Trans- *, ligand displacement reaction

The title complex (as the chloride) was first reported by Kyuno and Bailar,1 who obtained cis-a and m-(i isomers by the ligand displacement reaction of cis-a-dichloro(N,N -bis(2-aminoethyl)-l,2-ethanediamine)cobalt(lll) chloride in gaseous and liquid ammonia. However, its trans isomers were not prepared until 1980.2... 

Such protonation without ligand displacement has been found in the complex trans-[Mo(NCPrw)(N2)(dppe)2], showing that under the right conditions of ligation the terminal nitrogen atom is sufficiently basic for protic attack (Reaction 4) (17). 

The first ligand substitution reaction at a Sn(II) center involved displacement of a Cp ligand with an imino reagent (a)k The product can be isolated in reasonable yield (42%) as a crystalline solid. The crystal structure of the complex demonstrates that the Sn2N2 four-membered ring is planar and that the Cp rings are trans. 

The reaction of lithium diphenylphosphide with a bis-ben2ylic halide has been employed in the synthesis of the diphosphine (14), which is of interest as a trans-spanning ligand. Displacement of halide ion from a vinylic carbon atom occurs in the reaction of cis- and fm/zi -iS-ctilorovinyldiphenylarsines with lithium diphenylphosphide, which proceeds stereospecifically with the formation of the corresponding cis- and rra 5-phosphine-arsines (15). Surprisingly, the reaction of lithium diphenylphosphide with a thirty-fold excess of cw-l,2-dichloroethene yields only the cis-diphosphine (16). ... 

Their preparation always involves an initial oxidative addition of a chiral hydrosilane followed by ligand displacement. Complexes 132, 162, 163 were obtained according to equations [73] -[75] and have square planar geometry around the platinum atom. All these reactions were proposed to proceed with retention at the silicon atom. The X-ray structure determination of 132 (eq. [73]) confirmed the trans arrangement around platinum and the assigned retention at the silicon atom (210). 

Palladium(II) and platinum(II) form a wealth of square planar complexes a tendency for Pt—Pt interactions (i.e. for the heaviest group 10 metal) is quite often observed. The mechanisms of substitution reactions in Pt(II) complexes and the trans-effect have been much studied and we return to this in Section 25.3. However, for the discussion that follows, it is important to note that mutually trans ligands exert an effect on one another, and this dictates the order in which ligands are displaced and, therefore, the products of substitution reactions. A word of caution do not confuse trans-eSect with trani-influence (see Box 22.9). 

It was observed long ago that cis-trans isomerization in planar complexes is catalyzed by traces of free ligands. Since a single displacement reaction is, as mentioned above, stereospecific and conserves stereochemistry, this is best explained in terms of the two stage mechanism shown in Fig. 21-13. 

By far the most well studied ligand-substitution reaction trans to the organic ligand in organocobalamins is the simple displacement of the pendant axial 5,6-di-methylbenzimidazole in acid, the so-called base-on - base-off transition of organocobalamins (Eqn. 63). 

Kinetic studies of H2 dissociation and substitution rates show that the potential energy surfaces for these reactions vary dramatically even with minor changes in ancillary ligands or for isomers (Table 7.7).5,69 Electronic effects, especially the influence of the trans ligand, appear to be more important than steric factors. For the Ir system, the ds isomer with H2 trans to Cl has a strongly bound H2 (dHH = 1.11 A) while the trans isomer with H2 trans to H contains a weakly bound H2 that dissociates nearly 10s times faster (see also Section 4.7.1). One of the few comprehensive quantitative studies of H2 substitution reactions shows displacement of H2 by L (MeCN, PhCN, ) fromCMHCHjXP) (M = Fe, Ru, Os) is first-order in concentration of complex and zero order in L, Le., a dissodative mechanism.70... 

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