Islam, V

Shah, M A A

Siddiqi, K S

Zaidi, S A A

English

NOPR

Indian Journal of Chemis ryVol. 22A, April 1983, pp. 355-356Stability Const nts of Neodymium III),Samarium( I), Gadoliniumfflfj,Terbium(III , Dysprosium(III) 8,LHolmium(III) qomplexes with 2-Amino-5-mercap~o-1 ,3,4-thiadiazoleK S SIDDIQI", M A ~ SHAH, V ISLAM & S A A ZAIDIDepartment of Chemistr~, Aligarh Muslim Universit , Aligarh202001Received 2 August 1982; revised 18 October 1982; Jccepted 5ovember 1982Neodymium(III), Sm(II~, Gd(III), Tb(II1), Dy(III) a~d Ho(II1)form I: I, 1:2 and 1:3 (mJtal-ligand) complexes with 2-amino-5-rnercapto-I, 3, 4-thiadiazol (AMTDA) in aq. ethanol (50~, v/v) andat ionic strength of 0.1 M ( aCI04). The stability constants of thesecomplexes have been deter ined potentiometrically. The changes infree energy, enthalpy and entropy have also been repo ted. Theorder of stabilities of the ch lates is:Gd > Tb > Nd > Ho > Sm >Dy.The corriplexes of 2-amino-5-mercapto-l, I 3, 4-thiadiazole (AMTD with the first and second rowtransition metal ions ave been reported 1 but ~workhas been done on its complexes with rare ea ths. Incontinutation of our e rlier work on bismuthio (1)2, wereport herein the fo marion constants of A TDAcomplexes of Nd(I J), Sm(III), Gd(III), b(III),Dy(III) and Ho(III in aq. ethanol (500~' v/v)employing Bjerrum-C lvin ' pH-titration tech ique asadopted by Irving an Rossetti". A solvent co rectionfactor was applied in tre pH-meter reading for e non-aqueous solvent usedf-Anhydrous rare earth chlorides (NdCI3, ITbCI3,DyCI3, HoCI3) were hsed as such or prepared fromtheir oxides (Sm203, Gd20))6. The appropriateamounts of these rar5 earth chlorides were di solvedin perchloric acid 011 desired normality to breventhydrolysis. The sOlufion of AMTDA (m.p. 245°,Koch-light, AR) wasl prepared in ethanol. Sodiumhydroxide (BDH), sodium perchlorate (Reidel) andperchloric acid (E. M~rck) solutions were prepared inCO2-free triply distilled water. IFor pH measurerents an Elico LI -to pH-meter(accuracy ± O.Op pH unit) equipped wittl glass-calomel electrode afsemblY was employed. Thecalibration of pH elec rode system was done ith thehelp of standard buffe s (PH 4 and 9.2). The tJrationsof the following solutions (total volume 50 ~'l) werecarried out under purred N 2 atmosphere in ethanol-water (50%, v/v) agarst carbonate-free NadH (1.6x 10 -I M) at constant ionic strength of 0.1 M/S,HS- C C-NH2II 1/N-N<I)/S"HS-C c= NHII tN-NH(II )/S"C,H/C-SHII N IN-NH( II I)(NaCl04): (a) 5 ml of perchloric acid, (b) 5 mlperchloric acid + 25 ml AMTDA and (c) 5 mlperchloric acid containing metal ion + 25 mlAMTDA.The ligand exists in three forms 7, (I) yellowishprismatic needle (m.p. 245°), (II) transparent whitecubes (m. p. 232°) and (I II) brownish yellow rectangularplates (m.p. 224°). Janniah and Guha+" have shownthat in solution, it exists in two forms (II and III) inequilibrium with each other, one form being in slightexcess over the other. Their interconversion involvesthe formation of a bridged structure (III). The ligand isamphoteric in character: it accepts a proton at theamino group. and loses a proton at the mercaptogroup thereby showing its potentialities of formingcomplexes with the metal ions.In the present work the acid ligand curve deviatesfrom pure acid curve near B=2.7. The values of nAwere calculated at B= 2.7 and 6.8 and were found to be0.838 and 0.274 respectively, indicating thedissociation of one proton only. Log K~ was obtainedfrom the proton-ligand formation curve by half-integral method. This value was compared with thatcalculated by pointwise method.For the calculation of metal-ligand stabilityconstants of rare earths, very dilute solutions wereused to prevent hydrolysis. The pL values werecalculated at various n values and the metal-ligandformation curves were obtained by plotting n againstpL. From these curves, the values of the stabilityconstants, log K1, log K2 and log K3 could be directlyread by half-integral method (Table I). The values werealso calculated by pointwise method. The highest nvalues obtained at pH = 6.6 in all the cases were lessthan 3.5, suggesting the formation of 1:1, 1:2 and 1:3metal ligand complexes. The abrupt change in n valueabove pH = 6.6 was not considered due to thepossibility of hydrolysis. Standard deviations werealso calculated by the known equation.355INDIAN J. CHEM., VOL. 22A, A RIL 1983Table I-Metal Ligand Stability Constants at J1 =0.1 M (NaCI04)Nd Sm Gd Tb Dy HologK, 25° 6.416±0.29 6.375±0.25 6.500±0.27 .375±0.26 6.475±0.17 6.425 ±0.2435° 6.176±0.31 6.176±0.15 6.240±0.25 .120±0.06 6.220±0.02 6 ..240±0.20log K2 25° 5.200±0.22 5.200±0.26 5.400 ± 0.08 .250±0.13 5.225±0.25 5.275±0.0235° 5.000±0.27 5.010±0.24 5.180±0.25 .035 ±0.27 5.015±0.26 5.095±0.26log K3 25° 4.200 4.175 4,275 4.200 3.900 4.05535° 4.040 4.005 4.015 4.025 3.735 3.905log x, 25° 15.816 15.750 16.175 15.825 15.60 15.75535° 15.216 15.191 15.515 15.280 14.970 15.240The overall changes in free energy (~G) at 25° and35°C, enthalpy (~H) and entropy (~S) have beencalculated using the standard equations and the valuesare given in Table 2. ~G values are almost similar while~H and ~S values differ considerably. The positivevalues of enthalpy change and increase in entropy 10.11have been attributed to the presence of metal chelatebond causing greater steric strain. The formation of 1:2and I: 3 complexes appears to be less endothermic andthe values of ~H and ~S are less positive than those ofthe I :I complex. This is completely in agreement 12with the expectation of less dehydration of the cationand probable degree of covalency in the metal-ligandbonding. The greater the steric hindrance of thecomplex, the greater will be the values of log K 1/K2.The deviation in the stability order is also caused by then-bonding tendency':' of the sulphur atom with themetal ion.One of the authors (M A A Shah) thanks the UGCNew Delhi, for the award of a teacher fellowship.ReferencesI. Gajendragad M R & Agrawal U, J inorg nucl Chem, 37 (1975)2429.2 Zaidi S A A & Islam V, Indian J Chern, 15A (1977) 155.356Table 2-Changes in Free Energy, INd Sm Gd21.500 21.400 21.98021.371 21.306 21.785nthalpy and EntropyTb Dy Ho21.510 2l.l90 21.42021.318 21.020 21.400-"'G2535'(kcal/rnol I)"'H(kcal/mol I)"'5 153.43(cal deg ~Imol -I)25.125 23.407 27.633 27.013 26.381 21.127147.67 163.46 159.94 156.54 140.143 Bjerrum J. Metal amine formation inlaqueous solution (P Haase,Copenhagen), 1941, 298; calV~' M & Wilson K W, J Amchem SOl', 67 (1945) 2003; Calvi M & Melchior. M G, JAm(~em Soc. 70 (1948) 3270.4 Irving M & Rossotti H S, J chem 5 r, (1953) 3397; (1954) 2904.5 Van itert L G & Hass C G, J A chem SOl', 75 (1953) 451.6 Braue G. A handb(;ok o/preparatiV}inorganic chemistry, Vol 2(4cademic Press, New York), I 65, 1147.7 Bambbs L L, Chemistry ofheterocyc ic compounds, 4 (1952) 143.8 Jann~,h S L·& Guha P C. J Am ch m SOl', 52 (1930) 4860.9 JannI~h S L & Guha P C, J Indian Inst Sci, 16A (1933) II.10 Martell A E & Calvin M, Chemistry ofmetal chelate compounds<1cademic Press, New York), I 52, 149-151.II Cottor F A & Wllkmson G, Ad anced inorganic chemistry(Second Willey Eastern reprint) 1978, 650.12 Degisbher G & Choppin G R, J ino:J nucl Chem, 34 (1972) 2823.13 Dutt K & Rahut S. J inorg nucl chem, 32 (1970) 1033 & 1035.-

Stability Constants of Neodymium (III), Samarium(I), Gadolinium(III), Terbium(III) , Dysprosium(III) & Holmium(III) Complexes with 2-Amino-5-mercapto-1,3,4-thiadiazole

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Stability Constants of Neodymium (III), Samarium(I), Gadolinium(III), Terbium(III) , Dysprosium(III) & Holmium(III) Complexes with 2-Amino-5-mercapto-1,3,4-thiadiazole

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