Bhatia, J. C.

Paul, R. C.

Sharma, O. D.

English

NOPR

India •• Journal of CbemlatryVoL 15A, November 1977, pp. 966-968Ligand Exchange Reactions in Some Pentaamminecobalt(III) ComplexesR. C. PAUL, O. D. SHARMA & J. C. BHATIADepartment of Chemistry, Panjab University, Chandigarh 160014Received 22 December 1976; rev ised 6 May 1977; accepted 19 May 1977The exchange of water with triethylphosphate (TEP) in aquopentaamminecobalt(III) per-chlorate, and of dimethylsulphoxide (DMSO) in (DMSO)-pentaamminecobalt(III) perchloratein TEP is a unimolecular reaction. The product of the exchange reaction, (TEP)-pentaammine-cobalt(III) perchlorate, has been characterized. Pyridine and chloride ion displace H.O/DMSOligand from the pentaammine complex. The rate of the ligand exchange is a function of the[substrate].THE kinetics of exchange of water with mono-dentate and bidentate ligands in aquopenta-amminecobalt(III) perchlorate in aqueousmedium have been reported--". Also the penta-amminecobaJt(III) complexes having DMSO, DMFand pyridine as sixth ligand have been described++.In this paper we report the synthesis of a newcomplex, (tfEP)-pentaamminecobalt(III) perchlorate(TEP=triethylphosphate) and kinetics of exchangeof ligand L(L = H20. DMSO, DMF) in [Co(NH3)5L](Cl04)3 with TEP, chloride ion and pyridine in TEPmedium.Materials and MethodsThe chemicals used were of reagent grade qualityand were further purified by the reported methods",(Triethylphosphate)-pentaamminecohalt(III) perchlo-rate was prepared from aquopentarnminecobaJt(III)perchlorate. The substitution of H20 by TEP wascarried out by pouring aquo-complex (3 g, 0·0065mole) and TEP (1·187 g, 0·0065 mole) onto 150 mlof water. The contents were heated up to 85 ° andconcentrated at 85° for 20 rnin with a flash evapo-rating system. The reactants were cooled in anice-bath, perchloric acid (1 : 3) added and the con-tents kept overnightat 4°. A violet red precipitateappeared which was filtered, washed successivelywith small amount of cold water, ethanol, etherand TEP and dried in vacuo (Found: Co 9·76. Reqd:Co, 9'44%); UV: 475, 337 nm.(Dimethylsulphoxide)-pentaamminecobalt(III) per-chlorate and (dimethyl formarnide) pentaamine-cobalt(III} perchlorate, were prepared by the re-ported methods+".Kinetics procedure - The kinetics of ligand dis-placement reaction were followed conductometri-cally (20°-45°) as well as spectrophotometric ally(at 25° only) in pure TEP. The specific rateconstants and the activation parameters were cal-culated. k; values, calculated also from the spectro-photometric data (reference at 475 nm) for thereaction at 25° (room temperature), are in good agree-ment with those obtained from conductometricmethod. Ther eacticn could not be followed spectro-photometrically at other temperatures becauseof certain limitations of the instruments available.966The visible and ultraviolet spectra of the mixtureswere recorded on Cary-14 and Beckmann DUspectrophotometers.Results and DiscussionThe study of kinetics of displacement of suchligands as DMSO, DMF and H20 by TEP from theaqueous solutions of their pentaamminecoba1t(III}complexes [Co(NH3hDMSO](Cl04la, [Co(NH3}sDMF](CI04h and [Co(NH3)5H20] (Cl03)4 (molar ratio ofthe complex and TEP is 1 :10) is not possible becausewater behaves as solvent as well as the nucleophile.Also the aqueous solution of the complex [Co(NH3)6TEP](Cl04la always gives [Co(NH3}5H20](Cl04)3'Therefore, water as solvent is unsuitable for ligandexchange involving TEP. Hence, the reactionshave been studied in pure TEP only.The infrared spectra of aquo, DMSO and DMFcomplexes are identical with those reported earlier+".and are reproducible in fresh TEP solutions.Evidence of the ligand character of TEP has beenobtained from the IR spectum of [Co(~H3lsTEPJ(Cl04h and of the spectra of solutions of other com-plexes after heating in TEP. These spectra showoverlapping of perchlorate ion absorptions withvO-C and vP=O around 1000 and 1200 crrr+respectively. However, vP-O at 815 crrr ' is strongand clearly defined. vP=O in free TEP at 1272crrr'! shifts to 1175 crrr! in the complex. This shifthas been interpreted in terms of coordination throughthe oxygen atom of TEP.Whereas the conductivity of DMF complex inTEP remains unchanged, a gradual change inconductivity with time has been observed for aquoand DMSO complexes in TEP and Am", (molar con-ductance of the system after 24 hr) values are 24·0and 20·7 ohrrr ! em- mole? respectively. Thereis a simultaneous increase in optical density(reference at 475 nm) of the reaction mixtures. Thechange in conductance of the system (which remainswithin the range for a 3:1 electrolyte) may be dueto slow formation of [Co(NH3}sTEP]3+ ions from[Co(NH3}sH20](Cl04h and [Co(NHahDMSOJ(Cl04hrespecti vely.The complex [Co(NH3)sDMFJ(Cl04la dissolvesin TEP on warming and gives [Co(NH3)5TEP]3+PAUL el al.: LIGAND EXCHANGE REACTIONS IN Co(III) COMPLEXESions (characteristic UV maxima at 475 and 337 nm)in solution. AM (or optical density) changes veryrapidly and therefore, precludes a kinetic study ofthe exchange reaction (1) either conductometricallyor spectrophotometric ally.fastCo(NHa)sDMFS++ TEP----7- Co(NHs)sTEP3-f +DMF... (1)The electronic spectra of solutions of [Co(NHs)5H20(C10a) 4 (characteristic maxima- at 490 and 345 nm)and [Co(NHs)sDMSOJ (C104)a (characteristic maxima"at 515 and 352 nm) in TEP do not change initiallybut when the solutions are allowed to stand forsome time or are heated, the maxima correspondingto [Co(NHs)s TEP](CI04)a (characteristic values 475and 337 nm) are produced. There is a slow changein conductance of the solution of aquo and DMSOcomplexes with time. Simultaneously the opticaldensity (reference at 475 nm) of the reactionmixture also increases. The interchange of DMSO/aquo ligand with TEP is represented by reaction (2).TEPCo (N Hah(DMSO/H20)3-f ----7-Co(NHs) sTEps+slow+DMSO/H20 ... (2)The DMSO/H20-TEP exchange may be due to themass effect of TEP as solvent. At any stage ofthe reaction anationjaquation or TEP-DMSO/DMSO-TEP equilibrium is not proposed because theconcentration of H20 or DMSO is too low to competewith reaction (2) for an opposing reaction. Theaquation of the starting ion and hydrolysis of TEPin the product is unlikely because an insignificantamount of water in the complex [Co(NHshDMSOJ(Cl04)a.2H20 cannot compete with a large excessof TEP as solvent. It has been shown thatin aqueous or acidic conditions" where formationof aquo complex from [Co(NHa)sTMPJs+ (TMP-trimethylphosphate) is the principal reaction, es~erhydrolysis is insignificant (only 12% compared WIththe lability of Co-O bond calculated as 88%).The reaction follows a rate lawRate = k[Co(NHah(DMSO/H20)J3+The observed rate does not indicate whether thereaction follows an SNI mechanism or a compactSN2 mechanism. The dependence of the rate onTEP concentration cannot be ascertained since itsconcentration is overwhelmingly large and remainsalmost constant throughout the reaction. However,our values of k, (Table 1) for the reaction (2) arecomparable in magnitude with those for first orderspecific rate constants for the systems=" [Co(NHs) sHmOJa+-H20, [Co(NHa)sH20Js+-SOi- and [Co(NHa) 5H20J3+-H2P04 which have been assigned an SNImechanism. By comparison a similar mechanismhas been preferred for reaction (2). However, asubstantial kinetics influence of the solvent cannotbe disregarded. This view is in line with the viewexpressed by Adamson'? and supported by ~igenll.The rate data at different temperatures YIeld forthe parameters AH~ and ASt the values 27·8 kcalmole'< and 22·3 e.u. for water complex and 23·5kcal mole'< and 26·9 e.u. for DMSO complex. The!1H; values require that cobalt(III)-OH2 and cobalt(III)-DMSO bonds in the activated complex beTABLE 1 - AVERAGE VALUES OF ks AND ACTIVATIONPARAMETERS FOR THE LIGAND EXCHANGE REACTIONS(Ms=0·2-0·8 mmolejlitre)L 10"ks (see-I) at /)"H~ /),.S~kcal e.u.mole'?30° 35° 40° 45°H20 4·63DMSO 3·8711-07 30·12 78·86 27·89·34 25·59 74·60 23·522·326·9TEF(NHs).CoLs++ct- ---..(NHsl.CoCI2++LH20DMSO3·21 1HO 27·40 26·0 23·37·33 12·79 30·27 29·3 -4·0SD of ks=1·25; tJ.Ht=0·27; and tJ.S~=1·2. Tempera-tures maintained with an accuracy of ±0.10.stretched to a critical value. There is no basis fordeciding whether a true intermediate is formed ondecomposition of the activated complex or someuse IS made of the entering TEP molecule in theactivation process. Positive values of ASt are aslarge as would be expected if the activated complexwere to have the properties of SNI process.In the presence of CHaCOCl or (C2Hs)4NCI solutionso~ ~Co(NI~a)5DMSO/H20](CI04)a in TEP finally ex-hibit maxima at 500 and 341 nm which are charac-te~ist~c for tht: cation [Co(NHahC1J2+. Besides theprincipal reaction (3)TEP[Co(NHa)5DMSO/H20Ja+ +Cl---7[Co(CNHa)sClJ2++DMSO/H20 ... (3)there exists a reduction process involving conversionof some octahedral Co (III) species to tetrahedralcobalt (II) species. This has been substantiatedby the slow development of a blue tinge and alsoby the appearance of absorption bands" at 660,610 and 595 nm.The first order specific rate constant for differentmole ratios of the substrate and chloride ion (Table 1)have been calculated. At lower [Cl-J, deviationsare insignificant and the reaction appears to be firstorder in [Co(NHa)s(DMSO/H20)a+J. The rate lawfor the entire range isRate = (k1+k2[Cl-J)[complex]AHt and AS~ for the replacement of H20 andDMSO between [Co(NHa)5 H20J3+ and chloride ion,and [Co(NHah DMSO]3+ and chloride ion are 26·0,29·3 kcal moler! and 23·3 and -4·0 e.u. respectively.The entropy values favour the involvement of chlorideion in the activated complex. Thus the values of AStdoes not rule out an SN2 mechanism in which a seven-coordinate activated complex may be formed. How-ever. stronger solvation of chloride ion comparedto H20 or DMSO might have some contribution tothe overall AoSt values for the reaction.The reaction (4)Co(NHa) 5(OH2/DMSO)3+ + PY-7Co(NHa)5Py3++DMSO/H20 ... (4)967INDIAN J. CHEM., VOL. lSA, NOVEMBER 1977appears to follow a path similar to reactions (1) and(2). For the dilute solutions of [Co(NHaMDMSO/H20)](CI04)s in pyridine an immediate change incolour has been observed. The resulting spectrumshows absorption maxima at 479 and 341 nm whichhave been assigned to [Co(NHa)6Py]3+ ion. How-ever, the reaction could not be followed kineticallybecause both conductivity and spectrophotometricmethods failed to register the rapid change inconductance/absorbance as a function of time. Thereaction has also been investigated as a functionof [pyridine] in TEP as solvent. The shifts in thepositions of absorption bands appear to depend on[PyJ. However, the overall reaction does not seemto be a simple one because the results are notreproducible within permissible limits of error.Hence the possibility of competitive equilibria involv-ing pyridine and TEP cannot be disregarded.968References1. FITZGERRALD, W. R. & WATTS, D. W., ]. Am. them.Soc., 90 (1968), 1734.2. JOUBERT, P. R. & VANELDIK, R., Tnorg, chem. Acta,12 (1975), 205.3. PIRIZMACCOLL, C. R. & BEYER, L., Inorg. Chem., 12(1973),7.4. NORDMEYER, F. & TAUBE, H., J. Am. chem. Soc., 90(1968). 1162.5. PAUL, R. C., SHARMA, O. D. & BHATIA, J. C., Indian J.Chem., 13 (1975), 692.6. LANGFORD, C. H., Lnorg, Chem., 7 (1968), 1032.7. CHRISTENSEN, R. J., ESPENSON, J. J. & BUTCHER, A. B.,Inorg, Chem., 12 (1973), 565.8. SCHMIDT, H. & TAUBE, H., Inorg, Cbem., 2 (1963),698,9. TAUBE, H. & POSEY, F. A., J. Am. chem. Soc., 75(1953), 1463.10. ADAMSON, A. W., J. Am. chem. Soc., 80 (1958), 3183.11. EIGEN, M., Advances in chemistry of coordination com-pounds (Macmillan, New York), 1961.•

Ligand Exchange Reactions in Some Pentaamminecobalt(III) Complexes

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Ligand Exchange Reactions in Some Pentaamminecobalt(III) Complexes

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