Indian JOlUTuli of Chemistry Vol. 38A, February 1999, pp. 166-169 Structural influence on the stability of some nickel(II) ternary complex chelates M Sivasankaran Nair*, J Pauline Anbarasi , P Thillai Arasu & M A Neelakantan t Department of Chemistry, Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli 627 0 12 , India Received II Mar ch 1998; revised 5 October 1998 Computer bas ed analysis of the pH titration data on the Ni(JI)-1 ,3-diaminopropane (A)-DL-2-aminobutyric acid, DL-3 -aminobutyric acid , 4-aminobutyric acid a nd DL-4-amino-3-hydroxybutyric acid (B) sys tems show the presence of NiAB and NiABH ternaty complexes . Both the pH-metric and electronic spectral mea surement s show that the 5 and 6 membered Ni(I1) ternary complex chelates are markedl y stable. The r esu lt s indicat e th at the amino group of th e amino acid secondary ligand is protonated in the NiABH species. The ever in creasing importance of ternary complexes es pec ially those involving ligands containing functional groups id entical with those present III enzy mes viz. - COOH, - NH 2, - CONH- etc. is obvious from the application of such complexes in many analytical and bi ological reactions' . A mixed (O,N) donor such as amino acids with its hard oxygen and rel ativel y so ft nitrogen donors will be able to form co mpl exes with N i(II ), which I ies in the bord er region between a hard a nd soft accepto r. Thu s, it appears to be interesting to study N i(ll)-diamine- amino acid t.:rnary complex systems, where the di a min e contains two amino nitrogen donor atoms. The ternary systems repo rt ed in this paper are: Ni(II)- 1 ,3 -diaminopropane (tp) (A)-DL-2, aminobutyric acid (2aba), DL-3-amino-butyric acid (3aba), 4- aminobutyric acid (4aba) and DL-4-amino-3- hydro xy butyric acid (a hb a) (B). The studies have been carried out by pH titrim etry at bi ologically releva nt conditions viz. at 37°C and /=0. 15 111 01 dm - 3 (NaC I0 4 ) in aqueous perchlorate med ium . The axillary data under th e prese nt experimental conditions for the parent binary com-plexes have been taken fr om the literature 2 . Electronic spectral measurements for the N iAB type of ternary complexes have al so been done. Materials and Methods The method of preparation of all the reagents and oth er experimental details have been described fDcpartment of Chemistry, National Engineer in g Co ll ege, K R Nagar, Kovilpatti 628 503 earlier 2 .3. All the ligands used were F1uka products of Puriss quality. The calculations were done with the aid of the computer program 4 MlN IQUAD-75 on a Cyber 180/830A computer, fixing the ac id di ssociation constants of the ligands a nd the parent binary stability constants estimated under identical conditions 2 . The electronic spectral meas ur emer;ts were carried o ut on a Perkin-Elme r UV-vi sible spectrophotometer model Lambda 3 B interf aced with NEXUS computer usi ng Perkin-Elm er co mpute ri sed spectroscopy software. The I: I: I solutions at a partic ul ar pH where the N iAB co mpl exes had maximum concentration as ev id enced by pH met ri c species di stribution plots were used for measurin g th e spectra. All the results are reco rd ed in Tables 1- 3. Results and Discussion The ternary complex species of stoichiomet ry NiAB and NiABH in addition to th e binary species HA , H 2 A, NiA, NiAH, NiA 2' HB , H 2 B, NiB and NiB] have been detected in the Ni(lI)-tp(A)-2aba, 3aba a nd 4aba (B) system s. The Ni(II )-tp(A)-a hb a (B) tern ary system gave ri se to the formati on of o nl y NiAB ternary s pe c ie s in add iti on to the binary species HA , H 2 A, NiA, NiAH, NiA 2, HB , H2B, H3B, NiB a nd NiB2 binary species. The stabi li ty constants for the ternary complexes are re ported in Table 2. Stability and structure of NiAB complexes The Log values obtained in the Ni(lI)-tp(A)- 2aba, 3aba, 4aba and ahba (B) ternary systems (Table 2) are comparable to the re specti ve lo g va lu es ; . ."