摘要:
1974 1273iron(ii) Catalysis in Substitution Reactions of Amminepentacyano- andAq uo pen tacya no - f er rate( I 11) lo nsBy Alan D. James, Robin S. Murray,” and William C. E. Higginson, Department of Chemistry, The University,Hull HU6 7RXSubstitution reactions of the ions [Fe11r(CN)5X]2- (X = NH, and H,O) by Y (Y = N3-, SCN-, OH-, and [Co-(CN)6]3-) are catalysed by [FeI1(CN),Xl3-. At iron(l1) concentrations greater than ca. 1 % of the total iron speciespresent, the rate-determining step of these reactions is substitution of the ions [Fer1(CN)5X]3-- by Y.IKON(II) and iron(111) complexes of general formula[Fe(CN),X] undergo substitution reactions as in equation(i) and the kinetics and equilibria of several reactions ofthis type liavc. been studied.l I t appears that rates ofsubstitution o f the iron(I1) species are usually morerapid tlian those of the iron(m) analogues.We have observed that rates of formation andhydrolysis of the binuclear species [FelIr,( CN),,J4- aremarkedly affected by the presence of iron(r1) complexes,and the liydrolysis reaction of the ion [FerI1(CN),J3- hasbeen shown3 to be catalysed by the presence of[FeT1(CN),!*-, although the mechanism of the latterreaction lias not been established.We have subse-quently investigated the importance of iron(1I) catalysisin substitution reactions of the ions [Fe1I1(CN),Xl2-(X = NH, and H,O) with a variety of substituents, Y.(a) J. Lcgros, J . Chinz. phys., 1964, 61, 923; (b) J. H.Kspenson and S. G. Wolenuck, Iwovg. Cltenz., 1972, 11, 2034;(c) B.Jaselskis, J . A m e v . Chenz. Soc., 1961, 83, 1082.A. D. James, K. S. Murray, and W. C . E. Higginson,i~npul~lishetl work.sj J . T>~iI’I(,.;sis-l~(.~ros, Coiiijt. r c ~ z d . , 1970, 270, 1768.Here we report the results of investigations where( a ) X = NH, and Y = N3-, OH-, and X N - , and( b ) X = H,O and Y = :CO(CN)~]~-.EXPERIMENTALThe salt Na,[Fe(CS),NH,] was prcparecl according to themethod of Hoffman,* and characterised from its \-isibleand i.r. spectra.“’ The usual method of preparing the saltNa,[Fe(CN),NH,] involves oxidation of X’a,[Fe(CN) ,NH,]with I!”, a t 0 OC,* but to avoid any possibility of sub-stitution by nitrite ion, we used the niinimum amount ofbromine water as oxidising agent, followed by precipitationof Na,[Fe(CN),NH,] with diethyl ether-ethanol ( 1 : 1 ) .The product was recrystallised from water-ethanol {Found :C, 31.3; H, 2-60; Fe, 19.1; N, 28.45. Sa,[Fe(CK)5SH,1,-2-5H20 requires C, 20.5; H, 2-70; Fe, 19-0; N, 28-60,(1),1.r. and visible spectra agreed well with prcvious s t u ~ l i e s .~ ~ * ~ ~ ~Solutions of the ion [Fe(CN),H,Ol2- were prepared by amethod adapted from Espenson.lb Instead of using theK. -4. Hoffmaiin, Annalen, 1900, 312, 1.E. Baran and A. Rluller, 2. anovg. Chent., 1969, 368, 144.W. Haberditzl, I<. D. Schleintz, and H. C. Bartcl, Z. N n f i w -L. Tosi and J. Danon, Inovg. Clcent., 1964, 3, 1510G. Brauer, ‘ Handbuch dcr Praparativcii ,\norgan ischcnfovsch., 1968, 236, 891.Chemic,’ Ferdinand Enke, Stnttgart, 1964, 13641274 J.C.S.Daltonpurple diiiieric ion [FeIII,(CK) as starting material, asolution of [Fe1I1(CN),PU'H3I2- (ca. 0 . 0 5 ~ ) in KOH ( 0 . 1 ~ ) wasallowed to hydrolyse to [Fe1r1(CN),0H]3-. The resultingbright yellow solution was then brought to pH 4 with aceticacid containing Br,, passed down a Sephadex G25 column,and a yellow fraction consisting of the aquo-complex wascollected. (Bromine was added to prevent the iron(I1)-catalysed dimerisation of the ion [Fe111(CN),H,0]2-.2)Thus solutions of up to 0 . 0 2 ~ in the ion [Fe111(CN),H,0]2-were obtained which were stable at 5 "C for several hours.The visible spectrum was very similar to that of the complexprepared by Espenson lb (Figure).a00.5L L 300h l n mVisible spectra of the ions: (a), [Fe(CN),H20]2- (1.00 x(b) [(CN),F~(NC)CO(CN),]~- (1.00 x : and (c) [Co(CX),J3-(1.0 'x 1 0 - l M )Reduction potentials were obtained from potentiometrictitrations of the pentacyano-complexes with Na,[IrCl,] ,2H,Oor ascorbic acid at I = 1 .0 ~ and are quoted in Table 1.All kinetic measurements were carried out under N, onfreshly prepared 0,-free solutions a t 1 = l*O&f on a UriicamSP 600 spectrophotometer.Rates of hydrolysis of the ion [Fe11(CN),NH,]3- with andwithout pyridine (py) present to give [Fe1r(CN)5(py)]3- and[Fe1I(CN),H,0I3- respectively were measured at pH 4; theinitial concentration of [Fe11(CN),KH,]3- was 3 xBoth the aquo- and pyridine complexes have a singleabsorption maximum above 350 nm, a t 440 (E 700) and365 nm (E 4 200 1 mol-l cm-l) respectively, whereas[FeI1(CN),NH3I3- has a maximum a t 405 nm (E 450 1 niol-lcm-l), The uncertainty of & 10% in values of k , obtainedfrom these data is in part due to complications caused by aslower dimerisation reaction of the ion [E;e11(CN),H,0]3- togive [FeII,(CN)For the reaction of the ion [FeI1(CN),H,0l3- with[Co(CN),]3--, solutions of the former were prepared byhydrolysis of [Fe11(CK'),XHJ3- or by reduction of[Fe111(CN),H,0]2- with ascorbic acid a t pH 4.The rateof disappearance of the 440 nni peak of the aquo-ion whenvarious concentrations of [Co(CN)J3- ( w ~ - ~ O - ~ M ) wereadded a t 9.4 OC was measured, and the second-order rateconstant k , was calculated from the observed first-order rate constant (= K,[CO(CX)~~--] + k-,).The product[ (CN) ,Ferl(NC) ColI1(CN) ,] 6- has an absorption maximuma t ca. 380 nin ( E 300 1 mol-l cm-l).Reactions between the ions [Fe111(CN),NH,]2- and K3-,SCS-, and OH-, a t 26 "C with different amounts of[Fe11(CN),NHJ3- added (0 < c < 0.6), were followedusing solutions of ca. 10-4~~-[Fe111(CN),NH3]2- a t 590, 560,and 390 nm, respectively, where the complexes [FelI1(CN),-SCN]3-, [Fe111(CN),E'-,]3-, and [Fe1I1(CN),OHI3- exhibitmaxima (E 2 680, 3 700, and 1600 1 niol-l crn-l, respec-tively). The concentrations of the ion [Fe11(CX)5SH,]3- inTABLE 1Equilibrium datax Y ExlV a EyIV a K,/1 mol-1NH, OH- 0-399 0.159C S & 5NH, SCN- 0-399 0.385 f (9 & 3) x 10H,O [CO(CN)~]~- 0.409 0.457g (2 -J= 1.5) x lo30.409 0-508h (G -Ji 3) x lo50.509 CN-9 [CO(CN)~~-] = O - l M .NH, N3- 0.399 0.265 4&3HzO PYa At 0 OC, I = 1.0h1, f0-005 V.b At 25.0 OC. C At pH 13.5.[py] = O-liLI.Values for the [Fe(CN),I3--iFe(C~),]~- couple are includedfor comparison. The potentials were originally measuredrelative to a calomel electrode; values in Table 1 wereobtained by adding 0.244 V. The constants I<, and K ,were calculated from the equation F(Ex - Ey) =2.303RT log,,K, where Ex and Ey are reduction potentialsof the [Fe111(CN),X]2--[Fe11(CN),X]3- and [Fe111(CN)5Y]-[FeII(CN),Y] couples respectively.The equilibrium constant K,, for the reaction between theions [Fe111(CS),H,0]2- and [CO(CN),]~- was estimatedspectrophotometrically at the absorption maximum of thedimer a t 422 nm (Figure).Solutions containing the saltK,[Co(CN),] (0.005-0.10~) and [Fe(CN),H,0I2- (ca. 1 0 - 4 ~ )were allowed to reach equilibrium at pH 4 and I = 1 . 0 ~ .A plot of 1/(A - A,) against ~/[CO(CN),~-], where A , is theabsorbance at 422 nm when [CO(CN),~-] = 0, gave astraight line, from the gradient and intercept of which K,,and the absorption coefficient of the dimer were calculated.K3 Ki K12/l rnol-l 6(2.8 f 1.0) x 10' (5 & 2) x 10' (3.9 & 0.5) x lo5(3.0 f 1.0) x lo2 (5 2) x lo2 (1.97 & 0.16) x I O 3 d(2.57 & 0.16) x 1O'd(1 & 0.5) x lo-' (2.17 & 0.20) x lo2(1 0.5) x 10-2 (6.6 & 0.8) x lo3 d1.8 5 1 3 1 1d From refs. l(a) and l(b). e [Y,-] = 1 .0 ~ . f [SCN-] = 1 . 0 ~ .solutions used were obtained by addition of N,- or SCS-*followed by oxidation to the iron(II1) complex with H20.)pand spectroscopic determinations a t 590 and 560 nnirespe~tively.~The reaction between the ions [Fe111(CN),H,0]2- (1 x1 0 - 4 ~ ) and [Co(CN),I3- at pH 4 was followed a t 422 nm, theabsorption maximum of [ (CN) ,FelI1 (NC) ColI1(CN),] 5- (E1 400 1 mol-l cm-l), with different amounts of [FeII(CN),-H,0I3- (0 < c < 1.0). The source of the ion [FeII(CN),-H,0I3- was again [FeI1(CN),NH,I3-. The dependence ofkobs on the concentration of the ion [Co(CN)J3- was in-vestigated a t c = 0.13. ,4ttempts were made to isolatesolid Na,[(CN),FeI1I(NC)CorII(CN)J. Aqueous brominewas added to a solution of the dimer [to prevent iron(r1)-catalysed hydrolysis] and the solution was then passeddown a Sephadex G-25 column, followed by evaporationB. Jaselskis and J .C. Edwards, Analyt. Chem., 1960, 32,381.We estimate the error in G to be &5%1974 1275under vacuum a t room temperature. A solution in waterof the resulting solid gave a U.V. and visible spectrum withmaxima a t 422, 360, and 292 nni, but analysis revealed anexcess of the ion [Co(CN),]3-. The i.r. spectrum of thissolid showed two peaks in the region of the cyanide stretcha t 2 135 and 2 185 cm-l in the ratio 8 : 1. Haim et aZ.1°have shown that bridging cyanide ligands absorb at higherenergy and we therefore assign the upper band to bridgingcyanide.RESULTSSince the presentation of our results is simplified byreferring to our suggested reaction mechanisms, we introducethe latter a t this stage.When X = NH, the proposedcatalytic path is as follows, where (1) is a slow substitution[FeI1(CN),NH3l3- + H,O" --wki[FeI1(CN),H20-J3- + NH4+ (1)and (3) a rapid equilibrium.[Fe11(CN),H,0]3- + Y z+!=In the case where equilibriumkIk--2[FeII(CN),Y] + H,O; K , = k,/k-, (2)I(,[Fer1(CN),NH,I3- + [FelI1(CN),YJ (3)[ FelI1( CN) ,NH,]2- + [FeII( CN) ,v +,(2) is established rapidly this leads to equation (4). An- - d[Fe1I1(CN),NH,2-] -dtk,K,'c[Felll (CW) ,NH,,-] / / 1 + p[Fe111(CN),NH,2-] +LGK,' } (4)1 + p[Fe111(CN),NH,2-jandintegrated form of (4) is (5). When K,'c 9 1 and 0.05 <- kiK3'Ct = - kobsK3't= (K3'c + 1) In [Fe111(CN),NH,2-] -K3'c In {(I/$) + [Fe111(CN),NH,2-]} +p[Fe111(CN),NH,2-] + constantc < 1.0, expression ( 5 ) can be expressed as (6).--k,ct = -kobst = [Fe111(CN),NH,2-]/[Fe111]~When X = H,O the proposed catalytic pathfollowed by equilibrium (7).Under conditions[FeII(CN),Y] + [Fe111(CN),H,0]2- +[FelI1(CN),Y] + [FeII(CN) ,H,0l3-k , > k-, and the rate of substitution is dependent on [YI,10 P. A. Doors, A, Haini, and W. K. TVilmarth, J. Inovg.Nuclear Chem., 1961, 21, 33.this leads to expression (8).-d[Fe111(CN),H,02-]dt= K,X,c[Y] [Fe111(CN),H,02-]/ {l + q[Ferrr(CN),H,O2-] +On integration (8) becomes (9).LGK71 + q[Fe111(CN),H,02-]} (8)4 = ( K , - 1)[FeIII]T where--K,h',C[Y]t = -kobsK7t == (K7c + 1) In [Fe1II(CN),H,O2-] -K,G In { (l/q) + [Fe111(CN),H,03-]} +q[Fe111(CN),H,02-] + constant (9)Unless otherwise specified, all experiments were carriedout a t 25 "C, pH 4 (acetate 0s plithalate buffers), andI = l .0 ~ (maintained by the use of KC1). Under theconditions chosen reactions (10) and (1 1) are essentiallyK,,[Fe111(CN),NH,12- + H,O+ =+[Fe111(CN)5H,0]z- + XH4+ (10)Kn[Fe1I1(CN),NHJ2- + OH- +complete (Klo and K,, are 4 x lo5 and 50 & 7respectively) . l b s c Equilibrium constants, K,,, for reactions(12) are quoted in Table 1.K n[Fe11T(CN),H,0]2- + Y [FeIII(CN),Y] -+ H,O (12)(a) Reactions of the [Fe1I1(CN),NH3l2- Ion with N,-aizd SCN-.-Preliminary experiments indicated that thepresence of trace quantities of iron(I1) species catalysedsubstitution reactions of the ion [Fe111(CN),NH3] ,-.Con-sequently Br, or IrCl,,- (ca. 1 x 1 0 - 4 ~ ) were added t oreaction mixtures of the ions [l?eIII(CN)5NH3]2- ( 1 x ~O-,M)and Y (Y = N,- and SCN-; 0 . 1 0 ~ ) to oxidise rapidlytraces of iron(I1) species present in the samples ofNa,[FeIII(CN),NH,]. The excess of oxidant was observedspectrophotometrically t o disappear over a period of 5-10min, presumably with oxidation of N,- or SCN- ion.However there was no spectral evidence for formation ofany new iron(II1) complexes during these rapid reactionsand we have no evidence that the products of these re-actions had an effect on the subsequent slow substitutionreactions of the ion [Fe111(CN),NH,J2-. The latter re-actions [equation (13)] were investigated spectrophoto-[Fe111(CN),NH,]2- + Y + H+ +metrically and values of the initial rates - (d[FeIII(CN),-NH,2-]/dt), where Y = N,- (0.10~) and SCN- ( O - ~ O M ) , of<1 x lo-' and <3 x mol 1-1 s-1 were obtainedrespectively. Rates of substitution were found t o increasewith time, and we ascribe this to slow formation of the ion[FeI1(CN),NH,l3-, possibly due to reduction of [FelI1(CX),-NH3I2- by N3- and SCN-.Plots of Robs [equation (5)] against the concentration ofadded iron(n) (at constant [Fe1I1IT) for data discussed insection (d) gave straight lines with positive intercepts.Values of co (0-025, 0.020, and 0.040 for Y = N3-, SCN-,[Fe111(CN),0H]3- + NH, (1 1)[Fe1I1(CN),q + NH,' (131276TABLE 2J.C.S.DaltonKinetic data for iron(I1)-catalysed substitution reactions of the ions [Fe1I1(CN),XH,] 2- and [Fe1*1(CN),H,0]2-1 0 3 [ ~ y ~ 104[Fe1Tr]T/~ c 103ko~s/s-1 102kl/s1 k,/l mol-1 s-1X = NH,, Y = N3-, at 25 "C100 2.0 0.076 1.5 1.9100 2.0 0.1 3 3.0 2.3100 2.0 0.25 5.2 2.0100 2.0 0.36 6.8 1.9100 2.0 0.47 9.0 1.91 000 1.5 0.66 1 1 9.0660 1.5 0.56 12 2.1330 1.5 0.56 1 1 2.0100 1-5 0.56 11 2.050 1-5 0.56 10 1.917 1.5 0.56 10 1.95.0 1-5 0.56 8.2 1.51 -3 1.5 0.56 11 2.0100 0-3 1 0.15 3.0 3.0100 0.62 0.15 2.7 1.8100 2.8 0.15 2.7 1.8100 1.9 0.16 3.0 2.0I A >X = NH,, Y = OH-, at 25 "C7h- r50 3.0 0.13 2.7 9.150 3.0 0.1 9 3.7 3.050 3.0 0.27 5.0 1.950 3.0 0.34 6.5 1.950 0.5 0.3 1 6.7 3 . 350 1.8 0.:: 1 G - 1 3.050 0.3 0.3 1 6.0 9.050 3.1 0-31 6.1 2.0100100100I00100100660330100100100100100100X = NH,, Y = SCS-, at 25 "CA2.0 0.14 2.83.0 0.19 3.62.0 0.20 4.02.0 0.32 5.83.0 0.44 9.71.0 0.46 9.01.0 0.46 8.31.0 0-46 8.71.0 0.46 8.50.5 0.58 121.0 0.88 121 -6 0-58 103.3 0.58 104.8 0.58 10X = H20, 1' = [Co(CN)J3-, at 9.4 "C3.3353 33 33310075503533:3 33 33 30.1 10.34"0.420.630.840.130-l:i0.130-180.130-1 30.130-139.34.85.4"9.011-55-04.31.62.61.61.61.5*>. 5TABLE 3Kinetic data for the substitution reaction of the ion L F ~ ~ ~ ( C N ) ~ H ~ O ~ ~ - by [Co(CN),I3-, a t 9.4 "C1 O5[Fe1I(CN) 5H,03-] / M 1 03[Co(CN) 6 3 -1 / 11 102{l?-2 -+ k,[Co (CN) G3--])/s-1 K,/1 mol-1 s-14.0 1 -0 0.44 3.364-0 3 *0 1.18 3-604.0 5.0 1.72 3-244.0 7.0 2.52 3-434.0 10.0 3.60 3.51974 1277and OH- respectively) corresponding to the contributionto c of traces of iron(I1) impurities in samples of the salt;Ya,[Fe111(CN),NH3] were calculated from the values ofthese intercepts.The values of c in Table 2 are the sum ofc,, and cl, the contribution from deliberate addition of theion [FeII (CN) ,NH3] ,-.(b) Reaction ofthe [Fe111(CN)5H,0]2- Ion with [Co(CN),I3-.-'l'he rate of the substitution reaction of the ions[Fe111(CN),H,0]2- and [Co(CN),I3- to form the binuclearspecies [(CN),F~(NC)CO(C~~),]~- was also found to beincreased by traces of iron(r1) impurities in samples ofNa,[Fe111(CK)5H,0].The rate of the uncatalysed reaction{ -d[Fe111(CN),H,02-]/dt), in experiments where [FeIII-(CN),H202-] L- 1 x 10-4, [IrC1,2-] = 1 x and[CO(CN),~-] = 3.3 x 10-2ivr, was found by spectrophoto-metric investigations to be <3 x 10-7 mol 1-1 s-l. The rateof this reaction did not increase with time. From thevalue of the intercept of the straight-line plot of hobs[equation (9)] against the concentration of added iron(I1)[section ( d ) below] a value of c, of 0.003 was obtained.(c) Reactions of the [FeI1(CN),NHJ3- Ion with H,O andPyridine, and of [FeI1(CN),H,OJ3- with N3-, SCN-, and[C~(CN),]~-.-Substitution reactions of the ion [FeII(CN),-NH,I3- by H,O and pyridine (py) l1 to give [FeI1(CN),H20J3-and [Fe1I(CX),(py)I3-, respectively, both follow first-orderrate laws and give rate constants of (1.8 f 0.2) x lo-, and(1.77 & 0.12) A lop2 s-l respectively.The value of hobsis independent of the concentration of py over the range4.5 x 10-3---ti.6 x 10-2~r (Table 4). The reaction withH,O was complicated by dimerisation of the product togive [FeII,(CX) 10]6-,2 whereas no dimerisation was observedin the reaction with py where K , > 3 x lo5 1 mol-l. Re-actions of the ion [FeI1(CN),H,0l3- with N,- ancl SCN-were found to be extremely rapid, confirming the observ-ations of TVilkins.lz The reaction of the ion [FeII(CN),-H,0I3- with [Co(CN)J3- was much slower, and we foundthat the rate of substitution was dependent on the con-centration of [Co(CN),I3- (Table 3).(d) Catalysis of Substitutioti Reactions of the Ions[Fe111(CN),NH,j3- and [FelI1(CN) 5HzO]2- by [FeII(CN) 5-SH3I3- and [17e11(CN),H,0]3-.-For the systems whereS = NH, and IT = ST,-, SCP?, or OH-, and X = H,Oancl Y = [Co(CS),13-, relevant values of K , and K , deducedfrom reduction potentials are included in Table 1.Valuesof K12 ancl = K,,/K, are also included.TABLE 4Kinetic data for substitution reaction of the ion[FeI1(CN),NH,I3- by pyridine at 25 "C1O4[FeII(CN) 5NH3R-] /&I 1O2[PY1/M 102k,/s-13.0 6.6 1-753 . 0 3.3 1.753.0 1.4 1.623.0 0.46 1.621.0 3.3 1.837.0 3.3 2.00(i) When X = NH, and 1- = S3-, SCX-, and OH-,values of kov,.; were found to be directly proportional to c,and independent of the concentration of I-, as predicted byequation (5).When Y == S,- and OH-, K , = (3 & 1) x10, and (2.8 & 1.0) x lo*, respectively and, except wliere[Y] is low, K,' ,N I<3 > 50 leading to a linear decrease inl1 €3. 1;. l'oina mcl J. hI. Malin, Inovg. Chew., 1973, 12, 1039.l2 1C. Straslw and R. G. Wilkiiis, Inovg. Chew., 1969, 8, 156.l3 Hairn and TV. K. Wilmarth, J . ,¶mev. Clieirz. Soc., 1961,83, 30'3.[Fe111(CN),NH32-] with time. \Then [K3-] < d x ~O-,Mthe term l/K,[N,-] becomes important and K,' < 6 . Inthese cases this simple kinetic behaviour was not observed,although a plot of the complete expression gave straightlines and values of K , so obtained were similar to those a thigher concentrations of N,- ion. When Y = SCN-,K , = 1-8 f 0-8 (Table 1). Values of K3Kobs were obtainedby plotting the right-hand side of equation (5) against theappropriate value of t.The values of hobs so obtained(Table 2) were relatively insensitive t o the value of K ,chosen. For example the calculated value of Iz,,bs wasincreased by only 10% on increasing the value of K , from1.0 t o 1.8. The values of 12, obtained from the data inTable 2 where Y = N3-, SCN-, and OH- are (1.94 &-0.15) x lo-,, (1.90 & 0.15) >< lo-,, and (2.0 -J= 0.1) x lo-,s-l respectively, in agreement with the directly nieabureclvalues of k , given in section (6).(ii) When X = H,O and Y = [Co(CX),I3 -, tlie productof reaction was the binuclear species [(CN)51;eI11(SC)Co111-(CN),I5-, the spectrum of which is shown in tlie Figure.The linkage isomer [(CN)jFeIII(CN)CoIII(CN),]j , which isformed by reaction of the ion [Fe1I1(CPU'),l3- with [Co(CN)JS-followed by oxidation, has been reported previously.l3The species [(CN)5Fe111(CN)Fe111(CN),]5-, formed by re-action of the ion [Fe111(CN),13- with [Fe11-C(CN)5H,0]2--,has also been reported.l*Under the conditions used in these experiments reaction(2) was essentially complete. Values of K7kobs wereobtained by plotting the right-hand side of equation (9)against the appropriate value of t, and straight lines wereobtained for values of K , within the limits 0-0-2. A valueof K , of 0.10 f 0.05 was obtained from reduction-potentialdata (Table 1). The rate of reaction was found to bedependent on the concentration of the ion [CID(CN)~]~- andvalues of hobs at 9.4 "C are given in Table 2.At the con-centrations of the ion [Co(CN),]3- employed in theseexperiments, formation of the binuclear adduct was only85% complete. This was allowed for in calculating therate constant k , for this system. The mean value ofh, was 3-5 j, 1.8 1 niol-l s-l, compared with the valueobtained directly for the substitution of the ion [FeT1(CX),-H,OI3- by [Co(CN),I3- of 3.43 5 0-12 1 mol-l s-1. The largcerror in k , is due t o uncertain% in the magnitude of I<,.DISCUSSIONThe results indicate that in reactions of the ion[Fe1I1(CN),NH,l2- with N3-, SCN-, and OH-, and[FelI1( CN),H,O] 2- with [Co (CN),]3-, where concentra-tions of [FeI1(CN),XI3- are such that c .> 0.01, sub-stitution occurs mainly via the catalytic path and directsubstitution of [Fe1I1(CN)JI2- by Y is insignificant bycomparison.This conclusion is particularly relevant tosystems where Y can act as a reducing agent. Oui-results indicate that iron(I1) species are formed inreactions of the ion [17eT11(CN)5NH3]2- with N3-, SCN-,and OH--. Significantly, this effect was not observed inthe reaction between the ion [Fe111(CN)5H20]2- and[Co(CN)J3-, where oxidation of the latter is unlikely.Hydrolysis of the ion iFeC112L has been shown to becatalysed by Fez (aq) .16 The rate-determining stepl4 G. Emschwiller and J. Lcgros, Compt. u e ~ d . , 1971, 273, 452.l5 li. J . Campion, T. J . Conocchioli, :~nd N. S n t i n . .I. Amev.Chcm. SOC., 1961, 86, 45911278 J.C.S. Daltonfor the catalytic path is an electron-transfer reactionwhich leads to a different rate expression to that reportedin this paper.Recently Espenson lb reported a kinetic investigationof the reaction between the ion [Fe111(CN)5H20]2- andSCN-. The reaction appeared complicated. We re-peated one of Espenson's experiments, initial concen-trations being [Fe111(CN)5H20]2- = 2 x 10-4, [SCN-] =0.08, [H'j = 0.0160, and I = 1 . 0 ~ (NaC10,) at 25 "C,and found that when Br, was added (ca. 4 x 10-4~) therate of substitution of the ion [FeIIJ(CN),H20l2- did notexceed one sixth of that previously reported. Wecalculate that the rate observed by Espenson wouldrequire only the presence of ca. 0.01% iron(I1) species(c = 1 x 10-4). This calculation was based on anestimate l2 of the second-order rate constant for sub-stitution of the ion [Fex1(CN),H20]3- by SCN- ofl o 4 1 mol-l s-l at 25 "C; we assumed that rates of sub-stitution by the ions N,- and SCN- were similar.\Ye thank the S.R.C. for a grant (to A. D. J.).[3/2106 Received, 13th October, 1973
ISSN:1477-9226
DOI:10.1039/DT9740001273
出版商:RSC
年代:1974
数据来源: RSC