Bandhyopadhay, (Mrs) Kalpana (Nee Dutta)

Chatterjee, Debabrata

De, G S

English

NOPR

Indian Journal of ChemistryVol. 25A, April 1986, pp. 350-353Kinetics & Mechanism of Substitution of Aquo Ligands fromcis-Diaquo-bis(ethylenediamine)cobalt(III) Ion by Glycinein Water-Ethanol MixtureDEBABRATA CHATTERJEE, (Mrs) KALPANA BANDHYOPADHA Y (nee Dutta) & G S DE-Department of Chemistry, University of Burdwan, Burdwan 713 104Received 2 August 1985; revised and accepted 22 October 1'985The title reaction has been studied spectrophotometrically. The following rate law has been established in the pHd[Co(en)igly)2 +] k•.KE[Co(en)iH20h 3 +].o.al[glyH]dt 1+KE[glyH]range 2.6 to 4.3 (where KEis ion pair equilibrium constant between two reacting species and k., the rate constant for conversionof outer-sphere complex into inner-sphere complex). The reaction rate is found to bepH dependent in the pH range 2.6 to 4.3.The nature of influence of solvent on K[ and k. has been deduced by studying the reaction in four different EtOH-watermixtures. KEvalues increasewith increase in the percentage of organic component, whereas ka values decrease. Activationparameters have been calculated and compared with the isotopic water exchange process. A mechanism involving outer-sphereassociative equilibrium between two reacting species followed by dissociative interchange of outer-sphere complex to producthas been suggested.The available reports! -5 on aquo-ligand replacementreaction of various anionic ligands with stable aquo-cations, in general, show that the anation of aquo-amine complex proceeds by a outer-sphere associationbetween the complex and the incoming ligand, prior tothe slow rate-determining ligand water dissociationwhereupon the nearby ligand slips into positionvacated by leaving water molecule. This outer-sphereassociation and the interchange rate constant areinfluenced by solvents" -8. The interchange process isshown to occur either by a dissociative or an associativepath. This controversy regarding mechanism led us tostudy the kinetics of aquo-ligand substitution in cis-[Co(enh(H20h]3+ by a series of amino acids likeglycine (gly), alanine (ala), fl-alanine (p-ala), proline(pro) etc., in which ligand reactivity is varied bychanging the electron density at the donor site. In thispaper anation of cis-[Co(enh(H20h]3 + by glycine isreported.Materials and Methodscis-[Co(enMH20h](N03h (Complex-I) was pre-pared by the method of Sergeson9 and characterised byelemental analyses. Complex-I exhibited absorptionmaxima at 360(loge = 1.81)and 495 nm(loge= 1.89)inconsonance with the data reported in literature!". Dueto hygroscopic nature of the product of the reactionbetween cis-[Co(enh(H20h]3+ and glycine, it wasdifficult to separate it in crystalline form. So Complex-I and glycine were mixed in different molar ratios, viz.3501:1,1: 5 and 1:10atpH 4.05 in three different reactionvessels and thermostated at 50°C for 24 hr. In eachreaction mixture [Complex-I] was kept fixed at 0.005mol dm -3. Each reaction mixture exhibited maximumabsorption at 495 nm with more or less equalabsorbances but higher than that of substrate complexat 0.005 mol dm -3 concentration. The composition ofthe product in the reaction mixture was found to be 1:1as revealed by Job's method of continuous variation.Further the composition of the product in solution as[Co(enhgly]2 +was found to be the same as that of theproduct obtained by Tsucida and coworkers+'. In allthe experiments AR quality chemicals, doubly distilledethanol and water were used.Kinetic runAll the experiments except the dielectric variationwere performed in 30%(vIv) EtOH-water mixture. Allspectral measurements were made on a Hilger-UVISPEK spectrophotometer at 495 nm, where asubstantial difference existed between the absorptionof substrate complex and product. Equal volumes ofsolutions of Complex-I and glycine of desiredconcentrations were mixed and equilibrated at thedesired temperature. The composition of reactionmixture was such that the pseudo-first order rate lawwas applicable and pseudo-first order rate constants(kobJ were obtained by plotting 10g(Aoo -AoIA", -AJagainst time, where Ao, At and A 00 are the absorbancesinitially, after time t and after the completion ofreaction respectively. The rate constant values arereproducible within ± 3%.Results and DiscussionCHATTERJEE et al.: KINETICS OF SUBSTITUTION OF AQUO LIGANDS FROM cis-[Co(enMH20h]3+Effect of varying [Complex-I] on rate constantIn the first set of experiments [Complex-I] wasvaried (from 0.0025 to 0.0075 mol dm -3) at constant[glycine] (0.075 mol dm -3) pH (4.05) and ionicstrength. The rate of reaction was first order in[Complex-I] at 55°C, leading to the rate expression (1)d[Complex-II] kOb.[Complex-I] ... (1)dtEffect of varying pH on rate constantAt fixed [Complex-I] (0.005 mol dm -3), [glyH](0.075 mol dm -3) and ionic strength (j.t = 0.03 moldm -3), the pseudo-first order rate constant at 55°Cincreased with increase in pH (PH of the solution wasadjusted by adding HN03 or NaOH). The values ofkobox (104) are 0.58, 1.05, 1.44, 1.57 and 2.59 at pH 2.6,2.8,3.1,3.6 and 4.2, respectively. Within the pH r\nge2.6 to 4.2 the glycine exists as the zwitterion (NH3-CH2 -COO -) in solution'f and the values of pKIand pK2 at 50° are 2.32 and 9.18 respectively. Sochange in pH did not alter the ligand reactivityconsiderably. But considering the acid dissociationequilibrium (2) K,cis-[Co(enh(H20h]3 +¢cis-[Co(enh(OHXH20)]2+ +H+ ... (2)thepKI value is observed 13 to be 5.8 at 25°C. AspH ofthe medium is increased the percentage of hydroxo-aquo complex increases. Water exchange rate withhydroxo-aquo ion is about 60 times more rapid thanthat with diaquo species owing to well known labilisingeffect of hydroxide ion adjacent to water ligand byvirtue of its lone-pair capable of exerting strongelectromeric effect. Hydroxide ion is also an-donorwhich facilitates the formation of hydroxo in-termediate species and increases the rate.Effect of varying [glyRJon rate constantThe [glyH] was varied in the range 0.025 to 0.125mol dm -3 at fixed [Complex-I] (0.005 mol dm -3),pH4.05, ionic strength (11) = 0.03 mol dm -3 and temp.= 40°C. The results given in Table 1 show that the rateincreased with increase in [glyH] and attained alimiting rate at higher [glyH] due to completion of ion-pair formation!".The reactions (3) and (4) can be suggested to explainthe variation of rate due to change in concentration ofincoming ligand. KFcis-[Co(enMH 20}z] 3 + +glyH¢cis-[Co(enh(H20}z]3+ 'glyH ... (3)IP3 + k,cis-[Co(enh(H20h] ·glyH ~[Co(enhgly]2+ +H20+H30+ ... (4)Complex-IIOn the basis of reactions (3) and (4) and consideringion-pair equilibrium and rate-determining step, ratelaw (5) is, thus, established.d[ Co(enh(gly)2+]dtka'Kd.cis-Co( enh(H 20)~ +],otal[glyH]1+KE[glyH]=kObs[Co(enh(H20h3+]total ... (5)where KE= ion-pair equilibrium constant, ka = anationrate constant and [cis-Co(enh(H20h3+]total repre-sents the total unreacted complex concentration insolution.Now,k - ka'KE[glyH] ... (6)obs-1"+ KE[glyH]or... (7)1 1 1- =- +---=--c--".kObS ka ka'KE[glyH]According to Eq. (7), the plot of 1/Kobsversus 1/[glyH]at a constant pH should be linear with an intercept = 1/ka and slope = l/ka· KE• This is found to be so (seeFig. 1) and the values of ka x (104) thus obtained are1.3,6.25,8.33, 14.'28 and 25.00 s -I at 40°, 49°,52°,55°and 60°C respectively. KEvalues are found to be 4.0 to7.5. dm ' mol -I in the temperature range studied. Inproposing the mechanism as represented by Eqs (3)and (4), it was assumed that only cis-diaquo complex isinvolved in the reaction, because under theexperimental condition the cis/trans ratio is around 5'8(see ref. 13).Effect of varying ionic strengthThe ionic strength of the reaction mixture wasadjusted by adding NaN03. At fixed [Complex-I](0.005 mol dm -3), [glyH] (0.075 moldm -3),pH =4.05and temp. = 55°C, the values of kobs increased slowlywith increase in ionic strength of the medium. Forexample at 11=0.10, 0.13, 0.15 and 0.20 mol dm -3, kobs(x 104)are 2.50, 2.55, 2.60 and 2.71 s -I respectiv,fly.Considering the net charge of zwitterion form (NH3-CH2 -COO -) as zero, the result goes against theTable 1- Variation of Rate Constant (kobJ with [glyH] atDifferent Temperatures[PH =4.05; Il= 0.03 mol dm -3; and [Complex] =0.005 mol dm -3][glyH] 104 kob.(S -I)moldm -340° 49° 52° 55° 60°C0.025 0.42 0.729 0.88 2.110.050 0.31 0.88 1.49 1.65 3.840.Q75 0.43 1.19 1.96 2.41 5.180.100 0.50 1.49 2.42 3.18 7.100.125 0.54 1.69 2.69 3.95 8.75351INDIAN J. CHEM., VOL. 25A, APRIL 19863,002.82.4c2.0"<tIS? B"",1.6.00~1.220 30 40 50 60Ij[glyH]Fig. I - Plot of Ijkobt versus Ij[glyH] at different temperatures. (A)60°C, (8) 55°C, (C) 52°C, (D) 49°C, (E) 40°C.accepted theory 1 5 for ionic reactions in solution. Thisanomaly can be explained by the fact that the non-substituting nitrate ion facilitates the metal-waterbond cleavage through a specific attack on thecoordinated aquo-molecule which is not associatedwith dipolar glycine. Hence labilisation of this aquo-molecule catalyses the entry of ligand glycine. Thistypical type of ionic strength effect was observed byHarris and coworkers:'.Effect of varying dielectric constant on rate constantThe influence of solvent dielectric constant on ratewas studied in four different EtOH-water mixtures at40°C. In each set, ligand concentration was variedfrom 0.05 to 0.2 mol dm -3 at fixed [Complex-I] (0.005mol dm -3), pH 4.05 and ionic strength (jJ = 0.03 moldm -3). It appears from Table 2 that pseudo-first orderrate constant (kobJ increases with decrease in dielectricconstant of the medium.As the reaction occurs through ion-pair equilibriumfollowed by interchange process, the evaluation of KEand ka at different dielectric constants is necessary. Thevalues of k. and Kf. in different EtOH-water mixturesare given in Table 3.The nature of dependence of Kf. on solventcomposition agrees with ion-pair formation, since KEvalues increase with increase in organiccomponent(EtOH) in the solvent, i.e. with decrease indielectric constant of the medium. In contrast, k,values decrease with decrease in dielectric constant ofthe medium. The influence of dielectric constant on352rate constant can be described by the equation ofLaidler and Eyringdink e2Z2d(l/D) = 2kT (l/rl -l/r·) ... (8)(considering the net charge of isoionic glycine as zero).Here rl is the effective radius of ion-pair and r" is thatof activated species. Generally r1 <r" for associativeactivation (where charge of activated species is thesame as that of reacting species). But in the present caserl >r·, because in activated state one coordinatedwater molecule comes out from inner-sphere toprovide for the incoming ligand and consequently thesize of activated species is lowered. In fact plot of log k.versus 1/D is linear with a negative slope (Fig. 2)whichconfirms the dissociative interchange process. It maybe assumed, however, that influence of dielectricconstant on k. is not very great, since the effectiveradius (rd of ion-pair is much greater than that of awater molecule, so that it will not differ greatly fromr", As the increasing influence of kf. on kobs is muchgreater than decreasing influence of k. onkObS> the overall rate of reaction increases with decreasein dielectric constant of the medium.Effect of varying temperature on rate constantThe reaction was studied at five differenttemperatures for different ligand concentrations in30% (v/v) aq. ethanol and activation parameters werecalculated using Eyring equation (Fig. 3), checked byleast square method and compared with the data forisotopic water exchange process (see Table 4). Thesmall difference in flHt and flSt values is due to thedifference in dielectric constant of the medium. In theTable 2 - Values of kObo in Different EtOH-Water Mixtures[Glycine] 10-4 kobo (s -I)(mol dm -3)0.0500.0750.1000.1250.1500.20012YJv/v) 24%(vjv)0.25 0.270.31 0.390.36 0.420.49 0.510.57 0.600.64 0.7230%(vjv)0.310.430.500.540.630.7436%(v/v)0.350.500.530.570.650.77Table 3 - Values of k. and kE in Different EtOH-WaterMixtures104k.(s-l)EtOH(%,v/v)122430361.531.401.331.253.775.107.509.000.4,...--------------,CHATTERJEE et al.: KINETICS OF SUBSTITUTION OF AQUO L1GANDS FROM cis-[Co(enh(HzOh]l+-1,--------------------.0.0oL-----,l' ..,...2--:'1...,.4--~-.......,......-~."../. x 10"oFig. 2 - Plot of log k, versus 1/D at 40°C, [Complex-I] = 0.005 moldm -3 and pH =4.05.Table 4 - Activation Parameters for Substitution and WaterExchange Processsn;(kJmol-l)System tlst(JK -Imol ")Ref.cis-[Co(en)2(H20)d3 +-OH2 120.58(exchange)cis-[Co(enh(H20h]3+ - g-glycine 128.54(substitution)+62.81 16+88.34 Thisworkcase of glycine substitution the medium is 30% (vIv)aq.ethanol whereas isotopic water exchange has beenstudied in aqueous medium. In 30% (v/v) aq. ethanolthe dissociation of H20 from the aquo complex issomewhat slower than that in pure water. Thus it canbe concluded that the substitution of aquo-ligand fromcis-[Co(enh(H20hJ3+ by glycine proceeds throughdissociative interchange (/d) mechanism.-2-4- 5,.98 3.02 3.06 3.10 3.14 3.18 3.22...!. X 103TFig. 3 - Eyring plot of log k. versus 1/T.AcuowledgementThe authors express their thanks to the authoritiesof the University of Burdwan for the award of afellowship to one of them (D C).ReferencesI Haim A & Taube H, lnorg Chem, 2 (1963) 1199.2 Van Eldic R & Harris G M, Inorg Chem, 18 (1979) 1997.3 Brown P M & Harris G M, Inorg Chem, 7 (1968) 1872.4 Pal M & De G S, Indian J Chem, 14A (1976) 763.5 Dutta K & De G S, Indian J Chem, 21A (1982) 477.6 Udovenko VV, Reiter L G & Shkurman E P, Russ J inorg Chem,18 (1973) 838.7 Udovenko VV, Reiter L G & Shkurman E P, Russ J inorg Chem,18 (1973) 1296.8 Bishnu M N, Chakravarti B, Banerjee R N & Banerjee D, J coordChem, 13 (1983) 63.9 Dwyer F p, Sergeson AM & Reid L K, J Am chem Soc, 85(1963)1219.10 Gillard R D & Wilkinson G, J chem Soc, (1963) 3193.11 Shimura Y & Tsucida R, Bull chem Soc Japan, 29 (1956) 311.12 Stability constants of metal ion complexes, Special PublicationNo. 17 (The Chemical Society, London) 1964 pp. 377.13 Bjerrum J & Rasmmssen S R, Acta chem Scand, 6 (1952) 1265.14 Basolo F & Pearson R G, Mechanism of inorganic reactions,(Wiley-Eastern, New Delhi)·1977 pp. 154.IS Glasstone S, A text book of physical chemistry, (Macmillan,London) 1977 pp. 1116.16 Kruse W & Taube H, JAm chem Soc, 83 (1961) 1280.353

Kinetics & Mechanism of Substitution of Aquo Ligands from cis-Diaquo-bis(ethylenediamine)cobalt(III) Ion by Glycine in Water-Ethanol Mixture

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Kinetics & Mechanism of Substitution of Aquo Ligands from cis-Diaquo-bis(ethylenediamine)cobalt(III) Ion by Glycine in Water-Ethanol Mixture

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