Substitution in cis complexes

Among the compounds that retain their optical activity and geometry on hydrolysis are 

An optically active cis complex can yield products that retain the same configuration, convert to trans geometry, or create a racemic mixture. Statistically, the product of substitution of a di-[M(LL)2BX] complex through a trigonal-bipyramidal intermediate should be one fifth trans if both intermediates were equally likely and one third trans if the axial B form is not formed at all. Experimentally, aquation of di-[M(LL)2BX] in acid results in 

Source Data from M. L. Tobe, in J. H. Ridd, ed., Studies in Structure and Reactivity, Methuen, London, 1966, and M. N. Hughes, J. Chem. Soc., A, 1967,1284. 

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The magnitude of J(Pt—P) for some phosphine-substituted SiPt complexes depends on the covalency of the Pt—P bond (136). Such a bond is relatively ionic in a system where it lies trans to a ligand of high trans influence and J(Pt—P) is then small. The coupling in cis complexes decreased in the order Cl > C > Si and since a comparable situation has been observed for methyl-Pt compounds it is suggested that the effect is one of a- rather than tr-interaction. 

Phosphine-substituted hydrido-silyl complexes [FeH(SiR)3(CO)sL] have been prepared by carbonyl substitution in cis-[FeH(SiR3)(CO)4] thus for example, reaction of 1 equivalent of dppm with cis-[FeH(Si(0He)3 (CO)4] leads to mer-[FeH(Si(OHe)3)(CO)3(dppm-P)] and the reaction of cis-[FeH(SiPh3)(CO)4] with Ph2P(CH2)4PPh2 (dppb) leads to mer-[FeH(SiPh3)(CO)3(dppb-P)]. ... 

In the absence of any cis-effect the rates of ligand substitution in these complexes would be approximately equal because substitution occurs trans to chloride in both cases. Further, as the ions have different charges, identical rates would not be anticipated but, once this difference is accounted for, the cis-effect becomes evident. 

Two examples of steric effects deserve attention. In aryl complexes cis-Pt(PR3)2ArCl, introducing ort/io-substituents into the phenyl group slows down substitution considerably, as these block the position of attack (Figure 3.82). 

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-... 

Complex 4a (see Fig. 1) differs from these catalytically active complexes only in the substitution of the complexed olefin molecules and hydrogen atom by a 7r-allyl group. The ligands in these square-planar molecules can adopt two different arrangements around the central nickel atom The olefin can either be trans (31a) or cis (31b) to the phosphine molecule. Because precedent exists for both these arrangements [e.g., 12 (84) and 30 (82)]. it is difficult to decide which of the two structures (31a or 31b) represents the catalytically active species. It is of course possible that the differences observed in the catalytic properties of systems having different ligands L and Y (Section IV) is due (at least in part) to differences in the population of 31a and 31b. 

The Fe(III) complexes of R-substituted salicylaldehyde thiosemicarbazone (R-thsa2- Fig. 6) are among the most studied spin crossover materials of this family. The crystal structures of several of them have been determined at various temperatures. The iron-donor atom distances are compiled in Table 2. The Fe(III) ion is in a distorted FeS2N202 octahedron formed by two thiosemicarbazone ligands, which are geometrically arranged in such a way that the S and O atoms are located in cis positions, whereas the N atoms occupy trans positions, i.e. each tridentate molecule coordinates in an equatorial plane [101]. 

Computational results were also obtained, in a different study for the possibility that the chloride rather than the ammonia in cis-(NH3)2PtCl2 was substituted by methane (148). In the same contribution, an analogous study examined the reactivity of the (bipyrimidine)PtCl2 complex (Fig. 4). 

For example, complex 37 with an imidazolin-2-ylidene and a methyl ligand in cis-position to each other decomposes to yield the 1,2,3-trimethylimidazolium salt 38, Pd°, and cod (Fig. 13) [124], Additional examples for the reductive elimination of 2-alkyl and 2-aryl substituted azohum salts from palladium or nickel NHC complexes have been reported [125, 126]. Today, reductive elimination reactions have been established as one important reaction pathway for the deactivation of catalytically active metal NHC complexes [126, 127]. 

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