The trans-effect in octahedral complexes. Part I. Kinetic studies of some trans-dihalogenobisethylenediaminerhodium(III) complexes

Abstract

The rates of replacement of X^– from some trans-Rh en₂LX⁺ complexes have been measured, where L^– and X^– are chloride, bromide, or iodide. The rates are independent of the nature or concentration of the incoming nucleophile. When L^– is chloride or bromide and X^– is iodide the reactions proceed by a catalytic mechanism but in all other cases non-catalytic reactions occur unless certain catalytic reducing agents are added. When X^– is chloride or bromide the trans-effect of L increases along the series Cl, Br, I whether it is measured in terms of rate constants or enthalpies of activation. The activation enthalpies decrease by about 2 kcal./mole when L changes from chloride to bromide, or from bromide to iodide. The mechanism of the reactions is discussed in terms of the relative softness, or class (b) character, of the reactant complexes and the transition states. The results are consistent with a wide variation in the extent to which Rh–OH₂ bond-making participates in the transition state. It is concluded that in this case the energetics of the S_N1 and S_N2 reaction paths might be so similar as to make meaningless any precise assignment of mechanism. The high trans-effect of iodide is ascribed to the ease with which it can compensate for either an increase or a decrease in electron-density at the rhodium.