Trans and Cis Effects

The examples in Tables 5.7 and 5.8 illustrate the magnitude of the trans effect and cis effect on the ligand substitution reaction of typical platinum(II) coordination and 

Associative versus Dissociative Substitutions of Square-Pianar Compiexes 

In addition, dissociative mechanisms for substitution of d complexes can occur when the geometry is distorted from square planar or if the coordination number is higher than four. For example, replacement of the monophosphine in the highly distorted [cis-PtfSiMePhjljfPMejPhlj] with a bisphosphine has been shown recently to occur by dissociative substitution. The dissociation is favored in this case because of the strong trans 

The typically faster rate for associative substitution does not preclude dissociation of ligand occurring as the first step of reactions other than ligand substitutions (Equation 5.10). For example, dissociation of ligands from square-planar, d complexes occurs as the first step of reductive eliminations (Chapter 8). Thus, dissociation of ligands from this class of compound can occur, but association to form a five-coordinate intermediate is typically faster. 

To illustrate the propensity of these complexes to react by associative processes, consider the exchange of alkanes and arenes in complexes of the t5rpe [PtL2(R)(R-H)] by associative or dissociative mechanisms. Although it is difficult to conceive of a more weakly bound ligand than the alkane in a a-complex, the rate of associative substitution of acetonitrile for the bound alkanes is faster than simple dissociation of the alkane (Equation 5.16).  

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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). 

Based on your knowledge of the trans and cis effects (Chapter 7), why Exercise 9-9... 

Deviations in ligand redistribution patterns from the statistical prediction may be due not only to the trans- and cis-effects (see Sections 1.4 and 1.5) but also to so-called stacking-effects. This term refers to the higher stabilities of mixed-ligand complexes, as compared to those of homoleptic systems, and results from unusual ligand-ligand interactions found in certain mixed-ligand complexes.12,13... 

We introduce the dimensionless bending coordinates qr = t/XrPr anti qc = tAcPc ith Xt = (kT -r) = PrOir, Xc = sJ kcPc) = Pc nc. where cor and fOc are the harmonic frequencies for pure trans- and cis-bending vibrations, respectively. After integrating over 0, we obtain the effective Hamiltonian H = Ho + H, which is employed in the perturbative handling of the R-T effect and the spin-orbit coupling. Its zeroth-order pait is of the foim... 

Cis, trans and metal effects in transition metal porphyrins. J. W. Buchler, W. Kokisch and P. D. Smith, Struct. Bonding (Berlin), 1978, 34, 80-134 (158). 

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

Buchler JW, Kokisch W, Smith PD (1978) Cis, Trans, and Metal Effects in Transition Metal Porphyrins. 34 79-134... 

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

Contrary to the carotenoid behavior during orange juice pasteurization, losses of 46%-54% in the all-trans-a- and all-trans-fi-carotene contents and the formation of m-isomers were also verified for the pasteurization of carrot juice at 110°C and at 120°C, both for 30 s (Chen et al. 1995). In addition, all cis- isomer levels increased, with 13-c -P-carotene and 15-d.v-a-carotene formed in the largest amount. Heating at 121°C for 30min caused further losses of 61% in al I-tran.v-a-carotene and 55% in all-trans-P-carotene (Chen et al. 1995). However, minor effects on the amounts of trans- and cis- isomers of a- and P-carotenes were observed after the acidification and the heating of carrot juice at 105°C for 25 s (Chen et al. 1995). 

We now focus on the effect of replacing fluorine by another halogen of a higher period, e.g. Br. The trans and cis conformations of 1,2-dibromo-cyclohexane are shown below along with the dominant orbital interactions ... 

Cis, Trans, and Metal Effects in Transition Metal Porphyrins... 

Table 6. Static cis-, trans-, and metal effects taken from M=0- and M-Z-stretching frequencies of fra s-dioxoosmium(VI) porphyrins, 0s02(P) [25] and oxometal(V) porphyrins, MO(P)Z [26] (Perkin-Elmer 625, KBr). (OEP) -> [12], (TTP) -> [20] (Table 2)...

The osmium porphyrins have thus furnished the most coherent series of compounds for investigating cis, trans and metal effects of the origins illustrated in Figures 1 and 8. Additional material from other sources will be presented in the following sections. 

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