Olefin complexes trans-effects

The dependence of the relative reaction rates on olefin geometry can be discussed with reference to Equation 14. As pointed out by Murray and co-workers (25), the complex formation occurs with cis-olefins rather than with their trans isomers for steric reasons hence, Kc (cis) > Kc (trans). However, during the formation of the primary ozonide by either path the olefinic carbon atoms change in hybridization, from sp2 to spz. The bond angles thus decrease from 120° to 109° in the cis isomers, this results in a compression of the substituents van der Waals radii. The repulsion between the substituents is increased, and so is the activation energy. Consequently, ki (trans) > ki (cis) and k2 (trans) > k2 (cis). In the final analysis, the geometry of the olefin has opposite effects (a) onKc and (b) on ki and k2. Present results seem to indicate that for large substituents the effect on Kc predominates since k (cis) > k (trans). 

The latter is outside the scope of organometallic chemistry, but within the first two topics the work involved three main themes olefin and acetylene complexes, alkyl and aryl complexes, and hydride complexes. As continuous subsidiary themes throughout ran the complex chemistry of tertiary phosphines and such ligands, the nature of the trans effect, and the nature of the coordinate bond. All the work from 1947 to 1969 was carried out in the Butterwick Research Laboratories, later renamed Akers Research Laboratories, of Imperial Chemical Industries Ltd., and I am indebted to that Company and particularly to Mr. R. M. Winter, the Company s Controller of Research, and Sir Wallace Akers, its Director of Research, who in 1947, made available to me the opportunity to develop my research in my own way, in those laboratories. 

Stereoselective bromiuation of olefins. Exclusive trans-addition of bromine to alkenes can be effected with this bromine-crown ether complex. Thus cis- and trans-jB-methylstyrene react with DBC-Brj to yield threo- and erylftro-l-phenyl-1,2-dibromopropane, respectively. Other brominating agents (Brj, PyBrj) are significantly less selective for this reaction. ... 

Complex 7 now possesses all the prerequisites for cw-ligand insertion reaction a close (cis) position to the interacting ligands, and activation of the r-bonded olefin by weakening the olefin-metal T-bond and the olefin rotation barrier through the trans effect of the tran -chloro ligand across the complex. 

The Pt—Sn complexes catalyze the hydrogenation of ethylene and some other olefinic compounds this action is doubtless connected with the ready dissociation of the complexes in solution, promoted by the trans-effect of SnClJ, which leaves vacant sites for coordination of olefin and of hydrogen (cf. also page 771). 

In these considerations, one fact has not been taken into account. In Henry s mechanism, the water molecule replaces the chloro ligand in c/s-position to the olefin which is a prerequisite for the oxypalladation. However, for the analog platinum complex, it could be shown that due to the trans-effect, water replaces at first the chloro ligand in the tra/is-position [17], and so the corresponding Pd complex Eq. (9.10) would be formed. 

After coordination of the ethylene to the tetrachloropalladate A, the strong trans effect of the ethylene ligand in complex B facilitates ligand substition to give the aquo complex C. The function of this neutral aquo complex is possibly that it exhibits less n backbonding from the metal to the olefin than the anionic complex, and the olefin therefore more readily undergoes nucleophilic attack in the former. 

In the case of complexes also containing ligands other than Cl, the geometry of the resulting olefin compound is dictated by the trans effect. 

Trans effects. In these square-planar olefin complexes, ligands which are trans to the olefin readily undergo displacement reactions [91,92,93]. The trans effect of ethylene is apparent in the greater stability of the white cir-bis-ethylene complex cu-(C2H4)2PtCl2 compared with the very unstable yellow trans isomer [48,14]. The Ironr-isomer readily isomerizes to the cir-isomer in ether solution [94]. 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

---