摘要:
TReactions of p-Hydroxo-dicobalt(iii) Complexes. Part XI. t KineticStudies on the Reaction of Bromide with the p-Amido-p-hydroxo-bis-[tetra-amminecobalt(iii)] ComplexBy (Miss) Siew-Wan Foong and A. Geoffrey Sykes," Department of Inorganic and Structural Chemistry, TheUniversity, Leeds LS2 9JTThe kinetics of hydroxo-bridge cleavage of the p-amido-p-hydroxo-bis[tetra-amminecobalt(~tt)] complex ( I ) bybromide in aqueous perchloric acid have been investigated spectrophotometrically with [Br-] = 0.5-2.0~,[H+] = 0.5-2.0~, and / = 2 . 0 ~ (NaCIO,). The product in the rate-determining step is the aquo-bromo-complex,(11). which i s in rapid equilibrium with the p-amido-k-bromo-complex (111). equation (i). The bromo-complex(111) and not (11) is predominant. Pseudo-first-order rate constants, keq, for the equilibration in (i) can beexpressed as in equation (ii), where kl and k2 are for hydroxo-bridge cleavage, and kb for the back reaction (productkeq = k,[H+][Br-] + k2[Br-] + kb (ii)of a rate constant and ;he equilibrium constant KB) is as determined previously. At 25 "C k, = (4.33 & 0.17) xFrom the temperaturedependence AHl$ = 17.3 f 0.8 kcal moI--l, ASIS = -1 6.2 f 2.6 cal mol-l K-l, AH,: = 25-7 * 1.8 kcal mol-1,l2 mol-2 s-l, k2 = (1.4 f 0.2) x I mol-l s-l, and KAKB = ca.1.1 I2 mol-2.and AS2$ = 10.8 k 5.7 cal rn0I-l K-l.an intermediate. At 25 'C K3 = ca. 0.03 I mol-l, and k-, > 2 x I O - ' S - ~ (at 1 * 5 "C), / = 2.0M (NaCIO,).Reaction (iii) is effective at [Br-] > 1 . 7 ~ . where (11) may be involved asDETAILS of hydroxo-bridge cleavage reactions of they-amido-p-hydroxo-bis[tetra-amminecobalt ( HI)] complexL -1( 1 )(I) in aqueous perchloric acid solutions with a variety ofanions including chloride,l nitrate,2 sulphate,3 ele en ate,^ph~sphate,~ and acetate,6 have been described previ-ously. The reactions have yielded new complexes,which have been characterized, while kinetic data arerelevant in considering the mechanism of bridge cleavageand whether this is SN1 or S N 2 in character.' Thechloride study previously described has similar featuresto the bromide study which is now considered. Withchloride Werner originally assigned an aquo-chloro-S.W. Foong, I<. D. Mast, M. R. Stevenson, and A. G. Sykes,M. B. Stevenson, R.S. Taylor, and A. G. Sykes, J . Chem.M. B. Stevenson, A. G. Sykcs, and €3. S. Taylor, J . Clzeni.f Part X is ref. 5.J . C h e m SOC. (A), 1971, 1266.Soc. (A), 1970, 1059.Soc. ( A ) , 1970, 3214.structure to the crystalline product.S Similarly heassigned the aquo-bromo-structure (111) to the bromo-complex obtained on treating the sulphate salt of (I)with concentrated HBr.S A bromo-complex has beenisolated in connection with the present study, and fromanalyses and i.r. spectrometry (first part oE paper), it isconcluded that this has a p-ando-p-bromo-structure(111). Aqueous solutions of (111) are believed to givesmall (<6%) amounts of (11) in a relatively rapidequilibrium,1° and subsequently a t low or zero bromideconcentrations (I) is formed. The reaction of (I) withbromide in aqueous perchloric acid solutions ( I 11= 2.0M)S.W. Foong and A. G. Sykes, J.C.S. Dalton, 1973, 504.J . D. Edwards, S. W. Foong, and A. G. Sykes, J.C.S. Dalton,K. L. Scott and A. G. Sykes, J.C.S. Dalton, 1972, 2364.K. S. Taylor and A. G. Sykes, J.C.S. Dnltoti, 111 the press.Ref. 8, p. 50.M. K. Hyde and A. G. Sykcs, J.C 5 Daltutt, i n thc press.1973, 820.8 A. Werner, Atznalt!~, 1910, 375, 451454 J.C.S. Daltonprovides the subject of this paper where (11) is again tobe regarded as a transient species. Thus at low bromide-ion concentrations ( < 1 . 7 ~ ) (11) is formed in the rate-determining step and reacts rapidly to give (111). Athigher bromide concentrations the bis-bromo-complex(IV) is formed but this species is not predominant untilconcentrations of HBr approaching 811.rCharacterization of the Bronzo-product.-I t was notpossible in this work to isolate the dihydrate of (11)reported by Werner.9 Solutions containing predomin-antly (IV) give samples of (111) as the least-soluble form.Bromide and perchlorate salts were isolated followingthe reaction of (I) with HBr (details in Experimentalsection).Analyses indicated no water molecules andare consistent with structure (111). This structuralassignment has been confirmed by i.r. spectrometryas follows. First of all a comparison of metal-chloride stretching frequencies for [Co(NH,),Cl]Cl, and[(N H3) ,Corn p.( NH,,Cl) *Co (NH,),] C1, was made (seeFigure 1).The second of these complexes is knownto have a p-amido-p-chloro-structure.ll Perchloratesalts give identical spectra to those of the chlorides.Metal-bromide stretching frequencies for bromideand perchlorate salts of [Co(NH,),BrI2+ and[(NH,),CO*~(NH,,B~)*CO(NH,),~~~ were then recorded.The bromide salt of the latter does not give satisfactoryresolution in the region 220-170 cm-l, possibly due tolattice vibrations. However the frequencies observedfor the perchlorate salts (see Figure 2) confirm thep-amido-p-bromo-bridged structure. The assignmentsmade (see Table 1) give v(M-Br)/(M-C1) ratios for mono-nuclear and binuclear complexes in the range 0.71-0.75, as observed for halide complexes of other metals.12The frequencies for the mononuclear complex are in goodagreement with values previously reported; 13-16 aquo-penta-amminecobalt (111) has no similar absorptionbands in the 300--180 cm-l region (see Figures 1 and 2).Ratios of frequencies observed for binuclear to mono-nuclear complexes are in the range 0.90-0.94.Similarratios have been observed for bridging and terminalhalide ions in complexes M,ClS3- (M = V, Cr),17 inwhich the metal is also in oxidation state (111) and thel1 R. Barro, R. E. Marsh, and W. P. Schaefer, 1noi.g. CJzem.,1970, 9, 2131.I<. Nakamoto, ' Infrared Spectra of Inorganic and Co-ordination Compounds,' Wiley, New York, 1970, pp. 214-218.l3 G. W. Watts and D. S. Klett, Iizovg. Chew., 1964, 3, 782.geometry octahedral. The lowering of metal-chlorideand metal-bromide frequencies for the binuclear coni-pared to mononuclear complexes is consistent withTABLE 1Metal-halide stretching vibrations.Numbers inparentheses indicate number of spectra recordedComplex Frequenc y/cm-l[Co(NH3) 5c1lc12 285 (3)[Co(NH3)&11 (c10,) 2~ ~ N H ~ ~ ~ C o ~ ~ . ( N H 2 ~ C 1 ~ ~ C o ~ N H 3 ~ d ( C 1 0 4 ~ ~ 268 (4)[Co(NH3) 5BrI (C104) 2295 (2) b[ (NHJ pC~*p( NH,, C1) *CO (NH3) 41 CI,, 4-5H20 269 (5)214 (2) c[ (NH3)4Co'p( NH2,Br) 'cO ( NH$'),lj (clod), 192 (3)With the chloride (but not the perchlorate) salt splitting isapparent as reported in references 15 and 16 (see footnote b ) .Previous values 283 cm-l for [Co(NH3),C1]Br2 and [Co(NH,),-Cl](NO,), and 290 cm-l for [Co(NH,),Cl]CI, (ref. 13) ; 284 cni-1for [Co(NH,),CI]Cl, (ref.14) ; 278, 284 cm-1 for [Co(NH3),C1]C1,(ref. 15) ; 279,285 cm-'for [Co(NH3),C1]Br2 (ref. 16). Thevaluegiven in ref. 12, p. 157, appears to be in error. CPreviousvalues for [Co(NH,),Br]Br, are 205 cm-l (ref. 14) and 203 cm-1(ref. 15) ; and for [Co(NH,),Br]Cl,, 201 cm-l (ref. 16). Ref. 12,p. 157, gives value 215 cm-l, anions unspecified.bridging halide ions. In the case of the bromo-complexno other information is available regarding the structure.Equilibration Processes.-Solutions of (I) ( Amax. 520 nm,E = 149 1 mol-l cm-l in ~M-HC~O,) on equilibration withHBr, concentrations in the range 0-3-1.7~, give (111)(A, 560 nm, E = 154 1 mol-l cm-l). The complex (111)I ( * ' I I I us/cm-l280 240 220 180FIGURE 1 Infrared spectra of bridging and terminal chloride incobalt(II1) complexes [(NH3),Co*p(NH,,C1)~Co(NHs),~C1,, 4.5HZO (-) ; [Co(NHS),Cl]Cl, (- - - ).The spectrum of thecomplex [Co(NH,),H,OJCl, (- * - * -), which has no bondedchloride, is also shown.Evidence for bridging bromide from infrared spectra.Spectra shown correspond to [ (NH,) ,Co*p(NH,,Br) Co (NH,) J -(ClO,), (-) ; [Co(NH,),Br]Br, (---) ; and [Co(NH,),H,O]-FIGURE 2c1, (- * - * -)rather than (11) is favoured as the dominant species insolution. Cross-over points are observed at 475 andl4 L. Sacconi, A. Sabatini, and P. Gans, Inorg. Chem., 1964, 3,1772.l 6 K. W. Bowker, E. R. Gardner, and J. Burgess, Inorg. Ckim.Acta, 1970, 4, 626.I. Nakagawa and T. Shirnanouchi, Spectrochim.Acta, 1966,22, 759.R. J . H. Clark, Spectvochim. Acta, 1965, 21, 9551974 1&5541 nm, and during the conversion of (I) into (111) (andthe reverse) a well defined isosbestic is observed a t473 nm, and a less well defined isosbestic (&4 nm) isobserved at 540 nm. Redox studies with V2+ indicateTABLE 2Pseudo-first order rate constants for the equilibration ofComplex (I) present (I) and (111); I = 2 . 0 ~ (NaC10,).initially[H+: 103[Complex] 103k,, - r ~ r - 1 - Temp."C M RI M S-'25.0 2.00 0.50 0.92 1.122.00 0.75 0.75 1.292.00 0.75 1.01 1.292.00 1.00 0.92 1-462-00 1 -50 0.51 1 *902-00 1 *75 0-75 2.152.00 2.00 0.51 2-331.50 0.50 0.90 0.891.50 1.00 0.75 1.211.50 1 *50 1.00 1-531.50 2.00 0.90 1-731 *oo 0.50 0.92 0-671.00 1 s o 0 0.89 0.961.00 1.50 0.69 1.101.00 2.00 1 so0 1.411.00 2 *oo 0.89 1.390.50 1.00 0.87 0.660.50 1 *50 0.87 0.790.50 1 -50 1-05 0.7330.0 1.50 0.50 0.90 1.791-50 0.80 0.90 2-081.50 1.00 0.92 2-311.50 1.30 0.92 2.561.50 1.50 0.86 2.851.50 3-00 0.86 3.4435.0 1.50 0.50 0.86 2.921-50 0.80 0.92 3.501.50 1.00 0.86 3.621.50 1.30 0.92 4.101.50 1.50 0.89 4-501.50 2.00 0.89 5-4040.15 1 -50 0.50 0.91 5.281-50 0.80 0.91 5.931 -50 1 -00 0.92 6.531.50 1.30 0.92 7.271 *50 1.50 0.90 8.131.50 2.00 0.90 9.102.00 1.75 1.01 9.12that there is never >6% of (11) and the amount presentmay be considerably less than this value.1° Directconversion of (I) into (111) is hardly possible and forthis reason alone intermediate formation of (11) isnecessary.The movement in the isosbestic a t 540 nmis probably due to a small build up of (11). Kineticplots of absorbance changes for the conversion (I) '-t(111) and (111) _t (I), wavelengths in the range 435-560 nm, indicate only a single rate-determining step, andsupport a reaction sequence (1) followed by the relativelyrapid equilibration (2). Figure 3 indicates the variation'OHIJ Lof absorption coefficients h = 560 nm, for equili-brated solutions of (I) with varying amounts of HBr.The value of Eobs (partially) levels off ca. I - ~ M - H B ~ , whencobs has the same value as for (111). With (HBr) > 2 . 7 ~ ,I?! n l L O t fI I I0 1 -0 2.0[HS~]/MFIGURE 3 The variation of cobs with [HBr] a t 25 "C, 1.= 560 nm,I = 2.0~1 (XaC104). The levelling off which is apparent at[HBr] = ca. 1 . 7 ~ corresponds to complete formation of (11)-(111). Data detained a t 35 (A) and 40 "C (0) are also indicatedsobs increases further, and (IV) with an absorptionmaximum at (or close to) 560 nm is formed 200 < E <400 1 rnol-l cm-l. The increase in absorption with[HBr] is similar in appearance to that reported for theaddition of HC1 to (I) (Figure 4).l It will be recalledthat in the case of chloride absorbance changes could befollowed on adjusting [HCl] in the range 2-81~1, and rateconstants for the observed re-equilibration gave ailapproximately first-order dependence on iC.1-j .I This isconsistent with the formation of the bis-chloro-analogueof (IVj.From the similarity in the two curves for HC12201 220-1000 2 L 6 8 1's[HX]/MFIGURE 4 A comparison of the variation of sobs with [HX]The changes occurring a t [HBr] 1 . 7 ~Ionic strength notand HBr in Figure 4 we conclude that the bis-bromo-complex (IV) is formed on increasing [HBr] > 1 . 7 ~ .We choose to express this further equilibration as in (3),since (111) rather than (11) is the dominant mono-bromo-species. The reaction may of course proceed by way(X- = C1- and Br-).are attributed to the formation of (IV).adjusted to constant valueof (11)1456 J.C.S. Daltonniol-l, ASl$ = -16.2 & 2.6 cal mol-l K-l, AH2$ =25.7 & 1.8 kcal mol-l, and AS,$ = 10.8 & 5.7 cal mol-lJ(IY)Kinetics of Hyn'roxo-bvidgc Cleavage.-The decrease inconcentration of (I) was monitored at 560 nm with[Br-] = 0-5-2.0~1 and [H+] = 0.5-2.0~.At ionicstrength 2 . 0 ~ the equilibrium (3) makes negligiblecontribution even when [Br-] = 2.0141, and is relativelyrapid (see later section). The ionic strength wasadjusted to 2 . 0 ~ with NaClO,, which has previously beenshown to be satisfactory for reactions of (I).l Plots oflog (O.D., - O.D.,) against time were linear to at least80% completion; values of the final absorbance O.D.,were as determined experimentally after 6-8 half-lives.Pseudo-first-order rate constants, k,,, were obtainedfrom these plots, and give a good fit to (4), where k,k,, = k,[H+j[Br-] + K2[Br-] + kband k, relate to the forward reaction in (1) and the rateconstant kb is for the back reaction and is as determinedpreviously.l*P* It has been shown that kb exhibitslittle dependence on [H+] over the range 0.5-2.0~1, andsince kb makes only relatively small contributions in theexperiments reported here the [H+]-l-dependent term isneglected.Plots of (kes - kb)/[Br-] against [H+] givegood linearity (Figure 5 ) from which k, (gradient) and(4)0 10 2 .o[H+] / MFIGURE 6 The hydrogen-ion dependence of kep after allowing forthe back reaction kb, I = 2 . 0 ~ (NaC10,); the experimentaldata give a good fit to equation (4). Number of runs whichhave been avcraged are indicated in parenthesesk, (intercept) were evaluated (Table 3) by a least-squares treatment with no weighting factor. Activationparameters for k , and k , were obtained using a non-linear least-squares prograiii~ne,~~ with each data pointgiven unit weighting, and are AH,$ = 17-3 & 0.8 kcal* Thc ratc constant ht, is It, in ref.18. I t is also cqual tok-,jks elsewhere in this papcs.1s M. B. Ste\rcnson, l i . I). Mast, and A. G . Syltcs, J . Chcnz. SOC.( A ) . 1969, 93'7.TABLE 3Summary of A, and k , values (forward reaction), and kbvalues (back reaction), I = 2 . 0 ~ (NaC10,)104kb a - Temp. 104k1 104k2"C l2 mol-2 s-1 1 mol-1 s-1 S-125.0 4.33 f 0.17 1.4 f 0.2 3.430.0 7-30 & 0.26 4.5 f 0.3 5.435.0 10.80 f 0.53 8.2 -& 0-7 8.640.2 17.62 -& 0.85 17.2 -& 1.1 13-4a From best line in graph of temperature dependence of k-l,ref. 18 ; for k , in that study read k-,/liB in this paper.K-l.Runs at 40 "C were acceptable (contrast thechloride study) because higher halide-ion concentrationscould be used without interference from (2), and underthese conditions (1) was sufficiently rapid to excludedecomposition of one or other of the binuclear complexesinvolved.Deteyminatiof$ of the Eqzdibvizwt Constant K.-Theoverall equilibrium constant K (= K&B) for equations(1) and (2) can be determined spectrophotometrically.The ratio of rate constants kl/kb is also a measure of thisquantity. Solutions of (I) were allowed to equilibratewith [H'] and [Br-] in the range 0-3-2.0~ and ab-sorption coefficients cobs were determined at 560 nm.Equation (5) relating changes in cobs with [H+] and(5) -- 1 - 1 1 1cobs - &I K(EIII - &i) [H-;][Br-] -I- -CBr-1 to K can be derived, where EI and €111 are for (I)and (111) respectively. Values of K were determinedfirst for each cobs and an average taken, and secondlyfrom a plot of (cobs - c ~ ) - l against [H-+]-l[Br-]-l (seeFigure 6).The first treatment gives K = 1.12 -J= 0.2912 niol-2 at 25 "C. The second, using a least-squarestreatment with weighting such that the line is required topass through the point [H+]-l[Br-]-l = 0 and Eobs == 155 & 3 1 mo1-l cm-l (no other weighting), givesK = 0.97 j= 0.10 l2 mol-2 at 25". A comparison of KTABLE 4Ektermination of the overall equilibrium constantK (= K a K ~ ) for equations (1) and (2) by spectro-photometric and kinetic method, 1 = 2 .0 ~ (NaC10,)Temp. K(Spec) K(Spec) k,/kb"C l2 rnoP l2 mol-2 l2 mol-226.0 1-12 f 0.29 0-97 f 0.10 1-2630.0 1-00 & 0.34 0.54 3 0-06 1.3535.0 1-21 0.27 1-08 & 0.09 1-2640-2 1.19 & 0-43 0.94 & 0.07 1.32-1verage of individual K values. Computed from graph,Figure 6, with intercept given by EIII = 155 5 3 1 inol-l cm-1.c Thc ratio kJkb from lrinctic studies gives K .from spcctrophotometric and kinetic studies is made inTable 4. The variation in temperature 25-40 "C isseen to have little or no effect on K.19 Los Alanios Report LA 2367 (1959) and Addenda by R. H.Moore and K. I<. Zciglcr1974 1457Estimation of Equilibrium Constant K,.-The extentof formation of (IV) in equation (2) is small with[HBr] < %OM, and it was not possible to determine K ,from measurements on solutions I = 2*0M as in the HC1study.We have assumed that changes in absorptionspectra with [HBr] > %OM are entirely due to theformation of (IV). Since the ionic strength was notadjusted, and concentrations of HBr up to 8~ wereused, it is necessary to take account of variation in the0 1-0 2-0FIGURE 6 The spectrophotometric determination of K usingThe points marked are for 25 (0) and 40.2 O C equation (5).(A), I = %OM (NaC10,)activities of the various species. An approximate valueof K3 was obtained assuming that activity coefficientsfor (111) and (IV) at ~O-,M concentrations effectivelycancel out, and that for bromide as with chloride (seeref. l), the activity of bromide is to a first approximationthe same as [Br-1.Hence equation (6) can be derived,where EIV is for (IV) and aw is the activity of water.Unfortunately values of ai\r are available only for the0.1 1 i'/ /V - 0.2 0.40 w m r 3FIGURE 7 The estimation of K , at 26 "C for reaction (3)from spectrophotometric measurements using equation (6)more dilute HBr solutions. Values of am for concen-trations of HBr up to 2 - 8 ~ can be calculated fromosmotic coefficients.20 Above this concentration it ispossible to base calculations of the activity of waterupon the activity of HBr as a function of concentration.21By extrapolating data given by Spreer and King222o R. A. Robinson and R. H. Stokes, Trans. Faraday SOC., 1949,45, 612:values of aiv at 25 "C have been obtained (see Table 5).A linear plot of (E,,,~ - r111)-l against aw/[Br-] is obtained(Figure 7) from which K, = ca.0.03 1 mol-l at 25 "C.From the intercept E ~ J = ca. 290 1 mol-l cm-l. At1-5 "C we estimate K , = ca. 0.02 1 mol-l.TABLE 5The formation of (IV) at high HBr concentrations(25 "C), equation (3)RI 1 mol-1 cm-1 am3.0 167 0.894.0 173 0.786.0 179 0.696.0 186.5 0.607.0 194 0.548.0 200 0.48[HBr] a Eobs 'Ionic strength not adjusted; I = [HBr]. a From best linein Figure 4. C Obtained by extrapolation of data in ref. 22.Rate of Eqztilibration K,.-Procedures similar to thosefor chloride were used. Solutions containing (IV) inS-OM-HBr at 1.5 "C were diluted to 2.OM-HBr in the hopethat it would be possible to follow the kinetics of theequilibration of (111) and (IV) at 560 nm.Volumechanges on dilution were negligible, but temperaturechanges were significant and had to be allowed for.Blank experiments with precooled solutions (using anice-salt mixture) were carried out in order to reproducethe desired final temperature (1.5 "C). The equilibrationwas complete within 3 min, so that the half-life is<30 s. Assuming the equilibration exhibits thedependence k,[Br-] + k-, (where K3 = k3/k-,), andK3 = ca. 0.02 1 mol-l a t 1.5 "C, a value > 2 x s-lmay be assigned to k , at 1.5 "C, I = %OM (NaC10,).DISCUSSIONAlthough three bromo-complexes (11)-( IV) arerelevant to this study, only one of these has beenisolated, and analyses and i.r. spectra indicate that thisis the p-amido-p-bromo-complex (111).The i.r. in-vestigation has been put on a sound basis by comparisonswith the corresponding chloro-complex which, from anX-ray crystallographic study,ll has been shown to havea p-amido-p-chloro-structure as in (111). It has alsobeen concluded that p-bromo- and p-chloro-structures(111) rather than aquo-complexes as in (11) are retainedin aqueous solution from pH measurements on solutionsof acid-free samples, from p.m.r. studies, and from thekinetics of reductions by V2+ and Cr2+.lo Furthermorekinetic plots for the conversion of (111) into (J),l and nowfor (I) into (111) give good linearity, and it is concludedthat (11) is a transient species.Attempts to isolate (11) and the chloro-analogue haveso far proved unsuccessful. A particularly promisingmethod of isolating the chloro-analogue of (11) followinga procedure used by Werner,= was thought to be the21 R. A.Robinson and R. H. Stokes, ' Electrolyte Solutions,'Butterworths, London, 1959, p. 481 ; 'International CriticalTables,' ed. E. W. Washburn, McGraw-Hill, New York, 1928,22 L. 0. Spreer and E. L. King, Inovg. Chew., 1971, 10, 918.23 Ref. 8, p. 47.vol. 3, p. 551458 J.C.S. Daltonaddition of dilute sulphuric acid to a freshly preparedsolution of the chloride salt of (111). Rapid andquantitative precipitation occurred, and analyses (H,5.75%; C1, 6.7%) were as expected for the sulphate saltof a chloro-complex containing two water molecules.The metal-halide stretching frequency of the sampleremained at 269 cm-l however, which corresponds to ap-chloro-structure as in (111) rather than (11).Some slight variation ( 3 4 nm) in the isosbestic at540 nm (but not that a t 473 nm) could be explained bythe build-up of small amounts of the aquo-bromo-complex (11).Sufficient checks have been carried outhowever which lead us to the conclusion that the build-up of (11) is unlikely to exceed 6%. Similar conclusionshave been possible also for the chloro-analogue. Otherside-reactions could alternatively account for theisosbestic shift and we do not presume that our observ-ations are necessarily an indication of the extent ofbuild-up of (11).At the time of earlier kinetic studies it was assumed,following Werner's lead, that the starting bromo-complexwas in fact (II).9 The kinetics reported can be re-assigned to the conversion (111) into (I) where the firststage involves a rapid equilibration (111) zz+= (11) [theformation constant for (11) is 1/KB], with (11) _t (I)rate-determining (rate constafits k-, and k-J.Thus at25 "C the measured rate constant for the dominant[Hi]-independent path is now the composite termk,/KR = 3.43 x s-1, and activation parameters areAH: = 16.1 -J-- 1.0 kcal mol-l and AS% = -20.7 -J-- 3-0cal K-l mol-I. For the [H+]-l-dependent path themeasured quantity is K-,KJKR, where I<, is the aciddissociation constant for the H,O ligand of (11).Our interpretation of the K, and K, terms is the sameas in the HC1 study.l Thus for k, there is rapid proton-ation of the hydroxo-bridge (7) prior to cleavage (8).1 JL JThe rate constant k , is, therefore, the product of theprotonation constant Kp and the rate constant €or (8).Since there is a good linear dependence in Figure 5(and in the corresponding figure for the chloride study)we conclude that K , < 0.03 1 mol-l. For the k , pathbridge cleavage occurs without prior protonation of thebridge. The difference in AH$ and AS: values for k ,and k , (84 kcal mol-l and 27-0 cal mol-l K-l) are similarin magnitude to those observed for HC1 (8.8 kcal rnol-land 25.6 cal mol-l K-l).l In view of this agreementthere would seem to be little doubt that k,, although itmakes a relatively small contribution, is a genuinekinetic term and is not attributable to medium effects.The formation of (IV) [equation (3)] is not effectiveuntil [HBr] > 1 .7 ~ at which stage formation of (111) isnearing completion. Formation of (111) (K = KaKB =ca. 1.1 1, mot2) is less favourable than formation of thechloro-analogue of (111) (ca. 11 1, mol-,) a t 25 "C. Theagreement of spectrophotometric and kinetic values ofK is regarded as satisfactory, particularly as rateconstants for (111) into (I) were determined in theabsence of free halide ions. Estimates of K3 for theformation of the bis-halogeno-complexes, e.g. (IV),indicate a less favourable value for bromide (ca. 0.031 mol-l) as compared to chloride (0.09 & 0.04 1 mol-1)at 25 "C. Formation of the bisthiocyanato-complex[ ( NHJ4( NCS) Co*NH,*Co (NCS) (NHJ4I3+ is much morefavo~rable,~~ and in this instance isolation and charac-terization of the product was possible.The acid dissociation constant for the aquo-ligand of(11) is not known (values ca.in01 1-1 reported inref. 318 are incorrect, see ref. lo), and the true rateconstant k-, for the [H+]-l-dependent pathway for(111) into (I) (the reverse of k,) cannot, therefore, beevaluated. It is not possible at this juncture thereforeto comment on the ratio k,/k-, and hence determine theoverall equilibrium constant K for this kinetic path. Itis, of course, still possible that the hydroxo-group of theconjugate-base form of (11) reacts slower than expecteddue to hydrogen bonding as suggested previous1y.lTABLE 6Summary of data for the interconversion of the p-amido-p-hydroxo- (I), p-amido-y-bromo- (III), and bis-bromo-(IV) complexes at 25 "C and I = 2 .0 ~ (NaCIO,) exceptwhere statedk l = 4-33 (f0.17) x lo-* AH,$ = 17.3 f 0.8l2 mol-2 s-l kcal mol-lAS,$ = -16.2 -& 2.6cal mol-1 K-1k2 = 1.4 (&0*2) x lo-, AH2$ = 25.7 & 1.81 mol-l s-l kcal mol-1AS,% = 10.8 & 5.7k- 1 y = 3.43 x 1 0 - 4 s-l a AH-,$ = 16.2 & 0.5cal mol-l K-lkcal mol-1cal mol-l K-lAS-,$ = -23.3 5 2.0k-,K,/KB = 0.0114 x lo-'1-1 mol s-l aK (=KAKB) = 1.1 l2 mol-* AH N 0 lrcal mol-lAS - 0.1 cal mol-lK-1k-3 >2 x 10-2 s-1K3 -0.03 1 mol-ld~e AH positivea From ref. 18, for k, and k2 in that study read k-JKB andk-,K,/Kx respectively in present paper.a Average of valuesin Table 4. C At 1.5 "C. dIonic strength not adjusted toconstant value. A correction has been made for wateractivity (see text). K3 -0.02 1 inol-l a t 1.5 "C.As in other previous studies there is no kineticevidence for ion-pair saturation giving a rate law with aless than first-order dependence in bromide.l* A goodlinear dependence on [Br-] is observed over the range2p S. W. Foorlg, M. B. Stevenson, and A. G. Sykes, J . Chem.SOC. ( A ) , 1970, 106419740.3--2.0~ in this study. However a dissociativemechanism is believed to be effective in this and otherhydroxo-bridge cleavage reactions of dicobalt (111) com-plexes, and ion-pairing of the incoming ligand is believedto occur. These effects are discussed in more detailelsewhere.'EXPERIMENTALSamples of the bromide salt of the p-amido-p-complexwere prepared from the chloride salt [(NW,),Co*p(NH,,OH)*-Co(NH3),]C1,,PH,0,1 by first dissolving the latter (0.86 g)in a minimum of 0.01~-HC10, (25 ml).The solution wasfiltered and saturated sodium bromide solution was addeddropwise until crystals formed. The product was filteredoff and washed with alcohol and ether; it was recrystallizedby repetition of this procedure (Found: H, 5.2; Br, 47.8;K, 1 8.7. Calc. for [ ( NH3),Co-~(NH,,OH) *Co (NH,),] Br4,-4H@: H, 5-15; Br, 47.2; N, 18.56%). In order t oconvert the v-amido-p-hydroxo-complex into the bromo-complex the former (0.3 g) was dissolved in a minimum of0.0lbi-HC10,. This solution was then filtered into anequal volume of concentrated HBr.Reddish browncrystals formed when the mixture was cooled to 0 "C; thesewere filtered off and washed with alcohol and ether (yield6976). A sample of the perchlorate salt was prepared bythe same procedure except that this time double thevolume of Concentrated HCIO, was used instead of HBr 25(yield 79:/,). A second sample of the bromide salt of thecomplex was prepared as described by Werner from thesulphate salt of the p-amido-p-hydroxo-complex. Thismethod entailed treating the complex with 47% HBr, andTABLE 7Summary of analyses (yo) and absorption coefficients(in 1 mol-1 cm-1) for bromodicobalt complexH N Br Co H,O C1Bromide saltSample 1 b 4-5 19.1 59-2 O C 155Sample 2 bid 4.1 18.7 1.53Werner 1 17.9 56.5 17.1Werner 2 e 56.8 17.0Calc.3.9 18.8 59.8 17.6(anhydrous)Calc. 2H,O 4.3 17.8 56.7 16-7Perchlorate saltSample 1 b 3.65 11.1 0 18.9 154Calc. 3.48 10.7 19.0(anhyclrous)This work. c Noloss in weight after leaving sample over P,O, for 6 days.8 Values taken from ref.9, presence of two water molecules indicated.Absorption maximum a t A = 560 nm.Prcparetl by Werner method, ref. 9.then recrystallizing the product by dissolving it in aminimum of ice-water and adding to the solution a thirdof its own volume of 47% HBr. The samples had analysesconsistent with their being the p-amido-p-bromo- and notthe aquo-bromo-complex (see Table 7).*5 R4. B. Stevenson and A. G. Sykes, J . Chem. SOC. (A), 1969,2979.26 See comments in R.S. Taylor and A. G. Sykes, Chem. Comm.,1969, 1137 and J . Chewz. SOC. ( A ) , 1969, 1425, regarding the isola-tion of the p-nmi tlo-p-chloro-complex as the perchlorate salt.A sample of the chloride salt of the p-anlido-p-chloro-complex was obtained by the procedure used elsetvhere.1*26Crystallization from BM-HCL a t 0 "C was slow (severalhours). The product was converted into the p-amido-p-hydroxo-complex [ (NH,) ,Co*p(NH,,OH) Co(Pu'H3) JCl,, 4H20as described previously. To prepare the perchlorate saltof the p-amido-p-chloro-complex the chloride salt of thep-amido-p-hydroxo-complex was dissolved in a minimum of0-01~-HC10, at 40 "C, and added t o twice its own volume ofconcentrated HC10,.26 Crystals were obtained on coolingthe solution to room temperature and then to 0 "C.Analyses figures and details of the spectra consistent withformulae [ (NH,),Co.p(NH,,CI) *Co( NH,) JCl,, 4.5H20 and[(NH,),Co~p(NH2,C1)Co(NH,),](C1O,),,H,O are given inref.1. There is some uncertainty as to whether the chloridesalt contains 4 or 5 water molecu1es.lSamples of the mononuclear complexes [Co (KH,) ,XI X,and [Co(NH,),X](ClO,), (X- = C1 and Br-), were preparedfrom penta-amminecarbonatocobalt (111) nitrate by standardproced~res.~~ The purity of solutions in O-~M-HCIO, waschecked against known visible absorption maxima, E =49.0 1 mol-l cm-l a t 1. = 533 nm for Co(NH,),C12+ andE = 53 1 mol-l cm-" at A = 551 nm for Co(NH,),Br2f. Thecomplex [Co(NH3),H20] (ClO,), was also prepared frompenta-amminecarbonatocobalt(~x~) nitrate (further detailsas in ref. 28).All reaction solutions were made up with triply distilledwater. Spectrophotometer cells were rinsed with triplydistilled water and dried over silica gel in a desiccator.Hydrobromic acid (Hopkin and Williams 46--48% AnalaRsolution) was purified by distillation after addition ofhypophosphorus acid to react with free bromine.21 Solu-tions were stored in darkness a t 0 "C. Kinetic experimentswere followed at the 560 nm maximum of the aquo-bromo-complex. When the concentration of bromide is increasedfrom 0 to 2-0M by the addition of sodium bromide, theabsorption coefficient of the p-amido-p-hytlroxo-complexincreases (linearly) from E = 117 to 124 1 mol-l cm-1; byincreasing the concentration of hydrogen ions from [H+] = 0to %OM E increases from 117 to 118 1 mol-l cm-l. Suchvariations were taken into account in all calculations.Solutions of perchloric acid and sodium perchlorate(B.D.H. AnalaR), and sodium bromide (B.D.I-3. LaboratoryReagent) were made up in triply distilled water. Thelatter was obtained by redistilling singly distilled waterfirst from alkaline potassium permanganate and thenacidified potassium dichroniate.High-resolution i.r. spectra 400-40 cm-1 were recordedon a single-beam Beckman-R.I.I.C. model FS 720 ititerfero-meter with a coupled wave analyser. Wax discs wereprepared, and spectra recorded using a black Polythenefilter.S. W. F. is grateful to the University of Ixeds for theaward of a Scholarship. We thank the Physico-ChemicalMeasurements Unit a t Harwell for recording infraredspectra.[2/2766 Received, 8th December, 1972127 See e.g. S. W. Foong, B. Kipling, and A. G. Sykes, J . Chem.Soc. ( A ) , 1971, 122.28 J..Doyle and A. G. Sykes, J . Chem. SOC. ( A ) , 1968, 2839.29 G. Bauer, ' Handbook of Preparative Inorganic Chemistry,'Academic Press, London, 1963, vol. 1, p. 288
ISSN:1477-9226
DOI:10.1039/DT9740001453
出版商:RSC
年代:1974
数据来源: RSC