2250 J.C.S. DaltonThe trans-Effect in Octahedral Complexes. Part V.l Intrinsic Kinetictrans-Effect of Hydroxide Ions in Octahedral Rhodium(rii) ComplexesBy A. J. Poe and C. P. J. Vuik, Department of Chemistry and Erindale College, University of Toronto, Toronto,Data are presented for the kinetics of the reaction of trans-Rh(en),(OH)I+ (en = ethylenediamine) with hydroxideions. The observed pseudo-first-order rate constants fit the expression koba = k, + k,[OH-1, and activation para-meters corresponding to k, and k, have been obtained. The value of AH:, gives a different measure of the kinetictrans-effect of the hydroxide ion, relative to those of ammonia and iodide, from that based on anation reactions,but this ambiguity can be resolved by appropriate allowance for the corresponding values of AHo for the reactions.When this allowance i s made.a measure of the intrinsic kinetic trans-effect of hydroxide ion in such rhodium(ll1)complexes is obtained and this is found to be very close to that of chloride. This intrinsic kinetic trans-effect isobtainable even though data for the corresponding reactions of the trans-chloroiodo- and - bromoiodo-complexes,ex., are not available.The reaction path corresponding to k, i s the first [OH-] -dependent path observed for such trans-complexes butit is not very important. even in this case, because the value of AS, i s relatively unfavourable compared with thoseusually found for base hydrolysis of amine complexes of cobalt(ii1) and rhodium(ll1).CanadaTHE reaction of iodide ions with tram-Rh(en),(OH)-OH?- has recently been studied.l Comparison of therate parameters with those for reaction of iodide ionswith Rh(NH3),0H,3+ (ref.3) and trans-Rh(en),I(ref. 4) provides a quantitative measure of the kinetictrnns-effect of hydroxide ion compared with that ofiodide and ammonia in these octahedral rhodium( 111)complexes, the different cis-effects of four ammonia andtwo ethylenediamine ligands having been shown to be~rnall.l,~ On the basis of both relative rate constantsPart IV, A. J. Poe and I<. Shaw, J. Chem. SOC. ( A ) , 1870,* Presented a t the Third Northeast Regional Meeting of the393.American Chemical Society, Buffalo, New York, October 1971.and enthalpies of activation the tvmts-eft'ect sequenceis I- > OH- > NH,.It has been shown, however,that kinetic parameters alone do not always providean unambiguous measure of kinetic trans-effects, butthat allowance has to be made for different thermo-dynamic parameters for the various reactions i n ~ o l v e d . ~ ~ ~Plots of AH$ against corresponding values of AHo fora wide variety of reactions of rhodium(II1) complexesenabled an unambiguous quantitative measure of the-\. J . Foe, I\. Shaw, and XI. J. Wendt, I I Z O Y ~ . Chini. =Ida.H. L. Bott and A. J. PoE, J . Chew. SOC. ( A ) , 1967, 206.RI. V. Twigg and A. J. Poe, Canad. J . Chem., 1972. 50, 1089.1967, 1, 371.13 *A. J. Po& and K. Shaw, Chem. Comm., 1967, 521972 2251trans-effects of ammonia, chloride, and bromide to beobtained with respect to that of iodide ion.Thesetrans-effects were named intrinsic kinetic trans-effectsbecause they were quite independent of the variousthermodynamic contributions to the rates of theindividual reactions. In order to place hydroxideion in this series, we have studied the reaction: tvans-Rh (en),(OH) I + __t trans-Rh(en),(OH),".OH-EXPERIMENTAL AND RESULTStrans-[Rh(en) ,ICl]ClO, Was prepared by a publishedmethod 7 (Found: C, 9.75; H, 3-5. C4H1,Cl2IN4O4Rhrequires C, 9.90; H, 3.3%). The electronic spectrum inaqueous 0-1M-sodium chloride solution showed peaks at490, 440, 301, and 241 nm with extinction coefficients of260, 160, 4300, and 38,000 1 niol-l cm-l respectively, thefree chloride being necessary to repress loss of chlorideions from the complex by hydrolysis.Reaction of thecomplex with iodide ions gave a solution of the trans-di-iodo-product which had an extinction coefficient a t466 nni within about lo/, of that for a separately pre-pared and analysed sample of the di-iodo-complex in asimilar iodide solution.Solutions of sodium hydroxide were prepared by dilutionof 2~-sodium hydroxide (Fischer Certified), and an-hydrous sodium perchlorate was prepared by dehydrationof NaClO,,H,O (B.D.H.) a t 210 "C.Kinetics.-Reaction of trans-Rh(en),IC17 with hydr-oxide ions was followed by observing the accompanyingchanges in the electronic spectra. The absorption maximacharacteristic of the iodochloro-complex decreased in in-tensity while new peaks a t 274, 402, and 446 nm wereobserved to grow to maximum intensities, and subsequentlyto decrease a t a substantially slower rate.This secondreaction was accompanied by the appearance of stablepeaks a t 296 and 348 nni. The first reaction was character-ised by isosbestic points a t 266, 289, 371, and 440 nm butthese were eventually replaced bv others, characteristicof the second reaction, at 341 and 356 nm. The maximumintensities of the peaks due to the intermediate productgave approximate values for the extinction coefficients of4000, 160, and 150 1 1iiol-l cm-l a t 274, 402, and 446 nmrcspectivel5-.The rate oi the initial reaction, with 0. lOsr-sodiunihydroxide a t 50.2 O C , was followed by observing theincrease in absorbance a t 277 nm and thc decrease a t310 nm, the infinite-time absorbances being taken, re-spectively, as the maximum and minimum absorbanceswere reached.These values are, respectively, too lowand too high because of the subsequent reaction, and thecorresponding observed rate constants of 9.9 x lo-* and7.6 x s-l are therefore, respectively, too high and toolow. The mean value of 8.8 x lo-" s-l is in excellentagreement with values for other reactions of trans-Rh(en),-IClf that involve replacement of the chloride ligand andformat ion of tram-iodo-prod uc ts.The spectrum of the initial product described above issimilar to that of basic solutions of truns-Rh(en),1(OH2)~+(prepared previ~usly,~ by reaction of tvans-Rh(en) ,I2+with silver nitrate, preparatory to a study of its anationreactions), but the peaks of the latter were neither as clearnor as intense.It seems probable that the earlier pre-paration led to mixtures of t~um-Rh(en),(OH~)~~+ andtran~-Rh(en),I(OH,)~' (this would not have affected thekinetic studies of the latter complex which reacts verymucli more rapidly than the diaquo-impurity l). Reactionof these solutions with iodide gave solutions with spectracharacteristic of the trans-di-iodo-complex. The presentkinetic and spectroscopic results suggest very stronglythat the initial product of the reaction of the traws-iodo-chloro-complex with hydroxide ions is the tral-ts-iodo-hydroxy-complex .The absorption peaks of the final product at 296 and 348nm are in good agreement with those a t 298 and 348 nmshown by solutions, at pH 13, of trans-Rh(en),(OH)OH,2prepared by the unpublished method of Basolo and Kla-bunde,l although the corresponding extinction coefficients(101 and 133 1 mol-l cm-l) are somewhat higher than thosefrom the latter preparation (88 and 117 1 mol cm-l), andthan those (88 and 128 1 mol-l cm") inferred from spectra,given by Klabunde,S of the products of the reaction oftvans-Rh(en),(OH)Cl' with hydroxide ions.The finalreaction can, however, be concluded to involve loss ofiodide from the trans-iodohydroxy-coniplex.The kinetics of this reaction were followed by measuringthe decreasing absorption a t 272 nm, absorbance changesof cn. 1 unit being easily obtainable in 1 cm cells. Groupsof four runs were followed simultaneously in situ in cellscontained in the thermostatted cell holder of a Perkin-Elmer 402 spectrophotometer. Ionic strength was main-tained constant with sodium perchlorate.Temperaturesof solutions in the cells were measured directly with aprecision thermometer, emergent stem corrections beingTABLE 1Observed pseudo-first-order rate constants for the reaction :OH -tvai.zs-Rh(en),(OH)I' __t trans-Rli(en),(OH),'. 1 =[OII-]0 1 )0.010.0 10.0 10.010.0 10.100.100.200.300.400.400.500.600.700-800.901.001.001 . 0 ~ ; [Complex] = 2.8 x 10-41r105kobal~-155.4 60.93-63 5-802-643.62 5.332.71 5.973-80 6-233.00 6.063.07 6.563.07 7.053.31 7.043.34 7.193-52 7.673.37 7-513.82 8.3266-6 70.3 75.7 81.0 "C'10.6 19.0 34.1 61.310.6 34.0 61.734.1 61.262.065.810.0 18.9 36.2 62.611.3 19.0 38.0 69.811.7 20.6 39.3 72.711.8 21.0 38.9 77.712.9 22.2 42.5 7G.213.1 23.4 44.4 78.513.2 23.3 47.0 75-813.8 25.3 46.3 84.213.0 27.1 48.3 84.816.9 29.1 53.5 9 5 419.64'3.953.8made.The reactions were first order in concentrationof complex, good pseudo-first-order rate plots being ob-tained for at least three half-lives. The observed rateconstants (Table 1) were found to increase with increasinghydroxide-ion concentration according to the equationE. J . Bounsall and A . J . Poe, J . Chew. SOC. ( A ) , 1966, 286.H. L. Bott, E. J . Bounsall, and -1. J. Po&, J . Chew. SOC. ( A ) ,1966, 1275.* R.Klabunde, Ph.l>. Thesis, Xorthmestern L'niversity,Evanston, Illinois, 19672252 J.C.S. Daltonhobs = k , + h,[OH-]. The effect is small but quite real,as was evidenced by groups of runs followed simultaneouslyfor which the absorbance changes were clearly more rapidthe greater the hydroxide-ion concentration. Values ofk , and K , (Table 2) were obtained by a weighted least-TABLE 2Kinetic parameters derived according to the rate law:Kobs = k,[OH-]105k2t/"C 105~,/~-1 1 mol-1 5-l a(kobs)/o~55.4 2.619 f 0.034 1.051 f 0.072 2.860.9 5.57 f 0.11 2.48 & 0.22 3.865.6 10.26 f 0.30 4.36 f 0.59 5.570.3 17.99 f 0.44 9.22 f 0.90 4.381.0 62.2 f 1.1 27.8 f 2.5 4.375.7 33-95 f 0.51 17.8 f 1.0 3-2k1(50 "C) = 1.2 x s-,, A W 1 = 27.93 f 0.16 kcal mol-l(116.9 f 0.7 kJ niol-l), A S , = 6.33 f 0.48 cal mol-1 K-1(22.3 f 2.0 J mol-l 1C-I) ; k,(50 "C) = 4.6 x 1 mol-1 s-l,AHt, = 29.9 -l 0.8 kcal mol-l (126.1 f 3.3 kJ mol-l), A S z= 9.5 f: 2.3 cal mol-l K-l (39.8 f 9.6 J mol-1 K-l)squares treatment in which the percentage uncertaintyof each value of Kobs at a given temperature was assumedt o be constant.The estimated uncertainties in k , and K,,and in an individual measurement of hobs a t a given tem-perature, were corrected for the appropriate number ofdegrees of freedom so that 95% confidence limits can beobtained by doubling the quoted standard deviations.ion relative to that of ammonia or iodide. The trans-effect order is then I- > OH- > NH, in terms of rateconstants, and I- > NH, > OH- in terms of activationenthalpies.The latter is a generally preferable measureof the trans-effect since it is independent of temperature,but the order obtained is different from that found forthe reactions of iodide ion with tran~-Rh(en),I(OH,)~+,tvans-Rh( en),( OH) OH,2+, and Rh ( NH,),0H,3+ (Table 3).The qualitative ordering of the trans-effect, as well as itsmagnitude, is therefore dependent on which reaction isbeing used to measure it. In addition, neither set ofdata allows one to compare the trans-effect of hydroxideion with that of chloride or bromide because the re-actions trans-Rh(en),XI+ + OH, * tram-Rh(en),-XOH;+ + I- (X = C1 or Br) are subject to catalyticeffects and cannot be studied for comparison.8These difficulties are similar to those obtained beforeand can be resolved in the same ~ a y .~ ? ~ A com-bination of the kinetic parameters for the forwardand reverse reactions of the equilibrium trans-Rh(en),-(0H)I + + OH, +, tran~-Rh(en),(OH)OH,2+ + I-leads to the parameters AHo = +4.1 & 0.2 kcal mol-1(17.1 & 0.8 kJ mol-l), ASo = 0.1 0.8 cal mol-l K-l(0.4 &- 3.3 J mol-l K-l), and K(50 "C) = 2 x lo-, mol1-1 a t I = 0 . 2 ~ . The kinetic parameters for theaquation reaction are not affected by a change ofTABLE 3Kinetic parameters a for reactions of some rhodium(II1) complexesAquation b of:AH$ A S 104~ (50 OC)cal mol-l K-l S-1 Referencetrans-Kh( en) ,( OH) I+ 27-93 f 0.16 5.33 2 0.48 0.12 This worktvans-Rh(en),I,+ 25-1 & 0.2 l r t l 1.24 8kcal mol-1Rh(NH,),12+ 26.2 f 0.2 --7*8 f 0.7 0.0024 3_4nation,e by iodide, of:104,+(50 'c)1 mol-l s-ltvans-Rh(en),(OH)OH,*+ 33-81 0-15 5.91 f 0-48 72.6 1Rh(NH,),0Hz3+ 24.4 h 0.2 -1.4 & 0.6 1-15 3tvans-Rh(en),I(OH,)*+ 18-0 & 0.3 0.3 & 1.1 5800 4a Uncertainties are standard deviations. b Ionic strength, 0-1-1.0~ (negligible dependence of kinetic parameters on ionic " strength).c Ionic strength, 0 . 2 ~ .The variation of O(kobs) with temperature is not systematicand it seems likely that the variation is itself a reflectionof random errors. Values of the activation parameters(Table 2) were obtained from a weighted least-squaresanalysis of the linear dependence of log(k,/T) and log(h,/T)on I/T, the uncertainties in log(k,/T) and log(k,/T) beingderived from the uncertainties found for R , and h,.DISCUSSIONThe reaction path governed by the rate constant k,can be considered to be a simple loss of iodide ion byaquation, the presence of hydroxide being requiredsimply to drive the reaction to completion by trans-forming the hydroxy-aquo-product into the dihydroxy-form.The values of k, (50 "C) and AHtl can be com-pared with corresponding values for the loss of iodideion from Rh(NH,),12+ and trans-Rh(en),I2+ (Table 3)t o provide a new measure of the trans-effect of hydroxideionic strength from 1.0 to 0.211 (at 69 O C , [OH-] =0 . 0 1 ~ ~ and I = 0 . 2 2 ~ : 104k = 1.65 and 1.06 s-l;a t I = 1.02M: 104k = 1-64 and 1.68 s-l). The intrinsickinetic trans-effect of hydroxide ion can now be obtained,in terms of relative enthalpies of activation, by com-paring the values of AH1 for the reactions of the tram-hydroxy-iodo- and trans-hydroxy-aquo-complexes withthe values of AH$ for the hypothetical reactions oftrans-iodo-complexes which have the correspondingvalues of AHo, i.e. 3.4-1 and -4.1 kcal mol-l (+ or- 17.1 k J mol-l) respecti~ely.~9~ These reference valuesof AH$ can be obtained by interpolation from a plot ofAH$ against AHo for the aquation of the complexestmm-Rh(en),IX+, and for the anation reactions of trapts-Rh(e11),10H,~+ with X- (X = C1, Br, and I).3 Such aplot is shown in the Figure.The values obtained are22.85 & 0.15 and 18-65 & 0-06 kcal mol-l (95.5 -+ 0.1972 2253and 78-0 + 0.3 kJ mol-I) respectively. Thus the AH1values for the hydroxy-complexes are 5-08 &- 0.27 and5.16 0.16 kcal mol-l (21.2 & 1.1 and 21-6 & 0-7 kJniol-l) higher than the reference values for the hypo-t hetical trans-iodo-complexes.These two values arenot significantly different, and the weighted average is5.14 & 0.14 kcal mol-l(21-5 & 0-5 kJ mol-l) as comparedwith the corresponding valuesof 5.02 & 0.18,4-72 & 0-15,and 3.20 0.20 kcal mol-l (21.0 & 0.8, 19.7 sf 0-6,I l d r l * l I I I 1 1 1 1 1 '-9 -7 -5 -3 -1 *1 *3 *5A N o / k cal mol-'Plots of AH: against AHo for the reactions: tvans-Rh(en),LX+ +OH, tvans-Rh(en),LOH,*+ + X- (L = C1, Br, or I).The line for L = I is a reference drawn through the appro-priate data points 3 and the areas for L = C1 and Br showthe uncertainties (standard deviations) in the relative valuesof AH$ as derived from the individual data point^.^ Theresults for the trans-hydroxy-complexes are shown by crosses,the lengths of the lines indicating the standard deviations ofthe \?slues.and 134 & 0.8 kJ mol-l) for the trans-chloro-, trans-ammine- and trans-bromo-complexes respectively ., Thevalues for the trans-chloro- and trans-bromo-complexesare also shown diagrammatically in the Figure where thestandard deviations for L = C1 and Br are indicatedby the vertical distance between the lines defining theshaded areas.The order of the intrinsic kinetic trans-effects is therefore OH- < C1- < NH, .< Br < I.This method of analysing the kinetic data allows anestimate of relative intrinsic kinetic tram-eff ect s,independent of any complications from different thermo-dynamic contributions to the kinetics, even thoughcorresponding reactions of complexes differing only intheir trans-ligand cannot be, or have not been, studied.lo iM.L. Tobe, J . Chem. SOC. ( A ) , 1959, 3776.l1 S. C. Chan and hi. L. Tobe, J . Chem. Soc., 1963, 5700.le AT. L. Tobe, Inorg. Chem., 1968, 7, 1260.Although activation parameters are available foraquation reactions of trans-Co(en),LX"" (L = NH,,OH, Br, or C1; X = Br or C1),10-12 these reactions arecomplicated by widely varying degrees of trans ----t cisrearrangement, and this is known to have a large effecton the activation parameters.12 Activation parametersfor aquation of trans-Co(cyclam)LCl+ (L = OH or Cl;cyclam = 1,4,8,1 l-tetra-azacyclotetradecane) are alsoavailable l3 and no rearrangement occurs during thesereactions.AH: (L = OH) Is over 6 kcal mol-l (25 kJmol-l) less than AHS (L = Cl) and, although correspond-ing values of AHo are not known, it seems likely that theintrinsic kinetic trans-eff ect of hydroxide ion is genuinelymuch greater than that of chloride in these cobalt(II1)complexes in contrast with the virtual equality in therhodium(m) complexes. It is not clear whether thiscan be ascribed to different G- or x-bonding effects butit has been pointed out that rhodium(Ir1) seems lesssusceptible to x-bonding effects than cobalt (111) .14These results may therefore be a consequence of thegreater importance, in cobalt(m) complexes, of the morepronounced x-donor character of hydroxide ion comparedwith that of chloride.The second-order term in the rate law has not pre-viously been observed for trans-dihalogenobisethylene-diaminerhodium(II1) complexes, only those with N-Hgroups co-ordinated trans to the leaving halide ionshowing pronounced second-order reactions with hydr-oxide This is simply due to the low concen-trations of hydroxide ion used previously.The activ-ation parameters show that the relative unimportanceof the second-order term is due to the fact that AS:,is only very slightly more positive than ASS,, AH:,being 2 kcal mol-l (8 kJ mol-l) higher than AH:,. Incases where base hydrolysis of amine complexes isrelatively much more pronounced this is usually due to amuch more favourable value of AS:, (up to ca. 40 calmol-l K-l on 170 J mol-l K-l more positive) that morethan offsets the difference benveen AH:, andThe significance of the different pattern in the presentrhodium complex is not clear and we are extending ourstudies of the base hydrolysis of such complexes to seehow general the pattern is.We gratefully acknowledge partial support for thisresearch from the National Research Council of Canadaand the S.R.C. through a Postgraduate Studentship (toC. P. J. V.).[2/920 Received, 25th April, 1972113 C. K. Poon and M. L. Tobe, J . Chem. SOC. ( A ) , 1967, 3069.14 S. A. Johnson, F. Basolo, and R. G. Pearson, J . A4mrr.Chem. SOC., 1963, 85, 1741.15 F. Basolo and R. G. Pearson, 'Mechanisms of InorganicReactions,' Wiley. New York, 2nd edn., 1967, ch. 3 ; M. L. Tobe,Accounts Chew Res., 1970, 3, 377