Indian Journal of ChemistryVol. 25A, February 1986, pp. 201-202

Formation Constants of Some TernaryComplexes of Copper(II) with

n-Bonding Ligands

SURESH PATEL & UMA CHUDASAMA*Department of Chemistry, Faculty of Science,M.S. University of Baroda. Baroda 390 002Received 15 April 1985; revised 26 June 1985;

rerevised 19 August 1985; accepted 9 September 1985

The formation constants of ternary complexes of the type. CuAL[where A = dipyridyl (dipy), 1,1O-phenanthroline (o-phen). 2-{2'-pyridyl)benzimidazole (PyBI), 2-{2'-pyridyl)imidazoline (PyIM).dipyridylamine (dpa), 5-nitro-l,l O-phenanthroline (5-NOro-phen)and L =anthranilic acid] have been determined in I: I (vIv) dioxan-water medium at ionic strength J1= 0.2 mol dm ..3 (NaCI04) andtemp. = 30°C. The effect of metal-ligand rr-interaction andsubstitution on the tertiary amine has been discussed.

Formation constants. K~:~L of ternary complexes ofthe type MAL where M =Cu(Il) or Ni(II). A= tertiaryamine and L = c-bonding ligand are greaterthan those expected from statistical consideration 1-3,

due to M-+A zr-interaction'':". It was, therefore,thought of interest to study the ternary systems whereL = anthranilic acid (in which the chelating amino andcarboxylate groups are attached to an aromatic ring)and A = dipy, o-phen, 2-(2' -pyridyJ) benzimidazole(PyBI), 2-(2' -pyridyljimidazoline (PyIM), dipyridyl-amine (dpa) and 5-NOro-phen all of which coordinatethrough two nitrogens and have delocalised rt-electrons.

All the reagents were of AR grade, except PyBI andPyIm which were prepared by a known method".

The titrations were carried out in I: I (vIv) dioxan-water medium at II =0.2 mol dm -3 (NaCI04) andtemp. = 30"C, using a digital pH meter (accuracy±0.01 pH units). The pH meter readings werecorrected for the non-aqueous solvent by the methodsuggested by Van Uitert and Haas 7.

The proton-ligand formation constants and theformation constants of the binary complexes CuL,CuL2, CuA, CuA2 were evaluated using the SCOGScomputer programme as detailed earlier",

For the determination of formation constants ofternary complexes, CuAL, the SCOGS computerprogramme was used under the following conditions:

(i) The ternary complex formation was assumed tooccur by the reaction:

[CuA]2+ + L~[CuAL] +

since formation of [CUA]2 + was complete in the lower

pH range. The species considered to be present in thesolution were L, LH, LH2, [CuA]2+ and CuAL. AsCu2+ and A were in equimolar proportions and as theformation of [CUA]2 + was complete, no free ligandspecies A, HA + and H2A2+ were considered. Theformation constants of the ternary complexesCu.PyBI.L and Cu.PyIm.L were determined using thisprogramme. The formation constants of Cu.PyBI,Cu(PyBI)z, Cu.Pylm, and Cu(PyIm)z could not bedetermined, as complete formation of [CuAY + wasassumed.

(ii) The ternary complex formation was assumed toproceed by the reaction:

Cu2+ +A +L~[CuAL] +

under the assumption that possible species in solutionwere LH2, LH, L, H2A 2+, HA +, A, Cu2+, [CuL] +,CuL2, [CUA]2 +, [CuA2]2+ and [CuAL] -.

The concentration-pH profile indicated that in thepH range ~ 2.0 to ~ 3.0, Cu(II) and CuA were themajor species while in the pH region ~ 5.5 to ~ 7.0 thespecies CuA and CuAL predominated. The formationof CuA2, CuL2 and CuL was very less.

The values of LllogK, i.e. 10gIQ~~L -logIQ~L aresmall and negative (Table I) due to Cu-+A rr-interaction which reduces electron density around themetal ion in [CUA]2 + and makes it comparable to thatin Cu2+.

The formation constants 10gJQ:~~L (see Table I)follow the order: Cu(5-N02-o-phen)L >Cu(PyBI)L> Cu(dipy)L > Cu(o-phen)L > Cu(dpa)L>Cu(PyIm)L.

2-(2' -Pyridyl)imidazoline (PyIm) coordinates via onepyridine nitrogen and one of the imidazoline nitrogenatoms. In this case n-interaction is possible only withthe pyridine ring, imidazoline ring being saturated isdevoid of delocalised n-electron cloud. Enhancedstability of Cu(PyBI)L is due to additional z-

Table I - Stability Constants of Ternary Complexes ofCu(II) in Dioxan-Water Medium

[PKrH=6.6; IOg~~L=4.97; logKc~t2=4.0; J1=0.2M (NaCI04;

temp. 30°C; Solvent = I: 1 (v/v) dioxan-water]Complex 10g~~~L AlogK

[Cu(5-N02-o-phen)L] 4.55(±0.03) -0.42[Cu(PyBI)L] 4.31(±0.09) -0.56[Cu(dpa)L] 4.22(±0.18) -0.75[Cu(dipy)L] 4.17( ±O.02) -0.79

[Cu(o-phen)L] 4.14(±0.04) -0.83[Cu(PyIm)L] 3.57(±0.19) -1.40

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INDIAN J. CHEM., VOL. 25A, FEBRUARY 1986

interaction involving nitrogen of benzimidazole ring.Cu(dipy)L and Cu(o-phen)L are of comparablestability because the extent of Cu-+A z-interaction iscomparable.

In the case of the complex Cu(dpa)L, chelatingimino group of the ligand, situated in between twopyridine moieties, enhances the electron density on thering because of the lone pair on imino nitrogen atoms.However, delocalisation of electron density in dpaoccurs over a larger area than in dipyridyl whichrenders the ternary complexes Cu(dipy)L andCu(dpa)L of comparable stability.

The highest stability of [Cu(5-N02-o-phen)L] is dueto the nitro group which makes 5-N02-o-phen astronger n-acid than o-phen..

It is observed" that ~logK value of Cu-A

202

anthranilate is more negative as compared to that ofCu-A-aminophenolate.

Thanks are due to Prof. P K Bhattacharya for usefulsuggestions. One of the authors (SP) is also thankful tothe M.S. University of Baroda, Baroda for the awardof a research fellowship.

References1 Sigel H, Chimia, 21 (1967) 489.2 Griesser R & Sigel H, Inorg Chern, 9 (1970) 1238.3 Chidambaram M V & Bhattacharya P K, J inorg nucl Chern, 32

(1970) 3271.4 Sigel H & Prijs B, Chimia, 29 (1975) 134.5 Patel P J, Patel V K & Bhattacharya P K, Inorg Chern, 21 (1982)

3168.6 Walter J L & Frieser H, Analyt Chern, 26 (1954) 217.7 Van Uitert L G & Hass L G, J Arn chem Soc, 75 (1953) 457.8 Gopalakrishnan K & Bhattacharya P K, J chem Soc Dalton Trans,

(1982) 353.