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 en,LX+ complexes have been measured, where Land Xare 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 Xis chloride or bromide the trans-effect of L increases along the series CI, 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-OH, bond-making participates in the transition state. It is concluded that in this case the energetics of the S , 1 and S, 2 reaction paths might be so similar as to make meaningless any precise assignment of mechanism. The high rrans-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.KINETIC studies of substitution reactions of octahedral complexes have been concentrated largely on cobalt (III), chromium(m), and, to a lesser extent, platinum(1v) systems.1 The reactions of cobalt (111) complexes have been studied mainly with the aim of determining the molecularity of the reactions, and the stereochemical rearrangements which occur have been a great help in this respect.2 Platinum(1v) complexes have been I (a) F.

Trans effect in water-exchange reactions of chromium(III) complexes

“…F=l/s(4FdM+Fiim) (5) where Vs is the normalizing factor. Substituting 4c for b and eq 3 and 4 in eq 5 gives upon rearrangement 1 -F= 0.8 exp[-/rcis((a + 4c)ja)t] + 0.2 expHtrans((a + c)/a)t]…”

The trans-Effect in Octahedral Complexes. VI. Base Hydrolysis of trans-Hydroxybromo and trans-Hydroxychlorobis(ethylenediamine)-rhodium(III) Complexes

“…The assignment of a transstructure to these complexes is made on the grounds (a) that the kinetic behavior (see below) shows that the solutions are "kinetically" pure, i.e. they contain only one species since it is highly unlikely that cis and trans complexes would react at the same rates, (b) the product of base hydrolysis is spectroscopically pure trans-[Rh(en)2(OH),]+ (2), and (c) that complete trans + cis rearrangement during the reaction of trans-[Rh(en),X,]+ with silver ions, and complete cis + trans rearrangement on anation of the aquohalogeno complexes and on base hydrolysis of the hydroxyhalogeno complexes is not in accord with the known stereochemical retention that occurs in all reactions of trans-dihalogeno complexes studied so far (5,(7)(8)(9). The only cases where substantial trans + cis rearrangements have been found in such diacido complexes are in base hydrolysis of the transdihalogeno complexes (10).…”

“…However, it has been shown (3) that an intrinsic kinetic trans-effect (i.k.t.e.) can be defined by comparing the activation enthalpy for a particular reaction with that found (AH,,,') for a hypothetical reaction against AH0 for reactions of the trans-iodo complexes, these being a useful reference since iodide has the strongest trans-effect of the ligands so far studied, and the data obtained for these complexes are among the most precise found (9,11 since the values of AH, * obtained (Table 5) can be taken to be the activation enthalpies for the showed, by comparison with other reactions aquation reactions of the trans-hydroxybromo involving interchange of iodide and OH, and trans-hydroxychloro complexes, the hyligands but with different groups in the position droxide being necessary only to drive the re- For personal use only.…”