Substitution reactions trans effect

It is well documented that the ligand in the position trans to the leaving group has a significant influence on the rate of substitution reactions. This effect has been most thoroughly studied in Pt(II) chemistry, where the ordmiig of trans labilization is... 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

In the second case, the phosphorus atom in the phosphine also has empty d orbitals that can accept electron density donated from the Pt2+. In fact, it is more effective in this regard than is the sulfur atom in SCN-. This results in the more stable bonding to SCN- being to the nitrogen atom when PR3 is in the trans position. In essence, the presence of tt bonding ligands in trans positions that compete for back donation leads to a complex of lower stability. As will be discussed in Chapter 20, this phenomenon (known as the tram effect) has a profound effect on the rates of substitution reactions in such complexes. 

Coordination requires vacant sites (coordinative unsaturation) (7) or facile ligand substitution. The trans and cis effect of ligands on substitution reactions has been discussed (2). Equilibria between free ligands and complexes have been studied, and information about the steric and electronic effects of ligands is available (5). 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

Steric hindrance is well known to slow down the rates of ligand substitution reactions in square-planar metal complexes. An example for which steric hindrance controls the aquation rate is complex 9. The effect of 2-picoline on the rate of hydrolysis of CP trans to NH3 (cis to 2-picoline) is dramatic, being about 5 times as slow as the analogous CP ligand in the nonsterically hindered 3-picoline complex (Table I) (44). 

The traras-effect has been defined as the effect of a coordinated group on the rate of substitution reactions of ligands trans to itself See Basolo, E. Pearson, R. G. Prog. Inorg. Chem. 1962, 4, 381. 

Some kinetic data on the trans effect are now available. More detailed systematic studies are needed, but quantitative information does provide the magnitude of the trans labilizing ability of various ligands for substitution reactions in these systems. For complexes of the type [PtNH3LCl2]", where the leaving Cl" is trans to L, the trans effect order of L is approximately (28),... 

The reaction of trans-Pt with two equivalents of GSH results in a monomeric complex in which two GS units are coordinated via the cys-S atom [Eq. (10)] (76,131,189). The unstable intermediate species, [trans-Pt(NH3)2(GS)Cl] and [frans- Pt(NH3)2Cl 2GS], could also be detected by Berners-Price (189). The reaction of [Pt(NH3)3Cl]Cl with two equivalents of GS- initially yields [Pt(NH3)3GS], which reacts further, eventually forming [irans-Pt(NH3)2(GS)2] and free NH3 [Eq. (11) (131). This is rationalized by the large trans effect of the coordinated sulfur and the presence of a second nucleophilic sulfur. Therefore, the amine ligand is easily substituted by a second GS" unit. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

Table 14 Trans Effect of L on Reaction Rates of Pyridine Substitution in trans-PtClL(PEt3)2 at 25 °C...

The mechanism proposed to account for the substitution reactions of the Ni(L)r complexes is shown in Fig. 60. A similar mechanism is proposed for substitution reactions with unidentate nucleophiles. Several of the five-coordinate intermediates shown in Fig. 60 were detected and their stabilities were estimated. It was suggested that the trans effect operates through the stability of the five-coordinate intermediate which in turn is correlated to the extent of involvement of the nickel 4pz orbital in n bonding. The implication of the 4pz orbital in n bonding should be considered cum grano salis, however, in view of recent theoretical calculations indicating that the involvement of the 4pz orbital in the n system is very small even in the Ni(MNT)2 complex (123). (See also Sect. 2E.)... 

In 1893 a third important regularity was observed by Nikolai Semenovich Kumakov (1860—1941).107 While investigating the substitution of ligands by thiourea and thioacetamide, Kumakov found that replacement occurs with all the ligands of the cis compound but only with the acid radicals of the trans compound (Scheme 2). Since the two isomers yield different products, this reaction, known as Kumakov s reaction or Kumakov s test, may be used to differentiate cis from trans isomers of dipositive platinum or palladium. Kumakov s classic reaction played a crucial role in Werner s proof of the square planar configuration of Pt11 and in Chemyaev s formulation of the trans effect. 

---