Dash, A. C.

Dash, M. S.

English

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IDdl ••• .10Ul'llal of ChemistryVol. 15A, April 1917, pp. 300-302Kinetics & Mechanism of Base Hydrolysis of Malonato &Succinato Complexes of Pentaamminecobalt(III) IonM. S. DASHDepartment of Chemistry, B.J.B. College, Bhubaneswar 6andA. C. DASH"Department of Chemistry, Utkal University, Bhubaneswar 4Received 21 April 1976; accepted 8 September 1976The base hydrolysis reactions of malonato and succinato complexes of pentaammine-cobalt(III) ion, [(NH3).CoCO,(OHs)nCOt]+ (n = I, 2), have been investigated at 30°, 35° and 40°in a medium of I = 0·3M (NaCl). At [OH-) = 0·02-0·2M, the rate law for the reaction is givenby -d In [complex)/dt = kl[OH-]. The values of the second order rate constant (in M-l sec=)at 30° and the associated dH: (in kcal/mote), ~S: (in e.u.) are 2·86(±0·08) x 10--, 29·2(±1·2),22(±4) for malonato complex and 1·18(±0·04) x 10-', 34·2(±3·5), 36(± 11) for succinato complexrespectively. The rate and activation parameters have been compared with the analogous datafor IOxalato, maleato and fumarato complexes of, pentaammioecobalt(III) [catfon, The resultsare in accord with the rate limiting Co-O bond fission of the ammine conjugate base[(NH.)(NH.).CoCO.(CH.)nCO,).EARLIER studies from this laboratory dealtwith the kinetics of aquation of pentaamrnine-cobalt(III) complexes of the dicarboxylicacids->. However, the kinetic data did not provideevidence in favour of the in tramolecular acidcatalysis due to the unbound carboxyl group in theaquation reactions of the complexes. It was, there-fore, thought worthwhile to examine the effect ofthe unbound carboxylate function on the rate andmechanism of base hydrolysis of the dicarboxylicacid complexes of pentaamminecoba1t(III) ion.The alkaline hydrolysis of oxalato, maleato andfumarato pentaa.mminecobalt(III) complexess-s havebeen studied earlier. We report in this paper thekinetics of base hydrolysis of the malon ato andsuccinate complexes of pentaamminecobaltrl H).Materials and MethodsAll the chemicals used were of an alar or extrapurequality. Solutions were prepared in distilled waterfrom a copper still. The spectral measurementswere marie in a Beckman DU 2 spectrophotometerwith 10 mm matched silica cells. The cobalt(III)complexes were prepared in acid form by thepublished method" and gave satisfactory elementalanalysis. The malonate and succinate complexesshow maxima (in nrn) and corresponding molarabsorbancy indices (in M-I em") in O'lM HCI04medium at 505(74'9), 350(58·2) and 505(68·2), 355(54·8) respectively.Rate measurements - The kinetics of base hydro-lysis of the cobah(III) complexes were investigatedat 30°, 35° and 40° at an ionic strength of 0·3Madjusted with sodium chloride. The concentrationof the complexes was varied in the range 7'6-9·9*To whom all correspondence may be directed.300x 10-4M. Reaction was initiated by adding 2 mlof a stock solution of the complex (at the reactiontemperature) to the pretl.errnostated reactionrnixture. The rate of base hydrolysis was followedby withdr awir g 5 ml of the reaction mixture atregular time intervals quell ching the reaction byadding dil. hydrochloric acid to PH 1 and thenmeasuring optical density at 260 nm at which thecarboxylatopen taarnminecobaltfl H) complexes arethe only significantly absorbing species. The re-action could be followed satisfactorily up to 40%completion beyond which the decomposition ofhydroxopen taamminecobalt (III) ion to insolublecobalt(III) oxide was sigriifican t an d the reactionmixture even after acidificatior- did rot yield clearsolution for the optical density measurement. Thepseudo-first order rate constan ts ('kobs) were, there-fore, calculated by the method of initial rates fromthe gradients of log D, versus time plots where D,stands for the optical density of the reaction mixtureafter acidification at time t. Each run at a givenalkali concentration was repeated 3 to 5 times.Results. ~h~ second orrl er ra.t;e.c.onstants (kl) obtained bydividing kobS WIth the in itial [OlIo] are presented inTable 1.pKa of the free carboxyl group of Hsmalonatoand H-succinato complexes is ~4 (ref. 2). Assuch both the complexes will exist as monopositivecations, (NH3)sCoC02(CH2)"CO!. in the alkalinemedium employed in tl.e kinetic study. A perusalof the rate data presented in Table 1 indicates nosystematic variation of the secon d order rateconstant (kI) with COHo] in the range 0·02-0·2M.The rate law for base hydrolysis can be best repre-DASH & DASH: BASE HYDROLYSIS OF .PENTAAMMINECOBALT(III) IONTABLE 1 - RATE DATA FOR BASE HYDROLYSISOF(NH3).COU3-II» AT I = 0'3M (NaCI)[OW]M10'k. (formalonate"}(M-' sec=)10'k. (forsuccinare=)(1'11-1 sec+)0·020·050·100·2030'OO(±0'1°)2·84(±0·14)3·0S(±0·12}2·92(±0·16}2·73(±0·lO}Av 2·86(±0·08)*3S.00( ±0'1 0)s·61(±0·2s}6·69(±0·10}6·88(±0·10}7·26(±0·s8)Av 6'70(±0'19)*4O·00(±0·1°)13·0(±0·6)13·6(±0·s)ls·0(±1·3)15-4(±0·S)Av 13'8(±0'S)*7·06( ±0·4s)7·7S( ±0·42)7·79(±0·30)9·30(±0·40)Av 7'99(±0'43)*1·37(±0·12)1-18(±0·04)H2\±0·08)Av H8(±0'04)*0·020·050·100·202·82( ±0·31)2·83(±0·22)2·6S(±0·30)H9(±0·31}Av 2'72(±0'08)*0·020·050·100·20*Weighted mean value,sent ed as shown in Eq. (1)-d In Ctldt = kObS = k1[OH-] ••• (1)where C, is the total concentration of the complexat time t and kl is tile second order rate constantfor the reaction (2)kl{NHahCoC02(CH2)IICO; +OH- ~ (NHa)sCoOH2++(CH2)II(COi)2 .. ,(2)The rate law (1) has also been found to be valid inthe base hydrolysis of maleato and fumaratocomplexes of pentaamminecobalt(III) cation at(OH-] ~ l'OM (ref. 4) a.id also for the corres-ponding oxalato complex at [OH-] ~ 0'1M (ref. 3)The average values of kl given in Table 1 were ob-tained by the weighted least squares ~ethod.Weighting factor for kJ was taken to be the reciprocalof its variance. The activation parameters werecalculated from the least squares gradients andintercepts of In (kiT) versus lIT plots, the f<;>rmerbeing weighted as the reciprocal of its var.lance,which was calculated from the known vanancesof the average value of kl and T (Table 1). Calcu-lations were performed by the least squares computerprogrammes adopted to an IBM 1130 computer. Thevalues of activation parameters are given ill Table 2.DiscussionThe rate and activation parameters for the basehydrolysis of oxalato, malonate, succinate, maleatoand fumarato complexes of pe.i taamminecobalt(III) ion have been collected in Table 2 for the sakeof comparison. The rate data for the latter twocomplexes (at I = 1M NaCI) have been correctedto 1= 0'3M by co.isider ing the retarding influ~nceof io.iic strength 0:) the rate of base hydrolysis ofthe oxalato complex (see Table 2). This metho.d-of ionic strength correction of the rate constant ISTABLE 2 - RATE AND ACTIVATIONPARAMETERSFOR THEBASE HYDROLYSISOF (NH.l.CoU3·n)+Ln· 10'k. (30°) t.H! t.St Ref.(M-1 sec-1) kcal/mole e.u.Oxalate> 6·2 (8·3) 31·7 (33-4) 32 (37) (a)Malonate+ 2·86±0·08 29·2±1·2 22±~ This workSuccinate!" 1·18±0·04 34·2±3-5 36±11 This workMaleate+ 0·57. 28·5 18·3 (a)0·76(b)Fumarate'? 1'58, 28·0 14·4 (a)2·11 (b)(a) I = 1M (NaCl) (ref 4). Values given in parenthesesrefer to I = 0'3M (NaCIO,) (ref. 3).(b) Ionic stre-ngth corrected to 0·3M by the relationship:k(l=O·3M)=k(I=lM) X [k(I=O·3M)/R(I=lM)]oxalate.reasonable as the rate illflue_:cillg effect of Cl- rela-tive to that of C104"ill .t~e base hydrolysis reactionis expected to be negligibles. The order of reacti-vities of the complexes as evidenced from the valuesof kl at 30° is oxalate > malonate > fumarate >succinate > maleate. The rate decreases as thecarbon chain intervening the free and the cobalt(III)-bound carboxylate groups increases. This isin accord with the fact that the electron withdrawingeffect of the un bound carboxylate anion influencesthe rate. The reactivity sequence fumarate >succinate > maleate also indicates that the orienta-tio.i of the free carboxylate anion with respect tothe cobalt (IJI)-bound carboxylate group has someeffect on the base hydrolysis rates. Tracer studieshave indicated that oxalatopentaamminecobalt(III)undergoes base hydrolysis via both Co-O and C-Obond fission, the latter arising out of a third orderprocesss->, The fo-rmer mechanism. however. ope-rate.s predominantly at [~H-J~ O·lM(ref. 3. 4). Theco.ijugate b~,>e mechaills~ with rate limiting Co-Obo.id breaking also applies to the base hydrolysisof maleato and fumarato complexes under consi-deration+The conjugate base mechanism may be depictedas shown in Scheme 1.QCB(NH3hCocO.RCO. ++OH- ~(NH.) (NH.).CoCO.RCO. +H20(CB)Ifast ~ kCB(NH3lsCoOH2+ -- H20 + (NH.)(NH3).CoI+ + R(CO.-).Scheme 1For all dicarboxylic acid complexes under con-sideration (see Table 2) a consta.i t value of QCBappears reasonable. The se.rsitivity of kl to thenature of the leaving group would then indicatehow both electronic and steric effects of the latteraffect the reactivities of the conjugate bases (sincek1 = .kC.BX QCBl. . ~he Charge of the leaving groupsremanung same It IS the Co-O bond weakening effectof the unbound carboxylate group which will governthe reactivity sequence of the conjugate bases ofoxalate, malonato, and succinate complexes. Theobserved reactivity sequence oxalate > malonate301INDIAN J. CHEM., VOL. 15'\. APRIL 1977> succinate supports this fact. The dissociativetransition state of the reactive conjugate base maybe visualized to be an ion-paired complex. [(NH2)(NH3)4C02+... (-02C)2R] and as such kCB (and hencek1) will decrease as the electrostatic in teractionbetween thel caving group and the cobalt(IlI) centrewill increase when the group R is varied. The cisand trans conformations of the carboxylate anionsin maleato and fumarato complexes, restriction torotation of the carboxylate groups about C=C bondin these complexes and absence of such factors inthe succinato complex indicate that the electro-static interaction between the carboxylate ai ionsand the cobalt(III) centre in the transition statewi.ll decrease in the order maleate > succinate >fumarate. The reverse order of reactivities[i.e. fumarate> succinate> maleate) observed forthese complexes thus lends support in favour ofthe dissociative transition state of the conjugatebases.Tr e activation enthalpies for oxalato (Co-O fissionpath), malonato, and succinate complexes are verymuch alike and are slightly higher in magnitudethan for fumarato and maleato complexes (Table2). The activation en tropies are large positive.302LlSt, however, decreases in the order oxalate >malonate ,......,succinate > maleate > fumarate.Much significance cannot be attached to theactivation parameter data without knowing thevalues of Lllli and LlSi for the conjugate baseequilibria. But it is worth notirg that the basehydrolysis reaction of the complexes under con-sideration (Table 2), like that of several other(NHs)5COX2+ complexes", are both enthalpy andentropy controlled. The effect of higher activationenthalpy is counterbalar.ced by large positive LlSifactor. Such a compensatory effect may be diag-nostic of a common rate limitirg step2.7.References1. DASH, A. C. & NANDA, R. K, Lnorg, Chem., 13 (1974),655.2. DASH, A. C. & NANDA, R. K, J. inorg, nucl . Chern .• 38(1976), 119.3. DASH, A. C. & NANDA, R. K, J. Indian chem. Soc., 52(1975), 298.4. ANGERMAN, N. S. & JORDAN, R. B., Inorg, Chem., 6(1967), 1376.5. JORDAN, R. B. & TAUBE, H., I Am. chem. Soc., 86 (1964),3891.6. BASOLO, F. & PEARSON, R. G., Mechanisms of inorganicreactions (Wiley, New York), 1967, 180.7. LEFFLER, J. E., J. org, Chem., 20 (1955), 1202.

Kinetics & Mechanism of Base Hydrolysis of Malonato & Succinato Complexes of Pentaamminecobalt(III) Ion

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Kinetics & Mechanism of Base Hydrolysis of Malonato & Succinato Complexes of Pentaamminecobalt(III) Ion

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