Indian Journal of ChemistryVol. 22A, February 1983, pp. 131·133

Transition Metal Ion Chelates of Some Substituted Salicylic AcidsPart II-A Thermodynamic Study

C R DHAT & D V JAHAGIRDAR*Department of Chemistry, Marathwada University, Aurangabad 431 004

Received 5 July 1982; revised and accepted 8 December 1982

The thermodynamic parameters !J.G, !J.H and !J.S have been determined for the complexation reactions ofMn(II), Co(lI), Ni(II), Cu(II) and Zn(II) with some substituted salicylic acids. The enthalpy change during chelateformation is separated into electrostatic and nonelectrostatic parts. Crystal field stabilization energy (CFSE)values of 1:1 chelates have also been calculated. It is observed that the increase in 10 Dq values for all these Iigandsis comparable with those of certain oxygen donating ligands like oxalic acid, malonic acid and salicylaldehyde. CFSEvalues are also obtained from spectral transitions for two copper chelates. These values are somewhat lower thanthe thermodynamic values.

In our earlier communication- we have attemptedto separate the electrostatic and non-electrostaticcomponents in the case of Cu(II) chelates of substitu-ted salicylic acids. In continuation of this work, wehave studied the chelates of Mn(II), CO(II), Ni(II) andZn(II) of various substituted salicylic acids. Thetemperature coefficient method was adopted to deter-mine the L,H values. Crystal field stabilization energy(CFSE) values of these chelates have been determinedand an attempt has also been made to compare thethermodynamic stability constant values with thoseobtained spectroscopically in the case of a few chelatesofCu(I).

Materials and MethodsThe experimental details of potentiometric measure-

ments, the methods of calculation of stability cons-tants, thermodynamic parameters and the errorlimits in L,H and 6S were the same as given earlier',The spectrophotometric measurements were madewith a Unichem SP 700 spectrophotometer.

Results and DiscussionIn the transition metal ion series, Cu(II) alone

forms 1:1 and 1:2 chelates'. The values of L,Hobtained for 1:1 metal complexes of substitutedsalicylic acids follow the order Zn(II) < Cu(II) >Ni(II) > Co(H) > Mn(II) (Table 1). The 6S valuesfor eu(II) chelates are relatively higher than thosefor the other transitional metal ion chelates (Table 1).In the case of Cu(II) and Ni(II), higher negativevalues of b,H indicate relatively greater covalentcharacter of the metal-ligand bond in these complexesas compared to the other metal ion complexes. ForCu(II), Ni(II), Co(Il) and Zn(II) enthalpy andentropy terms are favourable for complex formation.It may, therefore, be concluded that except in thecase of Mn(II), the stabilities of metal complexes ofsubstituted salicylic acids are partly due to covalentcharacter of the metal-ligand bond and partly tofavourable increase in -entropy. A higher negativevalue for £::,H is normally expected as compared toa lower L,S value. The relatively high entropy

Table 1-Nonelectrostatic (non) and Electrostatic (en Thermodynamic; Quantities Associated with Reaction of Bivalent MetalIons with Salicylic acid and 3,5-Dinitrosalicylic acid in Aqueous Medium at 25 ± 0.02°C

[Values of !J.G and !J.H in kJ mol=' and of !J.S in J K-l mol=]

Metal -!J.H AS C a -!J.Gnon -!J.Gel -!J.Hnon !J.Hel ss:Chelates of salicylic acid

Mn(I1) 25.9 64.0 -609.53 6.56 33.5 21.3 33.5 7.6 97.4Co (II) 28.5 71.9 -659.24 6.67 36.8 23.0 36.8 8.4 105.4Ni(II) 29.3 74.8 -677.55 6.64 37.6 23.9 37.6 8.8 ]08.3Cu(II) 31.0 101.2 -.842.36 6.01 41.4 29.3 41.4 10.5 134.6

27.2 63.1 -591.00 7.11 52.7 17.2 33.5 6.3 94.5Zn(II) 23.4 89.9 --771.73 5.11 33.1 27.2 33.1 9.6 123.3

Chelates of 3,5-dinitrosalicylic addMn(II) 11.7 20.9 -340.0 5.73 15.9 11.7 15.9 4.2 54.4Co(II) 12.6 26.8 -376.7 5.84 17.2 13.0 18.4 4.6 60.2Ni(I1) 16.3 24.7 -3636 6.98 20.5 12.6 20.9 4.6 58.1Cu(II) 16.7 73.6 -669.7 4.60 25.1 23.4 25.1 8.4 107.1

10.5 60.2 -585.9 3.75 17.6 20.5 18.0 7.5 93.7Zn(II) 8.4 33.5 -423.8 3.89 13.8 14.7 13.8 5.4 66.9

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INDIAN J. CHEM., VOL. 22A FEBRUARY 1983

values in CU(Il) complexes may be attributed to Jahn-Teller distortion of the octahedral geometry in Cu(ll)resulting in greater covalent character of the metal-ligand bond and consequently greater effective neu-tralization of cationic and anionic charges.

Separation of DH into electrostatic and nonelec-trostatic parts-The nonelectrostatic (non) andelectrostatic (el) thermodynamic quantities associatedwith reaction of transition metal ions with salicylicacids have been computed as described earlier! andare set out in Table I. Only representative data aregiven. The parent acid and 3,5-dinitrosalicylic acidhave been chosen as representatives because salicylicacid is the most and 3,5-dinitro the least basicamongst the substituted acids under study. Thevalues of all the constants, for the remaining acids,follow the same sequence and also have valuesintermediate between the corresponding chelates ofsalicylic and 3,5-dinitrosalicylic acids.

All the transition metal ions have numericallysimilar DHel values as compared to DHnon values.It is also evident that the order of magnitude of DHamong metal ions carrying the same charge is deter-mined chiefly by the magnitude of DHnon. 6Hnonfollow the order: Mn(H) < Co(Il) < Ni (II) <Cu(1l) > Zn(II) being due to the ligand field stabiliza-tion energy. The nonelectrostatic enthalpy changesarise from changes in the CFSE accompanyingcomplex formation.

The complex formation in aqueous solutioninvolves the replacement of the metal-water bonds bymetal-donor bonds. The heat change upon complexformation will be related to the difference betweenthese bond strengths. The change in the heat ofhydration will be mostly a result of ion dipoleinteraction, while the heat change on complexationmust be the combined effect of the electrostatic andcovalent interactions together with a structural con-tribution and ligand field stabilization. Electrostaticheats (6Hel) of the salicylic acid complex formationshow a linear relation with the heats of hydration,(DHh) .of the related metal ion. The correlationsuggests that most of the salicylic acid complexesare essentially similar in size and geometry of thecoordination sphere to the corresponding aquoions.

It is known that the softer metal ions have agreater affinity for the softer donors/. Values ofDHnon are plotted against the quantity En* introdu-ced by Klopman" as a measure of hardness andsoftness of a metal ion in aqueous solution. A softmetal is characterised by large negative value of En *and vice versa. The transition metal ion chelatesof salicylic and substituted salicylic acids exhibit asatisfactory linear relation between fj,Hnon and En';'.DHnon increases with the softness of the metal ion.

The a values or the ratio of D Gnon/ D Gel aregenerally constant for Mn2+ through Zn2+ chelates ofsalicylic acid, the variation being maximum around2.0. The magnitude of D Gnon/ D Gel is sufficientlylarge and the minimum temperature in the InK valuesis beyond 100°C for chelates of C02+, Ni2+ and Cu 2+and a bit less, i.e. between 80-100° for chelates of"Mn2t- and Zn2+. DH under these conditions wouldalways be negative. The metal ions are intermediatesand except salicylic acid (pKOH = 13.3), the substitu-ted acids (pKOH = 10 to 11) are also intermediates.These therefore exhibit good chelating tendenciesfor the metal ions under examination.

Calculation of crystal field stabilization energies-It can be seen from Table 1 that DS values are nearlyconstant for the 1:1 complexes of Mn(H), Co(Il),Ni(ll) and Zn(II) with a particular substitutedsalicylic acid. DS value for 1:1 copper complex is,however, higher in the metal series, but in no casethe value differs from the average by more than 25 J.However, as per George and McClure4 DS valuesmay be regarded as constant as the difference in 6.Swithin 25.1 J for MnCH) to Zn(Il) for salicylic andsubstituted salicylic acids. The DH values relativeto the value for Mn(H) and for Co(Il), Ni(IJ), Cufll)and Zn(III) and of CFSE values (aH) for Co(II),Ni(II) and Cu(ll) for each substituted salicylic acidhave been calculated by the method of George andMcf.lure",

The sn and 10 Dq values for CoCII), Ni(U), cetmcomplexes with the substituted salicylic acids are setout in the Table 2.

The formation of 1:1 complex between a metalion and a ligand in an aqueous solution can berepresented by a general equation (1)

Table 2-Crystal Field Stabilization Energy CoH) Values and 10Dq Values for Co(Il), Ni(U) and Cu(II) Chelates ofSubstituted Salicylic Acids

[Values of 'OH and Er in kJ mol:"; 10Dq values are given in parenthesis and are expressed in crrr": number of ligandand water molecules are I and 4 respectively]

aH of chelates ofLigand s, (Mn-Zn)

Salicylic acid 200.8

3-Bromo-SA 196.7

3,5-Dibromo-SA 200.8

3.5-DichJoro-SA 200.8

5-Nitro-SA 200.8

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Co(II)

104.6(12,500)100.4(12,000)]04.6

(12,500)104.6

(12,500)104.6

(12,500)

Ni(Il)

146.4(10,200)

154.8(10,800)142.3(9,900)146.4

(10,200)142.3(9,900)

Cu(JI)

121.3(16,900)125.5(17,500)125.5

(17,500)121.3

(16,900)121.3

(16,900)

DHAT & JAHAGIRDAR; TRANSITION METAL CHELATES OF SUBSTITUTED SALICYLIC ACIDS

MlH20)6n+(aq) -r mL ~ M(H20)6-mLmn+laq)-I-mH20 ... (1)

The I: I complex formation of salicylic acid witha metal ion results in the removal of two watermolecules by the ligand from the hydration sphereof the metal ion, e.g.

M(H20)6 -I- L ~ M(H20)4L-I-2H20t:,H values for aquo ions are available in the

literature' and the corresponding lODq values are12,000, 9,900 and 14,500 cnr+ for C02+, Ni2+ andCu2+ aquo ions.

The ratios of CFSE for M(H20)4L/M(H20)6[M = Cu(II), Ni(U) and CoCIl)] are 1.2, 1.0 and 1.0respectively. This means that nearly 20 % increasein 10Dq values is observed for Cu(IT) in 1: 1 com-plexation, while there is no indication of any signi-ficant increase for either Co(Il) or Ni(II). This extrastabilization of Cu(II) complexes may be attributedto Jahn- Teller distortion usually observed in coppercomplexes.

Salicylic acid behaves as an oxygen donor. The10Dq values observed for it and various substitutedsalicylic acids can be compared with 10Dq values ofcertain oxygen donating ligands like oxalic acid,malonic acid and salicylaldehyde. The increase inlODq values reported in the case of all these ligandsis of the same order as that obtained for salicylicand substituted salicylic acids. This observation isin agreement with the conclusion of George andMcClure that the contribution of oxygen atom isthe same no matter whether it is in a water molecule,a carboxylate ion or a phenolate ion.

The order for CFSE is 3HNI > stt-; > in«; Thepresent experimental results are in agreement with

the observation+ that Er and CFSE for many com-plexes are determined solely by the atoms directlybonded to the metal ion; the structure of the rest ofthe ligand group has far less influence.

Comparison of CFSE values obtained from ther-modynamic parameters and spectral transitions-TheAmaxvalues for I:1 copper complexes of 3-bromo- and3,5-dinitro-salicylic acids have been obtained fromspectrophotometric measurements. The thermody-namic and spectrophotometric values of CFSE for1:1 copper complexes of 3-bromo- and 3,5-dinitro-salicylic acids are 20.9. 20.9 and 15.5, 16.8 kJmol-1respectively. The thermodynamic values are some-what higher than the spectroscopic values. In theCFSE estimations, Cu2+ chelates have been consideredto be having octahedral geometry although the fieldsare undoubtedly less symmetrical especially for Cu(IT)and Ni(II) as a result of the Jahn-Teller effect. Inthe literature! higher thermodynamic values arereported in the case of NiF2, NiCI2, NiBr2, COF2and CrCI3. The experimental data reported byFiggis" for M(H2062+ in the case of transition metalions also indicate that the lODq values obtained fromspectrophotometric measurements are lower thanthose determined from heats of hydration.

References1 Dhat C R & Jahagirdar D V, Indian J Chem, 21A (1982)

792.2 Pearson R G, JAm chem Soc, 85 (1963) 3533.3 Klopman G, JAm chem Soc, 90 (1968) 233.4 George P & McClure D S, Progress in inorganic chemistry,

Vol. I (Interscience Publishers, New York), 1959, 381:5 Figgis B N, Introduction of ligand field (Interscience

Publishers), 1966, 227.

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