Ethylene trans effect

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

Results from preparative studies support both S-Me2SO and ethylene having high trans effects. Thus, the products of both reactions (6) and... 

The characterization and calorimetric results for a series of displacement reactions (equations 231-233) show little difference between CeHg and CH2C12 solvent.644 The relative displacement energies are P(OPh)3 ethylene > cyclooctene > cis-butene > styrene > cyclopentene > nitrostyrene > cyclohexene. These data are used to give a thermodynamic description of the trans effect. 

However, the study failed to reveal bis(ethylene)platinum chloride in the mixture. A similar tratir-chlorine substitution should be easier with the strong trans effect of an olefin ligand. Chart and Wilkins have prepared this compound (84) and found that it rapidly releases ethylene at room temperature ... 

They reasoned that the [H ] and [Cl"] terms in the denominator arise from Reactions 18 and 19 and the [H ] and [Cl"] terms in the numerator from Reaction 20. These steps are proposed to arrive at cfs-PdCl2(C2H4)(OH)" since it was felt that this stereochemistry was required for the further reaction of the hydroxo 7r-complex. Because of the trans effect of the ethylene, the original complex would be expected to be trans. 

This relevant finding is attributed to the greater nucleophilic discrimination factor of DMSO in its complex, whereas the trans effect is attributed to transition-state stabilisation arising from the x-acceptor properties of the ligand, S-bound DMSO being considered a better o-donor and x-acceptor than ethylene (22). 

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). 

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. 

Reactions in [PtCl4] -ethylene-[SnCl3] systems are dominated by the large 7r-acceptor and trans effect properties of the [SnClg]" ligand. This is also true for analogous systems containing other alkenes. A preliminary... 

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