The Trans Effect

The ligand arrangement around an atom after a substitution reaction may or may not be similar to that of the starting material, even for inert complexes. An example of such a change is provided by the formation of rr zns-[Cr(en)2(NCS)2] on heating [Cr(en)3] with solid ammonium thiocyanate at 130 °C. Similarly, ds-[Co(NH3)4(H20)Cl]S04. is converted into tr zns-[Co(NH3)4Cl2]HS04 by the action of a mixture concentrated hydrochloric and sulfuric acids at room temperature. 

One can see from this list the difficulty of proposing a general explanation 

Although the fit of the data to the line is not perfect, it is adequate given the level of accuracy of the 

One of the fascinating characteristics of substitution in square planar complexes is illustrated by the following equations  

In the first of these reactions, all of the entering pyridine goes in the position trans to CO. If there 

From the reactions shown, it becomes apparent that NH3 must have less influence on the position trans to it than does Cl-. Moreover, the first reaction shows that CO has a stronger influence than does Cl-, and the last reaction shows that N02- has a greater effect than Cl-. By conducting reactions such 

Here we can see that in K[Pt(NH3)Br3] the length of the Pt-Br bond opposite the Pt-NH3 bond is about 242 pm, whereas the Pt-Br bond opposite the other Br is 270 pm in length. A similar effect in seen in K[Pt(NH3)Cl3], but the difference in bond lengths is smaller in this case because Cl- exerts a weaker trans effect that Br-. In the anion ofZeise s salt, [Pt(C2H4)Cl3], 

If a complex such as PtX4 has one of the ligands X replaced by Y, a single product, PtX3Y, results. However, replacement of a second X by Y can lead to two different products  

Specific examples of this behavior are shown in the following equations  

If the order of introducing N02 and NH3 into the [PtBr4]2 complex is reversed, the result is quite different. In that case, the reactions can be shown as follows  

As described earlier, the trans effect is indicated by the fact that the products obtained when NH3 and N02 are substituted for Br- in [PtBr4]2- depend on the order of addition. If NH3 is added first followed by N02 , the product is cA-[PtBr2NH3N02] , but if N02 is added first, the product is trans- PtBr2NH3N02r. Clearly the stereochemistry of the product is the result of a trans effect. If this trans directing influence is manifested in this way, it should also be evident from other properties of square planar complexes. In fact, there should be kinetic as well as thermodynamic and structural evidence to indicate the difference in trans influence exerted by different ligands. In the case of square planar complexes, much 

Studies on many replacement reactions using square planar complexes have been carried out. For the reaction 

Long before the trans-effect was recognized by Chernyaev, empirical rules (Peyrone s rule, Jorgensen s rule, and Kurnakov s rule) were known in coordination chemistry that were based upon a large body of experimental data and that enabled predictions to be made concerning substitution reactions at platinum(II). Chernyaev was able to formulate a chemical basis for these rules and incorporate them into a single, comprehensive framework. Each of the three empirical rules is discussed in the paragraphs that follow. 

Experimentally, this generalization was demonstrated by Jorgensen and given its stereochemical interpretation by Werner. In memory of Peyrone, who was the first to prepare cis-[PtCl2(NH3)2l, this rule was later called Peyrone s rule. 

Note that Pt(II) adopts a coordination geometry different from that of Co(III). The ligands in these Pt complexes lie at the corners of a square with the metal at the center. This is called the square planar geometry (1.13). 

An important application of the trans effect is the synthesis of specific isomers of coordination compounds. Equations 1.3 and 1.4 show how the cis and trans isomers of Pt(NH3)2Cl2 can be prepared selectively by taking advantage of the trans-effect order Cl NH3. This example is also of practical interest because the cis isomer is an important antitumor drug, but the trans isomer is ineffective. In each case the first step of the substitution can give only one isomer. In Eq. 1.3, the cis isomer is formed in the second step because the Cl trans to Cl is more labile than the Cl tranS to the lower transeffect ligand, ammonia. On the other hand, in Eq. 1.4, the first Cl to substitute labilizes the ammonia trans to itself to give the trans dichloride as final product. 

A trans effect series for a typical Pt(U) system is given below. The order can change somewhat for different metals and oxidation states. 

In 1926, Chernyaev introduced the concept of the trans effect in platinum chemistry. In reactions of square-planar Pt(II) compounds, ligands trans to chloride are more easily replaced than those trans to ligands such as ammonia chloride is said to have a stronger trans effect than ammonia. When coupled with the fact that chloride itself is more easily replaced than ammonia, this trans effect allows the formation of isomeric 

FIGURE 12-12 Stereochemistry and the tmns Effect in Pt(II) Reactions. Charges have been omitted for clarity. In (a) through (f), the first substitution can be at any position, with the second controlled by the traits effect. In (g) and (h), both substitutions are controlled by the lability of chloride. 

Predict the products of these reactions (there may be more than one product when there are conflicting preferences). 

This episode provides general conclusions of importance some of our current ideas are likely to be wrong—we just do not know which ones. The literature must thus be read critically with an eye for possible flaws in the results, inferences, or arguments. Nugent has reviewed a series of ideas, once generally held, that subsequently fell from grace. Another lesson from Werner is that we must take objections seriously and devise critical experiments that distinguish between possible theories, not merely ones that confirm our own ideas. 

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

PtLaX + Y PtLjY -h X the rate law is generally found to be of the form 

The term is independent of Y and would, therefore, appear to be dissociative, but it is in fact found to be solvent-dependent and so it is thought to be associative. (It is also found to be sensitive to steric effects in the same manner as the ki pathway.) A plausible pathway for the kx route is slow solvolysis followed by fast substitution 

Retention of configuration occurs in these substitution reactions, as expected for a process involving a 5-coordinate intermediate in which the entering and leaving ligands are simultaneously bound. 

Kinetic study [141] of complexes of the type trani-Pt(PEt3)2XCl was of great value in establishing the strong tran -effect of hydride (Table 3.13) examination of the data for a wide range of reactions gives rise to a series 

In reaction (a), after the first ammonia is replaced, the second replacement is trans to the first Cl. In reaction (b), the second replacement is trans to d (replacement of ammonia by chloride is also possible, resulting in formation of the reactant [PtClJ ). The first steps in reactions (c) through (f) are the possible replacements, with nearly equal probabilities for replacement of ammonia or pyridine. The second steps of (c) through (f) depend on the trans effect of Cl . Both steps of (g) and (h) depend on the greater lability of chloride. By using reactions such as these, specific isomers can be prepared. Chemyaev prepared a wide variety of compounds and established the order of trans-effect ligands  

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Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

The preparation of the isomeric forms of Pt(NH3)2Cl2 is discussed in terms of the trans-effect in section 3.8.9 [67]. 

In the case of the trans-complex, only the two chloride ions are substituted, the trans-effect of ammonia being too low to give substitution with the result that white needle crystals of trans-[Pt(NH3)2(tu)2]Cl2 are formed [73],... 

The trans-effect is, therefore, a kinetic labilizing effect rather than a thermodynamic one. An approximate series is ... 

Theoretical explanation of the trans-effect (and /rans-influence) has centred on two theories, one based on cr-bonding the other on 7r-bonding. The tr-bonding argument considers two frans-ligands sharing a metal p orbital (Figure 3.83). 

Explanations of the trans-effect and trans-influence have considered a- and rr-bonding, often to the point of mutual exclusion. 

A 7r-bonding explanation notes that several ligands high in the trans-effect series are good -acceptors and thus siphon off 7r-density, making the region trans to it electron deficient and thus attractive to ligands that are electron rich. 

The classic application of the trans-effect lies in the synthesis of the cis- and trans-isomers of Pt(NH3)2Cl2, known as Peyrone s salt and Reiset s salt after their respective discoverers in 1844. 

The cis- and trans-isomers of [Pt(NH3)(N02)Cl2]- have been synthesized from PtCl - merely by choice of the order of ligand substitution (Figure 3.87). (In the second step, chloride trans to chloride is more labile.) The second substitution is dictated by N02 having a higher position in the trans-effect series than chloride [144],... 

Application of the trans-effect to synthesis ofplatinum( IV) complexes... 

Like the isoelectronic Pd2+ and Pt2+, Au3+ exhibits both trans-effects and trans-influence. Table 4.13 (above) lists structural data for a number of complexes AuL3L, showing how the disparity in Au-X distances between cis-and trans-X depends on the position of L in the trans-effect series for the compounds listed, the effect is least noticeable in AuC13NH3 as these two ligands are proximate in the series. 

The trans-effect can be used synthetically. In the reaction of Br- with Au(NH3)4+, the introduction of the first bromine weakens the Au—N bond trans to it so that the introduction of a second bromine is both sterospecifically trans and rapid. (A similar effect occurs in the corresponding chloride.) The third and fourth ammonia molecules are replaced with difficulty, permitting the isolation of AuBr2(NH3)2 (second-order rate constants at 25°C are k] = 3.40, k2 = 6.5, k2 = 9.3 x 10-5 and k4 — 2.68 x 10 2lmor s l at 25°C) [141]. 

Factors responsible for this order include the trans-effect, charge neutralization, and statistical effects. 

As the trans effect theory indicates, there should be some relationship between lability of a ligand and its role as a labilizing group in another position in a complex. In an octahedral complex reacting via a dissociative mode of activation, the transition state has five strongly bound ligands. This state will be stabilized... 

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