Indian Journal of ChemistryVol. 23A, April 1984, pp. 312-320

Kinetics & Mechanism of Acid, Base & Mercury(II) Ion Catalyzed Hydrolysesof cis-(Isothiocyanato)bis(ethylenediamineXimidazole)coba1t(III) &

cis-( Azido) bis(ethylenediamine Ximidazo le)co ba1t(III) Cations

ANADI C DASH· & BRUNDABAN MOHANTYDepartment of Chemistry, Utkal University, Bhubaneswar 751004

andSURESH K MOHAPATRA

Department of Chemistry, Regional College of Education. Bhubaneswar 751007

Received 2 August 1983; accepted 18 November 1983

The kinetics of the title reactions have been investigated in aqueous perchlorate medium of 1=0.3 mol dm -3. The rate andactivation parameters for the aquation reaction: cis-[CoX(imH)(en),Jn + + H20 ->cis-[Co(OH ,XimH)(en)'ll+ + x(n.31 + [X= NCS , N, (n = 2): NCSHg +, N,Hg + (n = 4)J and also for the acid-catalyzed aquation of the azido complex, cis-[Co(N3)(imH)(en)2J2 + + H + + H20->cis-[Co(OH2)(imH)(en)'J3 + + N,H. are reported at 60-80°C. The Hg(lI)-adduct, cis-[Co(N,Hg)(imH)(en),JH is _103 times thermodynamically less stable than its S-bonded analogue, cis-[Co(NCSHg)(imH)(en),J4 +; but the former undergoes water substitution at the cobalt(II1) centre much faster than the latter.pH titration at 25"C (I =0.3 mol dm -.1) yields pKNIl= 10.3±O.I and 10.6±O.I for the coordinated imidazole in the case of N,and NCS complexes respectively. The overall base hydrolysis of both the complexes (0.002 .;;[OH -J .;;0.2 mol dm 3. 35-50T)reveals a two-term rate law which is interpreted in terms of the aquation and base hydrolysis of the imidazole(I-) conjugatebase. cis-[CoX(im)(enl,J + . The rate and activation parameters for both the paths are reported. The rates of aquation of thecomplex species are found to increase in the order: [CoX(im)(en),J + > [Co(XHg)(imH)(en),]4 + > [CoX(imH)(en)'f + (X = N,or NCS -). The coordinated imidazole(I-) species is found to labilize Co - X bond -10' limes stronger than imidazole.

Recent studies on the hydrolysis of cis-[CoX(imH)(en)2]2+ (X=CI, Br)l, rx-cis-[CoCI(imHXtrien)]2 + (trien = triethylenetetramine)?and cis-[CoCI(imH)(pn)2]1 + (pn = 1,2-diaminopropane):' cations revealed that NH-deprotonated imidazole( 1-) species aquate - 103 timesfaster than the corresponding acid analogues. Similarrate contrast has been observed in the aquation of cis-[CoX(bzmH)(AA1)] 2 + (X =CI, Br; bzmH= benzimidazole; AA =en or pn)3A and thecorresponding NH - deprotonated benzimidazole(I-)species. The reactivity difference by a factor of - 103

has also been reported for the aquation reactions of apair of cis or rrans-[CoCIA(en)JIl + cations whichdiffer by one unit of positive charge" H. Thebenzimidazole( 1-) species, cis-[CoX(bzmXen)2] + (X= CL Br), however, were found to aquate slower" thanthe corresponding hydroxo complexes, cis-[Co(CI;BrXOHXenhJ ". In order to clarify thelabilizing action of the imidazole( 1-) and imidazolemoieties, and with a view to understanding the S NIcbmechanism of the base hydrolysis ofacidopentaminecobalt(lII) complexes more kineticdata on the aquation of cis-[CoX(imXen)zJ + speciesare necessary. The title investigation is an effort in thisdirection. The Hg(II)-catalysed aquation has beenstudied in order to assess the sensitivity of the aquation

312

rates of the complexes towards charges in the leavinggroups.

Materials and MethodsThe cis-(chloro )bis(ethylenediamine )(imidazole)-

cobalt(III) chloride/perchlorate were prepared asdescribed earlierl.

cis [Co(NCS'I.JmJ-fj.J'nh]( CIa4)2Perchlorate salt (4.8 g) of the chloro complex was

dissolved in hot water (pH = 3, 70 C) and treated withNaNCS (1.2 g). The reaction mixture was maintainedat 70C for 3 hr when the purple colour of the solutionchanged to red. It was then cooled to roomtemperature. On adding HCl04 (70''',) and furthercooling the content in an ice-bath, an orange red solidseparated out which was recrystallized from aq.HClO,t. The crystals were washed with ethanol, etherand stored over fused calcium chloride (Found.Co,12.0; NCS, 11.2; ,19.6. [Co(NCSXimHXenh](CI04hrequires Co, 11.7: CS, 11.5: N, 19.4~';,).

cis-[ CoN ](im fIX en) J(NO .\)2/Cl2cis-[CoCl(imHXenh]Cli (lOg) was dissolved in

minimum volume of hot water and sodium azide (2 g)added to it. The mixture was heated to 70C for a fewhours when the colour of the solution changed to deeppurple. Concentration of the reaction mixture

DASH et al.: HYDROLYSIS REACTIONS OF c~nnCOMPLEXES

followed by the addition of cone. HCI and cooling thecontent to O°C yielded the chloride salt of the desiredcomplex which was recrystallized from aq. HCI. Thechloride salt was converted into nitrate bycrystallization from aq. nitric acid. The azidobis(ethylenediamineXimidazole)cobalt(IIn dinitratewas purified by repeated crystalIization from aq. nitricacid. The glistening deep purple crystals were colIectedon a sintered glass funnel, washed free of nitric acidwith ethanol followed by ether, air-dried and storedover fused calcium chloride (Found: Co, 14.1; C, 19.6;H, 5.1; N03-, 29.8. [CoN3(imHXen)z](N03h requires:Co, 14.3; C, 20.3; H, 4.8; N03-, 30.0~{,).

Physical measurementsThe UV -visible spectral measurements were made

on a Beckman DU 2 spectrophotometer. IR spectrawere recorded in KBr phase on a Perkin-Elmer 398infrared spectrophotometer. PMR (90 MH:i) spectrawere recorded on a Perkin-Elmer R 32 NMRspectrometer in D20 medium using TMS as anexternal reference. pH measurements were made withECIL (India) pH meter model 5651. Standard buffersof pH 4.0, 6.86 and 9.2 were used for calibration of thepH meter. Sodium perchlorate (Riedel) and perchloricacid (AR) were used for adjusting ionic strength andacidity respectively.

Characterization of complexesThe isothiocyanato complex, cis-

[Co(NCSXimHXenh](CI04)z, exhibited Ama. at 310 (s,1920 ± 18) and 492 nm (s, 265 ± 5 dm? mol-I cm -I)(for literature support see ref. 9). A strong band at 2120em -I (vC - N) and a band of medium intensity at 842em -I (vC -S) corroborated the presence ofN-bondedthiocyanate=!", The ()(NH) band of the coordinatedethylenediamine appeared at 1610 (sh) and 1585 em -Ias a split band consistent with the cis-stereochemistryof the complex 11. This is further supported by themethylene proton signal of the ethylenediamine ring at152·.9. The presence of imidazole I2.13 was indicated bythe IR bands at 1555 and 1515 cm -I and the C-Hproton signals at b 7.2, 7.4 and 8.1. Of the three bandsat 870, 885 and 900 em -I the first one could beattributed to the imidazole ring stretching model 3 andthe other two to- the CH2 rocking mode of thecoordinated ethylenediamine, characteristic of the cis-[CoX(amineXenh]2+ cations 1.I 1.14.

The UV -visible spectra of the chloride and nitratesalts of the azido complex displayed maximumabsorption at 508 (/:,300) and 305 nm (/:,8040), whichmay be compared with the analogous data for the cis-and trans-[CoN3(NH3)(en)d2+ and[(NH3)5CoN 3]2 + (see ref. 15). The IR spectrum of thechloride salt of the azido complex displayed a strong

band at 2030 em -I due to the Co - N 3 group I 6 , bandsat 1590 and 1605 em -I (sh) characteristic of NH 2-deformation mode of the coordinated ethylenediamineexpected of the cis-isomer I I and bands of mediumintensity at 1550, 1520, 1450 cm -I diagnostic of thecoordinated imidazole 13. Bands at 870, 880 (sh) and900 em -1 were also observed similar to those for theNCS-complex. PMR spectrum of the nitrate salt of theazido complex displayed signals at 15 7.4, 7.65 and 8.35for the protons of imidazole moiety bound tocobalt(III) centre and at 3.08 for the NCH2CH2Nprotons of the coordinated ethylenediamine. Thesignal for the NCH 2C1:I2N protons of the cis-bis(ethylenediamine)cobalt(III) complexes have beenreported I 1.12.17 to occur below 153.0. This signal forcis-[CoCI2(enh] + and cis-[Co(CN)2(en)z] +, however,have been reported+" to occur at 153.2. Based on theUV-visible, IR and PMR data we tentatively assigncis-configuration to the complexes underinvestigation.

KineticsThe kinetics of acid, base and Hg(In-catalysed

hydrolysis of the complexes were studied spectro-photometrically under pseudo-first order conditions at1=0.3 mol dm -3 (CI04-). In the aquation studies thedecrease of absorbance due to the complex, cis-[CoX(imHXenh]2+ (or its Hg(In-adduct for X= NCS), with time was followed by the batch samplingtechnique? at 310, 370 (spontaneous aquation), 290(Hg(II)-catalysed aquation) (X = NCS) and340 nm (H +- and Hg(II)-catalysed aquation'(X = N 3)'The concentrations of the complexes were adjustedbetween 1.3 x 10 -3 and 3.2 x 10 --4mol dm -3 to yield0.4-0.7 units of initial absorbance (Ao). Thespontaneous aquation "of the NCS-complex wasextremely slow and only < 10% of the aquation couldbe followed; the Hg(II)-catalysed aquation was,however, folIowed upto 12-50% at 60-80°C. The H+-and Hg(In-catalysed aquation of theazido complexwas folIowed to the extent of 10-50% under theexperimental conditions (see Tables I and 2). Theinfinity absorbance (A "',) was determined as folIows.The cis-[Co(OH~imHXenh]2 + cation was generatedby the Hg(In-catalysed aquation of the correspondingchloro complex, adjusting the composition of thesolution as appropriate to the kinetic run and thenmeasuring the absorbance (Aoo) at the appropriatewave length. The base hydrolysis reaction was studiedat 0.{)()2~[OH -]T~0.2 mol dm -3 (30-50°C) with[complexJT=(2-3) x 10 --4 mol dm -3. The thermalIyequilibrated reaction mixtures were withdrawn (5 cnr')at known time intervals, acidified to pH 1 with HCI04and the absorbance was measured at 31O(X = NCS) or335 nm (X = N 3)' The absorbance-time data (A I-t) for

313

INDIAN J. CHEM., VOL. 23A, APRIL 19S4

Table I-Rate and Activation Parameters for Spontaneousand Acid-catalysed Aquation of cis-[CoN3(imHXen)2]2 +(a)[H +] 106kotJ.s -I) atmol dm "?

75.0±O.l°C1.71±0.21(b)2.553.60±0.IS(b)4.45 ± 0.21Ib)6.35 ± O.l5(b)

0.75±0.05IS.6±0.3

SO.O±O.I°C3.154.60±0.2O

DASH et aJ.: HYDROLYSIS REACTIONS OF Co(lIl) COMPLEXES

data for the Hg(II)-catalysed aquation (see later),however, indicated appreciable binding of the azidocomplex by Hg(II). Apparently, therefore, the lack ofspectral evidence for the species CoN 3Hg4 + isattributed to the absorption coefficient of this adductbeing equal to that of the azido complex in thewavelength range studied.

Determination of pKNHThe azido and isothiocyanato complexes were

titrated potentiometrically against 0.1 mol drn >'NaOH at 25°C. The pH titration data for the acidifiedcomplex solution (jcomplex], = [HCI04]T = 0.003, 1=0.3 mol dm -3) when compared with those of theHCI04 solution of identical acidity and ionic strengthclearly revealed that the ionization equilibrium (Eq. 3)is significant at pH >9.

KNHcis-[CoXXimHXenh]2+ ~ cis-[CoX(im)(en)z] +

+H+(X = N3-, NCS -) ... (3)

The values of pKNH as calculated" from the pH data inthe range 9-11 using Kw=1.0x 10--14 mol? dm :" (1=0) and fw =Is« =0.71 (ref. 19) are 1O.3±0.1 and10.6±0.1 (25°C, 1=0.3 mol dm --3) for the azido andisothiocyanato complexes respectively.

Results and Discussion

Spontaneous and Hg(lJ)-catalysed aquation ofcis-[Cd..,NCS'/.JmHXenh]2 +

The spontaneous aquation of cis-[Co(NCSXimHXen)z]2+ was studied in 0.1 mol dm -3HCI04 medium (1=0.3 mol dm --") at 75", 80° and85°C. Test for Co(II) by Kitson's method+? wasnegative in the aquation reaction. The stoichiometry ofthe spontaneous aquation may. therefore, berepresented as

kaqcis-[Co(NCSXimHXenh]2+l-iO cis-[Co(OH2)

2

(imHXenh]3+ + NCS - ... (4)The values of 106 kaq (s -I) are 1.3 ± 0.2, 2.4 ± 0.2 and4.1 ±0.2 at 75°, 80° and 8SoC respectively which yi-elded LlHt = 111 ± SkJmol-1 and LlSt = - 39 ± 15JK-1mol -I. These data compare well with the analogousdata for the isothiocyanatopentaamminecobalt(III)ion [kaq (80°C) (s -I), LlHt (kJ mol -I), LlSt (J K -Imol :"): I.2xI0-6, 126, -3 respectively] 5. Theaquation of the isothiocyanato complex is catalysed byHg(II), the Hg(II)-adduct, cis-[Co(NCSHgXimHXen)z]4+, being the catalyticallyactive intermediate. The stability constant of the

Hg(II)-adduct (7.7 x 104 dm ' mol:' at 29.5cC and 1=0.3 mol dm -3) is comparable to that of[(NH3)5CoNCSHg]4+ (9.8 x 104 dm ' mol ? at 25°Cand 1= 1.0 mol dm -3)21. The Hg(II)-catalysedaquation of the isothiocyanato complex was studied at60c, 70° and 80°C with [complexj-v- Ll z x 10-4,[Hg2+]T=O.OI and 0.03 and [H+] =0.1 mol dm -3 (1=0.3 mol dm -3). The pseudo-first order rateconstants were independent of [Hg2 +]T andsignificantly higher than kaq which, consistent with thestability constant data, indicate that the binuclearspecies, cis-[Co(NCSHgXimHXen)z]4+, is the aquat-ing species.

kcis-[Co(NCSHgXimHXen)z]4+ ~ [Co(OH1)

H20

(imHXenh]3+ +HgSCN -I- ••• (5)The values of 105 kHg (s -1)=0.86±0.OS (60°C), 1.94± 0.13 (70°C) and 3.70 ± 0.30 (80°C) yielded LlH;=69.0±2.1 kJ mol :' and LlSt= -136±6 JK-I

mol-I. The rate acceleration kH,Ikaq = 10 at 80°C(- 220 at 25°C) is predominantly due to significantlowering of the activation enthalpy [LlHt(kHg)-LlHt(kaq)= -42 kJ mol :"] despite the lessfavourable value of the activation entropy [LlSt(kHg)-LlSt(kaq)= -97 JK -I mol "}, The differentialdegree of solvation of the transition states of theaquation of the isothiocyanato complex and its Hg(II)adduct may be judged from the calculated value of [S*(RNCSHg4+) - S* (RNCS)2+] from Eq. (6)

S*(RNCSHg4 -1-) - S*(RNCS)2 -I- = LlSt(kHg)- LlSt(kaq) + LlSo+ SO(Hg;q+) ... (6)

where S*(RNCSHg4+) and S*(RNCS2 +) stand for thetransition state entropies of the binuclear species andthe thiocyanato complex respectively, ASG= entropychange associated with the adduct formationequilibrium (Eq. 1), and SO(Hg;q+)= aqueous entropyof Hg;q+ . The value of SO(Hg;q+) is - 22.6 JK -I mol-I(2SCC)22. The value of LlSoand the activation entropiesCASt terms) being known we obtain [S*(RNCSHg4+)- S*(RNCS)2+] = - 174 ± 37 JK -I mol >". This valuemay be attributed to the relative degree of freezing ofsolvent around the 1d transition states of the aquationprocess (i.e. R 3 -I- •.. NCS - and R 3 + ... NCHg "),

Spontaneous, acid and Hg(lJ)-catalysed aquationof cis-[ CoN 3{imH)(en)z] 2-1-

Unlike the isothiocyanato complex, the azidocomplex undergoes aquation via spontaneous (ko) andacid-catalysed (k I) paths. The kobsversus [H -1-] plots inthe range O.OSs; [H +] mol dm -3 ~ 0.3 are linear withpositive intercepts and slopes in accord with the ratelaw:

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INDIAN J. CHEM., VOL. 23A, APRIL 1984

-din [CoN3]T/dt =k-s; =ko+kl[H "l ...(7)The values of ko, k 1 and the associated activationparameters are collected in Table 1. It is worth notingthat both NCS and N 3 complexes undergospontaneous aquation at comparable rates (ko(NCS)/ko(N3)= 1.7, 1.7 and 1.4 at 75-, 80° and 85

cCrespectively). This is in accord with the fact that the Co- X bond that is broken is the same in both the cases.The observed acid catalysis for the azido complex is,however, due to the fact that the coordinated azide is amuch stronger base (PK =4.65 and -0.9 for N3H andHNCS at 25cC respectivelyr':' than the coordinatedthiocya~ate. The importance of the basicity ofN 3-andNCS - IS also reflected in the activation parameterdata: AH!(ko path) for the azido complex is - 20 kJ~o~ -I ?igher than that for the isothiocyanato complexindicating greater energy requirement for the cleavageof Co - N3 bond as compared to Co - NCS bond. Theenergy requirement is, however, compensated by theentropy factor [AS!(ko path) = + 29 ± 15 and - 39± 15 JK -I mol "! for N3 and NCS complexesrespectively). The activation parameters for thespontaneous aquation of [CoNiNH3)s]2 + (AH!= 141 kJ mol :", AS! = + 53 JK-1 mol :")" are alsosignificantly higher than for the aquation of[(NH3)SCoNCS]2 + (quoted earlier). The activationenthalpy and entropy data for the second order H +-catalysed aquation path of the azido complex arecomparable with the analogous data for several acid-catalysed aquation reactions of coba1t(III) com-plexes+?". The low valueof AS! = -7 ± 8 JK -I mol " 'indicates that structural rearrangement at thecobalt(III) centre is probably not involved in thetransition state of the acid-catalysed path.

The Hg(II)-catalysed aquation of the azido complexwas studied over the temperature range 65-85~C andthe relevant data are collected in Table 2. Under theexperimental conditions, the rate data fit the rate law.

-dln[complex] /dt =k = k~bs+kugK[Hg2+] (8)T obs 1+ K[HgH] ...

where kobs=k~bs (=ko+kl[H+]) at [Hg2+]=0 andkUK' K are the rate constant of aquation andequilibrium constant of formation of the binuclearspecies, cis-[Co(N 3Hg)(imH)(enhJ4 + respectively:

cis-[Co(N3)(imH)(enh]2 + + Hg2 + ~cis-[Co(N 3Hg)(imH)(enhJ4 +

kUg ! H20

HgN; + cis-[Co(OH 2)(imH)(enh]3 +Equation (8) is transformed to the reciprocal form:

(k +k'. J-1-(k k' )-1obs obs - Hg - ob:+:(kUg-k~bJK: --I[Hg2+]1 ... (9)

316

and the values of kHg and K were calculated from theleast square gradients and intercepts of the plots of(k k'. J -) . [ 2+ -Iobs- ob against Hg ] . The species cis-[Co(N3Hg)(imH)(enh]4+ is found to be _103 timesless stable than its isothiocyanate analogue[K(RNCSHg4+)/K(RN3Hg4+)=800 at 40°C, R=Co(im~)(en)2]. For the corresponding~ntaammmec?balt(III) species, the reported stabilitydifference IS of comparable magnitude[K(RNCSHg4+)/K(RN3Hg4+)= 1.3 x 104 at 25°C and1=1.0 mol dm -3] (ref. 15). The species cis-[Co(NCSHg)(imH)(enh]4+ is enthalpy stabilized tothe extent of -16 kJ mol ? as compared to its azidoanalogue; ASo for the binuclear complexationequilibria is negative and virtually the same ( - - 40JK -I mol -I) for both the species.

The species, cis-[Co(N 3Hg)(imH)(enhJ4 + aquates3-6 times faster in the temperature range of 6O-80°Cthan cis-[Co(NCSHg)(imH)(en)2]4+. The rate differ-ence of s~milar magnitude for the correspondingpentaammmecoba1t(III) species (factor of 10 at 25°qhas also been reported I 5. One notes that AHt for theaquation of cis-[Co(N3Hg)(imH)(en)2]4+ is -40 kJmol :' higher than for cis-[Co(NCSHg)(imH)(~nh]4+,the corresponding difference in the activation entropy[AAS!(RN 3Hg4 + - RNCSHg4 +) = -128 JK-1mol -I] is, however, highly positive as has beenobserved for the spontaneous aquation of both thecomplexes. These facts, are presumably related to theb~sicity of the leaving groups and are in agreementWith the general observations that in the aquation ofoctahedral coba1t(lIl) complexes the effects of theactivation parameters on the rate are mutuallycompensatory. A low negative value of AS! for theaquation of the azido binuclear species is alsocompatible with the Id mechanism.

Base hydrolysis~he azido and isothiocyanato complexes were

subjected to base hydrolysis at 30°C for lOt in 008I

3 1 2 .mo dm - NaOH with [complexh=0.OO43moldm-3and then the mixture was acidified to pH 2 withHCI04· The absorption spectra of the acidified aquoproduct are shown in Fig. I. The spectrum of the cis-[Co(OH2)(imH)(enh]3 + cation generated by theHg(I Ij-catalysed aquation of the corresponding chi orocomplex is also included for comparison. No significantdifference in the spectrum of the aquo productge?erated via different routes are evident. Theacidified, base hydrolysed reaction mixture of NandNC~ complexes were also titrated potentiometricallyag~mst 0.125 mol drn "? NaOH. Only one inflectionpoint ar~u~d pH 7 ([OH -]T/[Co3+h=0.99±0.02)characteristic of aquopentaminecobalt(III) ion wasobserved [the imidazole NH is too weak (PKNH > 10) to

DASH et al.: HYDROLYSIS REACTIONS OF Co(III) COMPLEXES

100

'ie 80u

'i"_0e 60

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INDIAN J. CHEM., VOL. 23A, APRIL 1984

Table 3-Rate and Activation Parameters for BaseHydrolysis of cis-[Co(NCS)(imH)(enh]2+(al

[OH -] 105 ko'" (s -I) atmol dm :"

3S.0±0.lo 40.0±0.lc 45.0±0.lo 50.0±0.IT

0.002 1.29 2.32 S.42 9.00O.OOS 2.27 3.43 9.54 19.60.01 3.65 5.36 15.0 28.70.02 4.93 10.5 21.2 SI.8 ±2.00.03 6.82 15.0 32.S 68.60.04 7.95 20.0 42.7 86.7O.OS 9.60 23.0 ± 1.0 47.2±1.8 108.0±4.00.08 14.3 35.0 80.3 172.00.10 17.1 44.S 98.6 2320.12 20.4 S3.2 lISO.IS 25.S 62.6 14S 3290.20 34.0 82.6 196

.Temp, 'C 10' k' (s - I) 10' kObH(dm ' mol-I s -I)

3S.0 154±0.16 160±240.0 2.34±0.49 408±S45.0 3.47±0.83 9S0±9SO.O S.IO±2.S 2165±37

sn; kJ mol ::' 63.7±1.8 141.4± I.7IlSt JK - mol"! -130±6 + 160±6

(aHcomplex]T=2.52 x 10-4, 1=0.3 mol dm -3.

respectively (see Scheme 2)

K~Hcis[CoX(imHXenhJ2+ ~ cis-[CoX(imH)(en)(en-H)] +

OH - I kca! OH[CoOH(imH)(enhf + ~[CoOH2(imH)(en)(en-H)f +

+xScheme 2

The value of K~H = 1a - 16 dm 3 mol - 1 (3a°C)27 isreasonable. The dissociation constant of thecoordinated imidazole (KNtJ being 10 -11 mol dm >'the reasonable upper limit value of K~II/ KNH at 3aDC isIa -5. For k' to be significantly different from k~~ (seeEq. 12) it is necessary that the conjugate base formedby the NH-deprotonation of the coordinatedethylenediamine must aquate at least 104 times fasterthan imidazole (1-) conjugate base. For the sameleaving group this appears to us an unusual reactivitycontrast as (i) both the conjugate bases have the sameoverall charge, and (ii) the coordinated imidazole (1-)species can stabilize the transition state (D or IJ via tt-bonding " (see structure I) as may be the case for theethylenediamine conjugate base. Tinner and Marty?",however, in their recent study of the base hydrolysis of

Table 4-Rate and Activation Parameters for Base Hydrolysis of cis-[CoNlimHXen)z] 2+(a)

Temp. [OH -] 104kob, [OH -] 104kOb' [OH-] 104kOb'(±O.I°C) (mol dm -3) (s -I) (mol dm -3) (s -I) (mol dm -3) (s -I)

30.0 O.OOS 0.18 0.01 0.2S 0.02 0.37O.OS 0.80 0.10 1.61 O.IS 2.350.20 3.12

35.0 0.0048 0.35 0.0096 0.S8 0.0191 1.010.048 2.35 0.096 4.40 0.145 6.S00.193 8.6S

39.6 0.0048 0.803 0.0096 1.22 0.0190 2.20±0.003

0.0462 S.04 0.0964 10.t 0.145 15.3±0.08 ±0.2 ±O.1

0.193 20.2±O.I

4S.0 0.002 0.85 0.003 1.08 0.004 1.26O.OOS 1.43 0.01 2.30 0.02 4.18O.OS 9.57 0.10 17.9 O.IS 26.20.20 35.1

30.0±0.lo 3S.0±0.1' 39.6±0.1' 45.0±0.1 ~C104k',s-1 0.082 ±0.0I3 0.16±0.02 0.2S±0.04 0.62±0.07104k~H' IS.2±0.1 43.9±2.1 103.4±0.4 172.1±0.8dm? mol " S-I

k' path: llHt = \OS± 7 kJ mol-I;'IlSt = +4±24 JK -I mol -I.k~H path: llHt=129±IS kJ mol:": IlSt= +129±48 JK -I mol :".(aHcomplex]T=2.S x 10 -4,1=0.3 mol dm -3.

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DASH et al.: HYDROLYSIS REACTIONS OF Co(lll) COMPLEXES

(halogeno )(2,6-bis-aminomethylpyridine)( 1,3-diamino-2-propanol-N .N'j-cobalul ll) ions put for-ward evidences which suggest that Co - N x-bondingby the amido conjugate base may not be the cause ofthe observed high reactivities of the acidoamine-cobalullf) complexes in the base hydrolysis path.

Furthermore the dipositive complex, cis-[Co(NCSXNH3Xenh]2+ is reported to undergo basehydrolysis - 15 times slower than the monopositivecis-[Co(NCSh(enh]+ cation '". In these reactions,NH-deprotonation is believed to occur to generate thereactive conjugate bases. Assuming that the conjugateacid species, cis-[CoX(imHXenhJ2+ (X=NCS, N3)undergo base hydrolysis at rates equal to those of thecorresponding imidazole (1-) species, and usingKw/ KNH = 10-3 mol dm -3, we calculate the values ofk~H' Kw/KNH as 1.6 x 10 -6 and 4.4 x 10 -6 S-I (35°C)for the NCS and N3 complexes respectively. Thesevalues are negligible as compared to the calculatedvalues of k' (see Tables 3 and 4). We are, therefore, ledto believe that k' must be reconciled with k~~, thespontaneous aquation rate constant of cis-[CoX(imXenhJ +.

The data thus indicate that the imidazole(I-) species,undergoes aquation - 103 times faster than itsconjugate acid analogue. It is also worth noting thatthe labilizing action of the coordinated im - ligand ismuch more pronounced than the rate acceleratingeffect of Hg2 +(k~~ > kHJ though the leaving groups inthe k~~ path will experience electrostatic attractionwith the cobalufll) centre in contrast to theelectrostatic repulsion between the coba1t(III) centreand the leaving groups, HgN; or HgSCN '. in theaquation of the Hgtllj-adducts. A plot of tlHt againsttlst for the first order aquation of a series of complexspecies, cis-[CoX(imH/imXenh]n+ is shown in Fig. 2.Considering the wide variation in the condition ofexperiments, overall charges of the reactants, and theleaving groups and also the nature of the Co - X bondbeing broken the observed linear correlation betweentlHt and tlst may be taken to be good. This furthersuggests that the mechanism of substitution at thecobalultl) centre for imidazole and imidazole(I-)species is independent of the nature of the leavinggroup; such a generalisation is compatible with thedissociative mode of activation (IJ of the Co - X bond.

The rate and activation parameters for the basehydrolysis of the conjugate base, cis-[CoX(imXenh] +

'-oE

~~%

INDIAN J. CHEM .• VOL. 23A. APRIL 1984

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