confine our attention to square planar and octahedral complexes, for which kinetic data are plentiful. 

Reaction 25.8 shows the substitution of X by Y in a square planar Pt(II) complex. 

Nickel(II) phosphine complexes have been reported to he efficient catalysts in carbonylation reactions. To investigate this reaction mechanism, we have studied the reaction of CO on the related Ni(II) complexes NiX2(PMes)n (n = 2,3) and [NiX(PMes)m]BFj> (m = 3,4). Pentacoordinate carbonyl nickel(II) species (without reduction of Ni(II) to Ni(0)) were isolated (1) by direct substitution of PMcs by CO in the pentacoordinate complex and (2) by addition of CO on the trans square-planar tetracoordinate complex. These compounds are trigonal-bipyramidal complexes with CO in equatorial position. The Ni-CO distance (1.73 A) is the shortest reported Ni-CO distance. Since these carbonylation reactions can be viewed as substitution of an equatorial PMes by CO in a TBP, they can be related to the substitution reactions in square-planar d metal complexes. 

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