Dutt, N. K.

Gupta, S.

Nag, K.

English

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Indian Journal of ChemistryVol. 14A. December 1976. pp. 11100-1003Stability Constants of the Complexes of Al(III), Ga(III), In(III),Fe(III) & Cr(III) with Malonic & Substituted Malonic AcidsNo K. DUTT, S. CUFL\ & K. NAC·Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032Receined 7 November 1975; arcrptet! 5 M'arch 1976The formation constants of AI(IlI), Ga(III), In(III), Fe(III) and Cr(IU) complexes withmalonic, methylmalonic, n-butylmalonic, phenylmalonic and bromomalonic acids have beendetermined. The stability constants of (log!}.)group IlIA metal complexes with most of the dicarb-oxylic acids follow the order Ga(III»AI(IlI»In(III), which is also followed by Zetfjr' values ofthese metal ions. Among the Fe(III) and Cr(III) complexes, the stability constants of theformer have been found to be greater. For a particular metal ion, the gradation in the stabilityconstants of its complexes derived from the acids is methyl,· /1- butyl phenyl »malonic> bromo,which is also the decreasing order of first dissociation constants (pK;;> of the acids.THE sustained in tcrest ill tI.IC study of. tlu-oxalate complexes of metal Ions In solut ionsis consequence of their being used as ~oddsystems ill the investigation 0:1 st ructure, .stahIllty.liability and other aspects of coordination COm-pounds'. As a result, stability constants of variousoxalato complexes have been determined>. Comparedto the oxalate complexes, howuver , much lessinformations are available regarding stability Con-stants of malonate complexes. The dissociatior.constants of Al.(III) and Ga(UI) malonates weredetermined by Dutt and Bose" by acid hydrolysisof the corresponding tris malonatcs and that ofFe(III) malonato complex \\ as determined polaro-graphically by Schaap. et al+. HO\l'ever, th~sl~authors failed to determine the stepwise formationconstants. In malonic acid. since the methylenehydrogen ator~s call be suitably substituted byelectron donating or elect roll withdrawing groups,it was thought of in tcrest to undertake a systematicstudy 011 tlu- solution chemistry of their metalcomplexes. In this paper WI' wish to report tileformation constants of AI(III), Ga(III), In(III),Fe(III) aTIII Cr(III) comp.lexes with malonic,mcthvlmalonic. n-hlltylmaloTllc, phen ylmalonic andbromornalonic acids.Materials and MethodsSodium perchlorate, perchloric acid, sodiumhydroxide and other chemicals used were of rea1?en tgrade. Maloll ic acid (BDH grade) was recrystallizedtwice from wat('r and dried at 105°, m.p, 135° (lit.m.p. 136°). The pcrchloratcs of aluminium, gallium,iron and chromium were prepared by d igcstion ofthe corresponding freshly prepared hydroxides withdil. perchloric acid. These were filtered al:d. thefiltrates appropriately diluted with doubly dist illedwater. The concen t ration s of AI(III), Ga(IIl) an dIn (III) were determined gravimetrically as theiroxinates and those of Cr(Ill) and Fc(III) were deter--To whom correspondence should be addressed.1000mill I'll 1>y standard volumct ric procedures. Execs sperchlo ric acid in t l«- stock solutions was determinedby passing all aljquot of the solution throu"hAmberl itv Jl{-120 (H+) and t it rat ii.g the total acidliberated. The results wr-r« clleckl,d by comparii!gwith thl~ pH 01 th« stock solutions.Ethyl merhylmalonar« (b.p. 196-98°/750 mm)was yrt·parl·cI5 by heatJllt; (at 130-150°) oxalopro-PIOIlICeste-r wlu ch III turn was obiainorl by rvfluxinga mixture of ethyl propionate, cliethyl oxalate andsodium vt hoxide.Ethyl-e-butylmalnnar., was prepared by refluxmgsodium t·tho;:ick with l1-hl~tylbromide al1d diethylrnalonat e. 11l!' l ract ion boi ling between 130-35°/20mm was collected.Ethyl phcnylrnalouau, (b.p. 158-62°/10 mm) wasobtained'. by Iwating (at 175°/15 rnrn) phe-nyloxaloacctic est or.Ethyl bromomal.mat« was preparcds by thephotocncrnical r('actioll of bromine with diethylmalonate in carbo II tdrachloride. The productboiling ill the ranz« 120-25°/15 mm was used.Sodium salts .of th-: acids wen' obtained byalkaline hydrolysis of the abov.: ('sb·rs. An ethanolir;solution of tli« ('st"r \\<lS rc-flux.«] wit h calculatedamount of sodium nvclroxido ip ethanol. Thesodium salt scparau-d was filterl'd and made freefrom alkali hy rqw<tted \I'ashillg with ethanol. Itwas then dissolved in a minimum amount of waterand rcpn-cipitnt.«I by adding a large excess ofethanol. Th« purity of sodium salts was checkedbv clu-mir al aualvsos"TitratiO'lI ...' TIH~ pH titrations were carried outat 300~: 0·2: aurl !J. :cc 0'1.1-£ (\'aCJO~) on a Camb-ridge (HI'nch typ\') pH-mekr having glass-calomelelcct rod« assembly. Th« dissociat ion constants ofmalonic acid wen: (!<-tt·rmillt·d by titrating its aq.solution aganst standard carbo!latt'-free NaOH.In th« case of sllhstitlltl'd malonic acids which werein the form of their sodium salts, the titrations werepcrformod against a standard solution of pcrchloricDUTT et al.: COMPLEXES OF TERVALENT METAL IONS WITH MALONIC & SUBSTITUTED MALONIC ACIDSacid. For determining the formation constantsof metal complexes, the gCilcral proccdur: consistedin titrating a 50 ml aq. solution having [metal ion] =,,,-,I X 10-3111, known volume of adckcl pcrchloricacid and excess of ligand against standard NaOHsolution. In several cases it was not possible toobtain sufficient number of ii values 10 dotermineeacn successive formation constants through a singletitration. In such cases, oxpcrimcn ts Wi"re performedusing different metal-ligand ratios (1:2 to 1 :10) andvarying the added [porchlor ic acid] from 10-2 to10-3N!. In the case of Cr(III) systems, due to slowreaction kinetics, a series of similarly prepared so-lutions with gradually increased amount of NaOHwere kept overnight, and pl-I of the solutionsmeasured the following day. Preliminary experimentsrevealed that equilibria were established in lessthan 16 hr.Computation of the [ormation constants --. Theformation constants were determined by Schroder'smethod? using the sets of nand [AJ valncs where[AJ refers to free [ligand] in tnc anionic form. IIIthe case where ligand was used ill th o acid form(viz. malonic acid) r.xprcssious (1) and (2) were usedfor n and [A] respectively.n = dM[CA~(2CA+E~CB~[1I+J)XJ~~;ritfi~I;;i~j~Y~~] ...(1)_ Kjl{!1.[AJ = (2CA+E -Cn- [H+J) X 2-·~H·-+'2+······}-;"'1::1+" ... (2)L J "IL" JWhen the sodium salts of substituted malonic acidswere used, it was borne ill mind tnat two equivalentsof perchloric acid were required to liberate thecorresponding dicarboxylic acid.Thus expressions (1) and (2) wen: modified togive Eqs. (3) and (4) respectively.tl = c~[CA~(E~Cn~[H+J)MX lH+]~.±J{~[.H._Jf J{~J{~ ] ... (3)2[H+}+ lq~H+JKiK'2[AJ = (E~CB-[H+J) X2[f{+]2+1(~[l-i+J ... (4)wnere CA stands for total [ligand], E is theconcentration of perchloric acid present in thesolution, Cn is the concen trat ion of alkali addedand lq, and lq are the first an] second aciddissociation constans of the ligand.In Eqs. (1-4), the contribntion due to [OH-] wasneglected, since formation of tne complexes iscompleted at pH < 7. Preliminary experimentsestablished that equilibria involving polynuclearcomplex formation can be Jleglcctccl.The generalized expression (5) for the formationfunction may be utilized to obtain dct erminants''which may be alternatively expressed by the linearforms (Eqs. 6-9 for systems where N =,. 3).N2: (n~n)[AJn ~n= 0 ... (5)n=ODo=AlA~A~+3AlA~A~-27AiA~A3+3AiA2A~+5A~A2A~+ 15ArA~A3D; ='A~A~+3A~A~+45AiA~-15A~A~ - 25A~A~D2=3A1A~-15AIA~--9A~A3+A2A~+45A~A3-- 25Ar A2 ... (8)D3= 5AIA~+3AIA~- 27Ai A3+ 15AiA2+ A2A~+3A~A3... (9)where Al=[A]'~~lj2; A2=[A]n=3j2; and A3=[A];'=5j2'The stepwise stability constants were determinedfrom relation (10).J{;~~D;/Di_l ... (10)Results and DiscussionThe acid dissociation constants (Kr and K2) ofthe dicarboxylic acids, viz. malonic(I), methyl-malonic(II), n-butylmalonic(III), phenylmalonictl V]and bromomalonic(V) acids, are given in Table 1.The acids distinctly fall into two categories:(a) containing electron-donating substituents such asII and III, arid (b) containing electron-withdrawingsnbstituents such as IV and V. The acids (II) and(III) are weaker than malonic acid and II is a weakeracid a'; compared to III. The alkyl groups affectprimarily the first dissociation constant (lq) , whilethe second dissociation constant (K~) is controlledmainly by the electrostatic field of tho carboxylateion. As expected, brornomalonic acid is a strongeracid than malonic acid. Phenylmalonic acid alsoclue to the electron-withdrawing effect of the phenylgroup should have been expected to be stronger acidthan malonic acid. Contrary to this, its dissociationconstant (PJ\.i = 2·96) is somewhat less than malonicacid (PI{y = 2·80). A possible explanation for thisanomaly may be traced to the hydrogen bondingof carboxyl hydrogen with the phenyl ring.The formation constants of the complexes of I-IVwi th Al(III), Ga(III), In (III), Fe(III) an d Cr(III)are given in Table 2. Obviously, the metal ionsfall into two categories. The first three metal ionshave the inert gas configuration, while Fe(III) andCr(III) have incompletely filled d-orbitals. Thedicarboxylic acids under consideration have O~-chromophoric groups and are hard bases. The metalions of the first category would form electrostaticbond with the ligand ions and the bond strengthshould have relation with the effective nuclearcharge and ionic radii of the metal ions. Onthe basis of electrostatic model. considering theanions as point charges, the bond strength isproportional to Zefdr2, where Zeff is the effective... (6)... (7)TABLE 1~ ACID DISSOCIATION CONSTANTS OFMALO:-lIC AND SUBSTITUTED MALONIC ACIDSAcid PK~ pK~5·455·615·765·733·76MalonicMethylmalonicn-ButylmalonicPhcnvlmalonicBrorrl'omalonic2·803·263·192·962·531001INDIAN J. CHEM., VOL. 14A, DECEMBER 1976TABLE 2 - STABILITYCONSTANTSOF THE COMPLEXESOF Al(III), Ga(III), In(IIl), Fe(III) AND Cr(III) WITHMALONICAND SUBSTITL'TEDMALONICACIDSA13+ Ga3+--------logK2logK3logK3Fe3+ Cr3+logK2logK3logK"log](,JogK2logK3logK •6·15 3·95 6·81 4·81MALONICACID5·55 3·77 3·08(log ~3= 12·40)6·39 5·97 5·70(Jog ~:I= 18·06)5·81 '"t·04 3·47(log 133= 13·32)METHYLMAlONIC ACID6·75 6·71 3·51(log 133= 16·97)7·64 6·47 6·16(log 133= 20·27)6·19 5·09 3·71(log ~3=' 14·99)7·15 6·98 5·96(log [33=20·09)7·25 6·36 4·74(log 133= 18·35)n-BUTYLMALONICACID6·71 5·69 2·80(log 133= 15·20)7·31 6·97 5·76(log 133= 20·04)5·86 4·38 3-14(log 133= 13·38)7·07 5·81 3·54(log ['3= 16-42)6·92 5·54 3·87(log 133= 16·33)6·66 6·54 5·01(log 133= 18·21)6·09 5·33PHENYLMALONICACID6·63 5·53 3·35(log 133= 15·51)6·65 5·70 3·28(Jog 133= 15·63)BROMOMALONICACID4·47 4·22 4·16(log ~3= 12·85)5·51 4·89 4·71(log 133= 15·11)5·08 3·81 3·39(log 133= 12·28)6·35 5-48 4·50(log 133= 1633)nuclear charge and r the radius of the metal ion.Though, tnis would be a naive picture, still thisseems to be a useful model for a comparative study.Thus, the values of Zefflr2 for AI(III), Ga(III) andIn(III) were calculated to be 34·0, 41·3 and 26·5respectively. It may be pointed out, as againstthe usual practice, the effective atomic numbersof tne metal ions have been used instead of simpleZ values because the attractive power of the nucleusfor tho electrons would be different for the metalas compared with its ion. Apparently all the basisof Zefflr2 values, the stability constants of AI(III),Ga(III) and In (III) with a particular ligand shouldfollow the trend Ga(III) >Al(III) >In(III).It would be appropriate at this stage to comparethe stability constants of the complexes of AI(III),Ga(III) and In (III) with malonic acid and itsderivatives. Tile comparison may be made eitheron tne basis of log Kl or log ~3 values. In few cases,e.g., the formation of tris-complcx species of AI(III)and Ga(III) with malonic acid and that of II1(III)with phenylamalonic acid could not be determined.Moreover, accumulation of error in each successiveformation constants would render the overallstability constant values more unreliable. In anycase, it may be seen from Table 2 that, in general,similar trend is followed by log K; as well as log ~3values, wherever available. The general trendas expected on the basis of Zeff/r3 values, i.o.Ga(III) > Al(III) > In(III), was followed inthe complexes with methylmalonic and n-butyl-malonic acids. In the case of phenylmalonic acidthe order is Ga(III) '" AI(III) > In (III) on thebasis of log KI• though on tne basis of log ~3 valuesit is AI(III) > Ga(III). This anomaly is probablydue to the steric effect of the phenyl ring. In thecase of bromomalonic acid the order is Ga(III) >1002In (1 II) > Al(III) on the basis of log K, values.However. 011 the basis of log ~3 values the expectedtrend, i.t'. Ga(III) > AI(III) > In(III), is obvious.In 1iteraturc, on e encounters very few systemsin which the stability constants of the above threemetal ion complexes have been compared. Izattet apo determined the formation constants of acetyl-acetonato complexes and observed a trend similar tothat obtained by us. Staryll also reported the sameorder of the overall stability constants of oxalatocomplexes. In several polyaminocarboxylic acidcomplexes, however, the following order of stabilityconstants In(III) > Ga(III) > Al(III) has beenreported1!-14.I t appears, therefore, that there exist two trendsin the formation constants of Al(III), Ga(III) andIn(III) complexes. While with ligands havingoxygen donors, viz. oxalic acid, malonic and sub-stitutcd malonic acids, acetylacetone, etc., the orderof stahility constants is Ga(III) > Al(III) > In(III).On tho other hand, with ligands having bothnitrogen and oxygen donors the stability constantsfollow the seqnence IIl(III) > Ga(IIT) > AI(III).It seems that nirropcn atom acts as a better donorfor IIl(ITI) ;).1)(.1 has least affinity for AI(III).It may he noted from Table 2 that the formationconstants of C.r(III) complexes with phenyl andbromomalonic acids coulil not be de ermined. Table2 also shows tllat the stability constants of Fe(III)complexes are greater than those of Cr(III) com-plexcs. Furrhor. ill literature though the stabilitycon sran ts for a l<1.rgcnumber of Fe(III) complexesare available, those of Cr(III) are meagre. Tileoxalato complexes have been studied for both themejal ions by different workers? using various tech-niques. Broadly, log ~3 of the Ierrioxalajo complexis greater than that of chromioxalato complex. WithDUTT et al.: COMPLEXES OF TERVALENT METAL IONS WITH MALONIC & SUBSTITUTED MALONIC ACIDSmalonic acid only, thc overall stability constantof Fc(III) complex has been determined polaro-graphically4. This value (log ~ = 15'7) is some-what lower than our value (log ~ = 18·0). Muroet al,l6 reported for Cr(III)-malo11ato complexes:log Kl = 7·06, log K2 = 5,79 and log K3 = 3,30,these values are somewhat higher than thoseobtained by us. With EDTA, the stability con-stants of both Fe(IIl) and Cr(III) complexes havebeen determil'edI2,16-IM, and in this case also Fe(III)complex has been found to be stabler than Cr(III)complex.It may be mcnj ioncd t nar although the electronicconfigura+ions of Fe(lII) and Cr(lII) have nosimilarity with those of AI(III), Ga(III) and IJI(III),except that all arc in t heir trivalent state,the stability constants of these two categoriesof metal ions are not widely diffcrcn r. Parti-cularly, one can rather find analogy ill theoverall stability constants of Fe(IIl) and Ga(III)complexes; tile Fe(III) compk-xes in most casesarc slightly more stable than Ga(IlI) complexes.Such trend iu the stability constan ts lias also beenobserved in Iijcrarurc> for ami nopolycarboxvla+cs.It is well known that the increases in basicity of aligand increases the stability of its metal complexes.The trend in basicity of the substituted malonicacids in terms of pIq is: me tnyl > n-butyl >phenyl > malonic > bromo. Inspection of tlleresults ill Table 2 reveals the following orders inlog Kl values of the substituted malonaro complexesof each metal ion: Al(III): methyl r-.J n -butyl >phenyl > malonic > bromo; Ga(IlI): mctuyl >n-bLJ.tyl > malo.iic > phenyl > bromo ; In (III) :methyl> n-butyl > phcl,yl > malonic > bromo;Fe (II!) : methyl > n-butyl > phenyl > malonic>bromo; and Cr(III): methyl> n-blltyl > malonic,From the above comparison it is apparent thatfor each metal ion the seqncilcc of log Kl values isbased on the basicity of the ligancls. In a case ortwo, methyl and n-butylmalo!1.ates have similarlog Kl values, this is not unexpected as one can seein Table 1 that though pIq of merhylmalonic acidis slightly higher than tnar of n-butylmalollic acid.Oil the otner hand, PK (pJ(i+PK~) of n-butyl-malonic acid is slightly higher than that of methyl-malonic acid.References1. KRISHNAMURTHY,K. V. & HARRIS, G. M., Chern. Rev.,61 (1961), 213.2. SILLEN, L. G. & MARTELL, A. E., Stability constants ofmetal-ion complexes (Chern. Soc. Sp!. Publication No.17, Supp!. o. 1), 1971.3. DUTT, N. K. & BOSE, P., Z. anorg. allg, Chem., 295(1958), 131.4. SCHAAP,W. B., LATINEN, H. A. & BAILAR, J. C., J. Am.chem. Soc., 76 (1954), 5868.5. Cox, R. F. & l'vICFLVAIN,S. M., cited in Org. Syntb.,Vo!. 2, edited by A. H. Blatt (John Wiley, New York),1947, 272, 279.6. ADAMS,R. & KAMM, R. M., cited in Org. Synih., Vol. 1,edited by A. H. Blatt (John Wiley, New York), 1947,250.7. LAVENE, P. A. & MEYER, G. M., cited in Org. Synth.,edited by A. H. Blatt (John Wiley, New York), 1947,250.8. PALMER, C. S. & MCWHERTER, P. W., cited in Org.Synth., edited by A. H. Blatt (John Wiley, New York),1947, 245.9. SCHR0DER, K. N., Acta chem. scand., 20 (1966),140t.10. IZATT, R. M., FERNELlUS, W. C., HAAS, C. G. & BLOCK,B. P., J. phys. Chem., 59 (1955), 70.11. STARY, J., Anal. chim. Acta, 28 (1963), 132.12. BOTTERI, E. & ANDEREGG, G., Helu. chim, Acta, 50(1967), 2349.13. L'ELPLATTENIER, F. & ANDEREGG,G., Helv. chim, Acta,47 (1964), 1792.14. MOELLER,T. & CHU, S., J. inorg. nucl, Chem., 28 (1966),153.15. MURO, H. & TSUCHIYA, R., Bull. chem, Soc. Japan, 39(1966), 1589.16. RYBACHIKOV, D. I., MAROV, 1. N. & K'O-MIN, K.,Russ. J. inorg. cu,«, 8 (1963), 326.17. ZHIRNOVA,M. M., ASTAKHOV,K. V. & BARKOV, S. A.,Russ. J. pltys. Chem., 41 (1967), 366.18. PECKSOK, R. L., SHIELOS, L. D. & SCHAEFER, W. P.,Inorg. Chem., 3 (1964), 114.1003

Stability Constants of the Complexes of Al(III), Ga(III), In(III), Fe(III) & Cr(III) with Malonic & Substituted Malonic Acids

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Stability Constants of the Complexes of Al(III), Ga(III), In(III), Fe(III) & Cr(III) with Malonic & Substituted Malonic Acids

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