Explanations of the trans Effect

FIGURE 12.15 Sigma-Bonding Effect. A strong cr bond between PtandT weakens the Pt—X bond. 

The order given here does not exactly match that for the trans effect, particularly for CO and CN, which have stronger trans effects than indicated. 

The overall trans effect list is the result of the combination of the two effects  

It is possible to prepare isomers of Pt(II) complexes with four different ligands. Predict the products expected if I mole of [PtCy is reacted successively with the following reagents (e.g., the product of reaction a is used in reaction b)  

FIGURE 12-13 Activation Energy and the trans Effect. The depth of the energy curve for the intermediate and the relative heights of the two maxima will vary with the specific reaction, (a) Poor trans effect, low ground state, high transition state. 

The Pt—X bond is influenced by the Pt—T bond, because both use the Pt pj and orbitals. When the Pt—To bond is strong, it uses a larger part of these 

Atwood, Inorganic and Organometallic Reactions Mechanisms, p. 54 Basolo and Pearson, Mechanisms of Inorganic Reactions, p. 355. 

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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. 

Two factors dominate the explanations of the trans effect, weakening of the Pt—X bond and stabilization of the presumed 5-coordinate transition state. The energy relationships are given in Figure 12-13, with the activation energy the difference between the reactant ground state and the first transition state. 

Trialkyl phosphines are rather good trans directors and, as expected, the reaction of BujP with [PtCl4]2- gives the trans isomer as a major product. However, when one uses PhjP in this reaction, only the insoluble cis product is obtained. Offer an explanation for this apparent violation of the trans-effect prediction. [Problem 13.5 may be helpfltl.)... 

A different explanation for the trans effect is based on the ability of a ligand to form multiple bonds to the metal ion. In Chapter 19, multiple bonding of such ligands as CO and CN- to metal ions by back donation of electron density from the d orbitals of the metal to vacant n orbitals on the ligands was described. The removal of electron density from the... 

Earlier explanations of the trans-stereoselectivity of nucleophilic additions to ethylenes cited frontier MO symmetry (Fukui, K. Tetrahedron Letters 1965, 2427), steric effects, or electrostatic factors (Benson, S. W., Haugen, G. R. J. Am. Chem. Soc. 87, 4026 (1965). 

This rule explains a host of experimental observations. For example, (CH3)2Hg is readily cleaved by dilute acid to form linear CH3Hg(H2O)+. But this cation resists cleavage, even by strong mineral acids. There is a simple explanation based on the theory of the trans effect. This effect occurs only for soft metal ions, and ligands of high trans effect are always soft ligands. 

The historical background and development of the trans effect and the trans influence have been outlined in an account of the evolution of Chernyaev s Rule. Photochemistry. Irradiation of c/5-[PtCl2(l-naphthylamine)2] results in loss of naph-thylamine. In the first excited state the electron density at the amine nitrogen is much less than in the ground state, and hence the bond to platinum will be much weaker. This explanation is supported by the observation that photosubstitution takes place on irradiation at 280 or 313 nm, in the absorption band of the coordinated naphthylamine, but not on irradiation at 453 nm, which is beyond the absorption band of this amine. Quantum yields at 280 and 313 nm are reported a qualitative idea of the kinetics can be gleaned from the published time variation of spectra. ... 

In 1926, Chugaev introduced the concept of the trans effect in order to correlate many of the reactions of Pt (II) complexes.Since then, numerous explanations have been presented to explain the effect in elec-... 

The addition of halogens and halogen acids to alkenes has been shown to be predominantly trans and where the results do not agree, explanations have been given in terms of steric factors. Dewar has proposed that in all electrophilic addition reactions where a classical carbocation is formed, cis addition is the rule and where there is the preponderance of the trans product, the effect is due to steric factors. 

The reactions presented in scheme (10) also account for effects exerted by the addition of Lewis bases or acids (as well as other electron donors and acceptors) to the polymerisation system on the microstructure of the polymers formed. As shown in Tables 5.4 and 5.5, some catalysts that are highly stereospecific for the formation of cis- 1,4-polybutadiene yield trans- 1,4-poly butadiene (or eb-m-1,At trans-1,4-polybutadiene) after the addition of a Lewis base or other electron donor to the catalyst system. A plausible explanation of the observed phenomena is that the added component occupies a coordination site at the transition metal, thus forcing the incoming monomer molecule to coordinate as an s-trans-rf ligand. When the additional catalyst component has a basicity comparable with that of the monomer, a competitive monomer/ Lewis base (electron donor) coordination takes place, as shown below [7] ... 

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