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A study of ligand exchange in some nickel complexes
J. inorg,nucl.Chem.,1971,Vol.33, pp. 529 to 533. PergamonPress. Printedin Great Britain
A
STUDY
OF LIGAND EXCHANGE NICKEL COMPLEXES
IN
SOME
D. C. PATEL and P. K. B H A T T A C H A R Y A Chemistry Department, M. S. University of Baroda. Baroda-2, India
(First received 16 February 1970): in revised fi~rm 13 July 1970) A b s t r a c t - T h e formation constants of nickel complexes of catechol and pyrogallol have been determined by Irving-Rossoni method and compared with that of nickel-ethylenediamine complex. On treating nickel bis and tris ethylenediamine complexes with catechol and pyrogallol, ethylenediamine is replaced. The resulting solids have been analysed and characterised by magnetic and spectral studies.
NICKEL is known to form chelate complexes with many organic ligands[1-3]. Ethylenediamine forms l : 2 and 1 : 3 compounds with nickel and their stabilities have been determined [4]. This work presents a study of the relative tendencies of ethylenediamine and di and tri hydroxy phenols to replace each other in nickel chelates.
Determination of stability constants The stepwise formation constants, were determined by lrving-Rossotti method [5] using a Metrohm pH Meter (accuracy ___0-05). The following solutions were prepared: 1. HC104 (0.05M, 10ml)+NaCIO~ (IM 9.5ml)+double distilled water (30.5 ml) : vol. 50 ml 2. HCIO4 (0.05M, 10ml)+NaCIO4 (1M, 9.0ml)+reagent (0.05M, 10ml) + double distilled water (21 ml): vol. 50 ml 3. HCIO4 (0-05M, 10 ml)+NaC104 (1M, 8.95 ml) +reagent (0-05M, 10 ml) +metal solution (0.01M, 5 m l ) + d o u b l e distilled water (16.05ml): vol. 50 ml. The ionic strength was maintained at 0.2M. The above solutions were kept in the thermostat at 25°C for sufficient time and then titrated against standard sodium hydroxide. The values of ~H and h were calculated using equations in the original paper [5]. In case of pyrogallol hH does not go below one even at pH 10. This indicates that the third hydroxy group does not dissociate in the range of complex formation. It does not take part in combination also because three-OH groups at 1, 2, 3 position in the benzene ring cannot bend to occupy three spatial positions around nickel ion. Further pyrogallol exhibits bidentate nature like A. Syamal and R. L. Dutta, J. Indian. chem. Soc. 45, 1 (1968). A. Chakravorty and R. H. Holm, lnorg. Chem. 3, 1010 (1964). A. Chakravorty and L. J. Theriot, Inorg. Chem. 5, 625 (1966). A. E. Martell and M. Calvin, Chemistry o f the Metal Chelate Compounds p. 518. Prentice Hall, New York (1956). 5. H. M. Irving and H. S. Rossotti, J. chem. Soc. 2904 (1954). 529 1. 2. 3. 4.
530
D.C. PATEL and P. K. BHATTACHARYA
catechol as observed in solid state studies. In the equation therefore Y (replacable hydrogen) has been considered to be 2 for both the ligands. In the region where t~H is 2-1 and 1-0, the values of PK1n and PKs n (constants corresponds to the association of two h y d r o x y hydrogens in catechol and second and third h y d r o x y hydrogens in case of pyrogallol) have been calculated by the linear plot of p H against log h n / 1 - A n at each point. The average values have been given in the Table 1. pL values at different points have been calculated using these proton ligand stability constant values. Log K1 and log Ks were worked out from the linear plot of pL against log 1 --~/h. The values of the formation constants for the nickel ethylenediamine system have been taken from the literature [4]. The values of log K1 and log K2 have been tabulated in Table 2. In case of nickel pyrogallolate precipitation starts after ~ = 0.6 and hence log Ks could not be calculated. Table I Ligand
PK~"
PK2~
Temp. (°C)
Catechol Pyrogallol
11.72 10-97
9.22 8-92
25 25
Table 2 Complex Ni(lI) catecholate Ni(lI) pyrogallolate [Ni(en)2]C12 [Ni(en)z]Cl~
log Kz log K2 log K3 Temp. (°C) 7.65 7-25 7.52 7.52
5.59 6.28 6.28
25 25 4.26
Isolation of the complex 1. [Ni(en)2]Cl2. (1.0g) was dissolved in minimum quantity of water and catechol solution (2M) was added till the p H was 7. The solution was scratched and allowed to stand for I hr. Bluish green solid separated out. It was washed with water, dried and analysed. Found: Ni = 17.31; N = 8.25%, [Ni(catechol)z] 2- enH22+ requires Ni = 17.42; N = 8.31%. 2. [Ni(en)2]Cle. (1-0 g) was dissolved in minimum quantity of water, pyrogallol solution (2M) was added till the p H was 7. A bluish green compound was obtained, this was washed with water, dried and analysed. Found: Ni = 15.83; N = 7.5%; [Ni(pyrogaliol)2] s- enHs s+ requires Ni = 15-91; N = 7.59%. 3. [Ni(en)3]Clz. (1-0g) was dissolved in water (15-0ml) and was mixed with catechol solution (2M) till the p H was 7. T h e compound having bluish green colour was obtained. It was washed with water, dried and analysed. Found: Ni = 17.25; N = 8.20%; [Ni(catechol)2]S-enH~ s+ requires Ni = 17.42; N = 8.31%. 4. T o the [Ni(en)3]Cl2 (1.0g in 15-0ml of water) pyrogallol solution (2M)
Ligand exchange in some nickel complexes
531
was added. T h e p H noted was 7. A bluish green c o m p o u n d was obtained. It was w a s h e d with water, dried and analysed. Found: Ni = 15.78; N = 7.45%; [Ni(pyrogallol)2] 2- enH._,2+ requires Ni = 15.91: N = 7.59. 5. T o the aqueous solution of nickel chloride (0-5 g in 10.0 ml) and catechol (1.2 g in 10-0 ml), ethylenediamine (2M) was added till the p H was 6. T h e compound obtained had bluish green colour. It was washed with water, dried and analysed. F o u n d Ni = 17.20; N = 8 . 1 5 % ; [Ni(catechol)2]2-enH=, 2+ requires Ni = 17-42; N = 8.31%. 6. T o the aqueous solution of nickel chloride (0.5 g in 10.0 ml) and pyrogallol (1.2 g in 10.0 ml), ethylenediamine (2M) was added. T h e p H of the solution was 6. A bluish green c o m p o u n d was obtained. It was washed with water, dried and analysed. Found: Ni = 15.75; N = 7-40%; [Ni(pyrogallol)2] 2- enHe 2+ requires Ni = 15.91: N = 7.59%. T h e pyrogallol complex has a tendency to change colour f r o m bluish green to b r o w n on e x p o s u r e to air. Since sensitivity of N i ( I I ) c o m p l e x e s to atmospheric oxygen is very unusual [6], the change in colour is possibly because of the oxidation of the ligand. Freshly prepared c o m p o u n d was therefore used in all the cases.
Magnetic studies Magnetic susceptibilities were determined at r o o m t e m p e r a t u r e using G o u y ' s method and were found to be as under: 1. [Ni(catechol)2] 2- enH2 '2+ 2.82 B.M. 2. [Ni(pyrogallol)2] 2- enH22+ 2.85 B.M. T h e values of magnetic m o m e n t were same for the samples of each c o m p o u n d prepared in the a b o v e three ways.
Spectral studies T h e absorption spectra of the c o m p l e x e s in aqueous solution were determined in the range 400 to 1000 m~. T h e spectra of samples prepared in three different ways are similar. Vmax(shoulder) [Ni(catechol)2] 2- enH22+ [Ni(pyrogallol)2] 2- enH22+
-23,000 cm-1; - 18,000 c m -1. - 2 3 , 0 0 0 c m - l ; - 18,000 c m -l.
T h e spectra in the i.r. region was obtained and following are the band positions: Compound [Ni(catechol)2] 2- enH22+
IN i(pyrogallol)2] 2- enH22+
p in cm -1 475,525,575,610,660,740,800, 1000",1075",1150,1300",1400", 1550,2900,3100",3300. Same asabove
*Less intense band. 6. c. R. Powers and G. W. Everett,Jr.,J.Am. chem. Soc. 91, 3468 (1969).
532
D . C . P A T E L and P. K. B H A T T A C H A R Y A DISCUSSION
Formation constants indicates that the order of stability is Ni-catecholate > Ni(en)2 > Ni-pyrogallolate. The values of formation constants are very close to one another. However, the addition of catechol or pyrogallol replace ethylenediamine and [Ni(phenol)2] 2- is formed. The ethylenediamine liberated forms the ion enH2 2+ and neutralizes the charge on the complex forming the compound [Ni(phenol)2] 2- enHz 2+. The reaction can be represented by the following equations: [Ni(en)~]Cl2 + 2C6H4(OH)2 i> [Ni(C6H402)212- enHz2+ + en + 2HC1 [Ni(en).~]Cl2 + 2C6H3(OH)3 --~ [Ni(C6H403)z] 2- enH2 z+ + en + 2HC1. The replacement of ethylenediamine can not be due to the decomposition of [Ni(en)z]C12 by excess acidity, because the formation of the precipitates starts even on the addition of two drops of catechol, the pH still being sufficiently high. The formation of the precipitates is also not immediate but appears slowly. This indicates that the replacement reaction may be proceeding through the formation of some unstable intermediate compound. The compounds [Ni(catechol)2] 2- enH2 ~+ and [Ni(pyrogaliol)2] 2- enH2 "+ have also been obtained by the addition of ethylenediamine to nickel + catechol and nickel+pyrogallol mixtures, indicating that ethylenediamine cannot replace catechol or pyrogallol but is simply in the outer sphere as an ionisable ion. The reaction of catechol and pyrogallol with [Ni(en)3]C12 result in the formation of the same compounds [Ni(catechol)2] 2- enH2 2+ or [Ni(pyrogallol)z] 2- enH2 2+ as are obtained from [Ni(en)2]C12. Since formation of intermediate [Ni(en)3 Cat] is not likely, the substitution of catechol or pyrogallol may be through prior elimination of ethylenediamine. Two catechol or pyrogallol molecules may be disposed in a square planar or tetrahedral way around the nickel ion. The structure of the compounds can not be definitely arrived at by magnetic study. The magnetic moment - 2.8 B.M. corresponds to two unpaired electrons. Since the value is close to the spin only value [7], the possibility of tetrahedral structure is eliminated. A square planar structure should normally be diamagnetic. Cases of paramagnetism corresponding to two unpaired electrons have been reported in square planar acetyl acetone [8] and salicylaldimine [9] complexes of nickel and this has been attributed to the existence of triplet polymetric state or the presence of tetrahedral form. According to Cotton[10] the magnetic behaviour in such system may be extremely complicated. The visible absorption spectra of the complexes in aqueous solution resemble that of square planar complexes[l 1]. There are two shoulders in the region 7. F. A. Cotton and G. Wilkinson, Advance Inorganic Chemistry. p. 738. Interscience, New York (1962). 8. L. Sacconi, P. L. Drioli, P. Paoletti and M. Ciampolini, Proc. chem. Soc. 255 (1962). 9. P, A. Cotton and J. P. Fackler, J. A m. chem. Soc. 82, 5005 (1960). 10. B. N. Figgis, Progress in Inorganic Chemistry (Edited by P. A. Cotton), Vol. 6. p. 206. Interscience, New York. 11. R.S. Drago, Physical Methods in Inorganic Chemistry p. 179. Reinhold, New York (1968).
Ligand exchange in some nickel complexes
533
23,000 cm -~ and - 18,000 cm-L Since the compounds are only partially soluble in water, extinction coefficients could not be found out directly. Further in aqueous solution dissociated catechol and pyrogallol develop slight colour due to the presence of ethylenediamine and interfere with the absorption due to the complex. T h e exact band assignment is therefore not possible. The i.r. spectra indicate the presence of the bands corresponding to stretching and bending modes of C - H (u = 2900 cm-1; 8 =- 1400 c m - 0 C - N (u = 1300 cm-~; 8 = 6 6 0 c m - 0 N - H (v -----3300 cm-~;8 = 1550 cm -~) C - O ( v = ! 1 5 0 c m - ~ ; 8 = 6 t 0 c m -~) as expected in catechol and ethylenediamine. The N i - N stretching frequency at - 5 0 0 c m - ~ [ 1 2 ] is absent indicating that ethylenediamine is not in the inner sphere. M - O stretching at 525 cm -~ is present. The absence of O - H stretching in catecholate confirms that the hydrogen of the - O H is-lost during chelation. T h e presence of it could not be detected in pyrogallolate also. This may be due to the lowering in the O - H stretching frequency as a result of hydrogen bonding between the - O H group of the two pyrogallol molecules and consequent merger with N - H stretching frequency or oxidation of the - O H group to quinonic forms. Acknowledgements--Authors' thanks are due to Professor S. M. Sethna. Head. Department of Chemistry for providing all the laboratory facilities. 12. D. B. Powell and N. Sheppard, J. chem. Soc. 1112 (1961).
A
STUDY
OF LIGAND EXCHANGE NICKEL COMPLEXES
IN
SOME
D. C. PATEL and P. K. B H A T T A C H A R Y A Chemistry Department, M. S. University of Baroda. Baroda-2, India
(First received 16 February 1970): in revised fi~rm 13 July 1970) A b s t r a c t - T h e formation constants of nickel complexes of catechol and pyrogallol have been determined by Irving-Rossoni method and compared with that of nickel-ethylenediamine complex. On treating nickel bis and tris ethylenediamine complexes with catechol and pyrogallol, ethylenediamine is replaced. The resulting solids have been analysed and characterised by magnetic and spectral studies.
NICKEL is known to form chelate complexes with many organic ligands[1-3]. Ethylenediamine forms l : 2 and 1 : 3 compounds with nickel and their stabilities have been determined [4]. This work presents a study of the relative tendencies of ethylenediamine and di and tri hydroxy phenols to replace each other in nickel chelates.
Determination of stability constants The stepwise formation constants, were determined by lrving-Rossotti method [5] using a Metrohm pH Meter (accuracy ___0-05). The following solutions were prepared: 1. HC104 (0.05M, 10ml)+NaCIO~ (IM 9.5ml)+double distilled water (30.5 ml) : vol. 50 ml 2. HCIO4 (0.05M, 10ml)+NaCIO4 (1M, 9.0ml)+reagent (0.05M, 10ml) + double distilled water (21 ml): vol. 50 ml 3. HCIO4 (0-05M, 10 ml)+NaC104 (1M, 8.95 ml) +reagent (0-05M, 10 ml) +metal solution (0.01M, 5 m l ) + d o u b l e distilled water (16.05ml): vol. 50 ml. The ionic strength was maintained at 0.2M. The above solutions were kept in the thermostat at 25°C for sufficient time and then titrated against standard sodium hydroxide. The values of ~H and h were calculated using equations in the original paper [5]. In case of pyrogallol hH does not go below one even at pH 10. This indicates that the third hydroxy group does not dissociate in the range of complex formation. It does not take part in combination also because three-OH groups at 1, 2, 3 position in the benzene ring cannot bend to occupy three spatial positions around nickel ion. Further pyrogallol exhibits bidentate nature like A. Syamal and R. L. Dutta, J. Indian. chem. Soc. 45, 1 (1968). A. Chakravorty and R. H. Holm, lnorg. Chem. 3, 1010 (1964). A. Chakravorty and L. J. Theriot, Inorg. Chem. 5, 625 (1966). A. E. Martell and M. Calvin, Chemistry o f the Metal Chelate Compounds p. 518. Prentice Hall, New York (1956). 5. H. M. Irving and H. S. Rossotti, J. chem. Soc. 2904 (1954). 529 1. 2. 3. 4.
530
D.C. PATEL and P. K. BHATTACHARYA
catechol as observed in solid state studies. In the equation therefore Y (replacable hydrogen) has been considered to be 2 for both the ligands. In the region where t~H is 2-1 and 1-0, the values of PK1n and PKs n (constants corresponds to the association of two h y d r o x y hydrogens in catechol and second and third h y d r o x y hydrogens in case of pyrogallol) have been calculated by the linear plot of p H against log h n / 1 - A n at each point. The average values have been given in the Table 1. pL values at different points have been calculated using these proton ligand stability constant values. Log K1 and log Ks were worked out from the linear plot of pL against log 1 --~/h. The values of the formation constants for the nickel ethylenediamine system have been taken from the literature [4]. The values of log K1 and log K2 have been tabulated in Table 2. In case of nickel pyrogallolate precipitation starts after ~ = 0.6 and hence log Ks could not be calculated. Table I Ligand
PK~"
PK2~
Temp. (°C)
Catechol Pyrogallol
11.72 10-97
9.22 8-92
25 25
Table 2 Complex Ni(lI) catecholate Ni(lI) pyrogallolate [Ni(en)2]C12 [Ni(en)z]Cl~
log Kz log K2 log K3 Temp. (°C) 7.65 7-25 7.52 7.52
5.59 6.28 6.28
25 25 4.26
Isolation of the complex 1. [Ni(en)2]Cl2. (1.0g) was dissolved in minimum quantity of water and catechol solution (2M) was added till the p H was 7. The solution was scratched and allowed to stand for I hr. Bluish green solid separated out. It was washed with water, dried and analysed. Found: Ni = 17.31; N = 8.25%, [Ni(catechol)z] 2- enH22+ requires Ni = 17.42; N = 8.31%. 2. [Ni(en)2]Cle. (1-0 g) was dissolved in minimum quantity of water, pyrogallol solution (2M) was added till the p H was 7. A bluish green compound was obtained, this was washed with water, dried and analysed. Found: Ni = 15.83; N = 7.5%; [Ni(pyrogaliol)2] s- enHs s+ requires Ni = 15-91; N = 7.59%. 3. [Ni(en)3]Clz. (1-0g) was dissolved in water (15-0ml) and was mixed with catechol solution (2M) till the p H was 7. T h e compound having bluish green colour was obtained. It was washed with water, dried and analysed. Found: Ni = 17.25; N = 8.20%; [Ni(catechol)2]S-enH~ s+ requires Ni = 17.42; N = 8.31%. 4. T o the [Ni(en)3]Cl2 (1.0g in 15-0ml of water) pyrogallol solution (2M)
Ligand exchange in some nickel complexes
531
was added. T h e p H noted was 7. A bluish green c o m p o u n d was obtained. It was w a s h e d with water, dried and analysed. Found: Ni = 15.78; N = 7.45%; [Ni(pyrogallol)2] 2- enH._,2+ requires Ni = 15.91: N = 7.59. 5. T o the aqueous solution of nickel chloride (0-5 g in 10.0 ml) and catechol (1.2 g in 10-0 ml), ethylenediamine (2M) was added till the p H was 6. T h e compound obtained had bluish green colour. It was washed with water, dried and analysed. F o u n d Ni = 17.20; N = 8 . 1 5 % ; [Ni(catechol)2]2-enH=, 2+ requires Ni = 17-42; N = 8.31%. 6. T o the aqueous solution of nickel chloride (0.5 g in 10.0 ml) and pyrogallol (1.2 g in 10.0 ml), ethylenediamine (2M) was added. T h e p H of the solution was 6. A bluish green c o m p o u n d was obtained. It was washed with water, dried and analysed. Found: Ni = 15.75; N = 7-40%; [Ni(pyrogallol)2] 2- enHe 2+ requires Ni = 15.91: N = 7.59%. T h e pyrogallol complex has a tendency to change colour f r o m bluish green to b r o w n on e x p o s u r e to air. Since sensitivity of N i ( I I ) c o m p l e x e s to atmospheric oxygen is very unusual [6], the change in colour is possibly because of the oxidation of the ligand. Freshly prepared c o m p o u n d was therefore used in all the cases.
Magnetic studies Magnetic susceptibilities were determined at r o o m t e m p e r a t u r e using G o u y ' s method and were found to be as under: 1. [Ni(catechol)2] 2- enH2 '2+ 2.82 B.M. 2. [Ni(pyrogallol)2] 2- enH22+ 2.85 B.M. T h e values of magnetic m o m e n t were same for the samples of each c o m p o u n d prepared in the a b o v e three ways.
Spectral studies T h e absorption spectra of the c o m p l e x e s in aqueous solution were determined in the range 400 to 1000 m~. T h e spectra of samples prepared in three different ways are similar. Vmax(shoulder) [Ni(catechol)2] 2- enH22+ [Ni(pyrogallol)2] 2- enH22+
-23,000 cm-1; - 18,000 c m -1. - 2 3 , 0 0 0 c m - l ; - 18,000 c m -l.
T h e spectra in the i.r. region was obtained and following are the band positions: Compound [Ni(catechol)2] 2- enH22+
IN i(pyrogallol)2] 2- enH22+
p in cm -1 475,525,575,610,660,740,800, 1000",1075",1150,1300",1400", 1550,2900,3100",3300. Same asabove
*Less intense band. 6. c. R. Powers and G. W. Everett,Jr.,J.Am. chem. Soc. 91, 3468 (1969).
532
D . C . P A T E L and P. K. B H A T T A C H A R Y A DISCUSSION
Formation constants indicates that the order of stability is Ni-catecholate > Ni(en)2 > Ni-pyrogallolate. The values of formation constants are very close to one another. However, the addition of catechol or pyrogallol replace ethylenediamine and [Ni(phenol)2] 2- is formed. The ethylenediamine liberated forms the ion enH2 2+ and neutralizes the charge on the complex forming the compound [Ni(phenol)2] 2- enHz 2+. The reaction can be represented by the following equations: [Ni(en)~]Cl2 + 2C6H4(OH)2 i> [Ni(C6H402)212- enHz2+ + en + 2HC1 [Ni(en).~]Cl2 + 2C6H3(OH)3 --~ [Ni(C6H403)z] 2- enH2 z+ + en + 2HC1. The replacement of ethylenediamine can not be due to the decomposition of [Ni(en)z]C12 by excess acidity, because the formation of the precipitates starts even on the addition of two drops of catechol, the pH still being sufficiently high. The formation of the precipitates is also not immediate but appears slowly. This indicates that the replacement reaction may be proceeding through the formation of some unstable intermediate compound. The compounds [Ni(catechol)2] 2- enH2 ~+ and [Ni(pyrogaliol)2] 2- enH2 "+ have also been obtained by the addition of ethylenediamine to nickel + catechol and nickel+pyrogallol mixtures, indicating that ethylenediamine cannot replace catechol or pyrogallol but is simply in the outer sphere as an ionisable ion. The reaction of catechol and pyrogallol with [Ni(en)3]C12 result in the formation of the same compounds [Ni(catechol)2] 2- enH2 2+ or [Ni(pyrogallol)z] 2- enH2 2+ as are obtained from [Ni(en)2]C12. Since formation of intermediate [Ni(en)3 Cat] is not likely, the substitution of catechol or pyrogallol may be through prior elimination of ethylenediamine. Two catechol or pyrogallol molecules may be disposed in a square planar or tetrahedral way around the nickel ion. The structure of the compounds can not be definitely arrived at by magnetic study. The magnetic moment - 2.8 B.M. corresponds to two unpaired electrons. Since the value is close to the spin only value [7], the possibility of tetrahedral structure is eliminated. A square planar structure should normally be diamagnetic. Cases of paramagnetism corresponding to two unpaired electrons have been reported in square planar acetyl acetone [8] and salicylaldimine [9] complexes of nickel and this has been attributed to the existence of triplet polymetric state or the presence of tetrahedral form. According to Cotton[10] the magnetic behaviour in such system may be extremely complicated. The visible absorption spectra of the complexes in aqueous solution resemble that of square planar complexes[l 1]. There are two shoulders in the region 7. F. A. Cotton and G. Wilkinson, Advance Inorganic Chemistry. p. 738. Interscience, New York (1962). 8. L. Sacconi, P. L. Drioli, P. Paoletti and M. Ciampolini, Proc. chem. Soc. 255 (1962). 9. P, A. Cotton and J. P. Fackler, J. A m. chem. Soc. 82, 5005 (1960). 10. B. N. Figgis, Progress in Inorganic Chemistry (Edited by P. A. Cotton), Vol. 6. p. 206. Interscience, New York. 11. R.S. Drago, Physical Methods in Inorganic Chemistry p. 179. Reinhold, New York (1968).
Ligand exchange in some nickel complexes
533
23,000 cm -~ and - 18,000 cm-L Since the compounds are only partially soluble in water, extinction coefficients could not be found out directly. Further in aqueous solution dissociated catechol and pyrogallol develop slight colour due to the presence of ethylenediamine and interfere with the absorption due to the complex. T h e exact band assignment is therefore not possible. The i.r. spectra indicate the presence of the bands corresponding to stretching and bending modes of C - H (u = 2900 cm-1; 8 =- 1400 c m - 0 C - N (u = 1300 cm-~; 8 = 6 6 0 c m - 0 N - H (v -----3300 cm-~;8 = 1550 cm -~) C - O ( v = ! 1 5 0 c m - ~ ; 8 = 6 t 0 c m -~) as expected in catechol and ethylenediamine. The N i - N stretching frequency at - 5 0 0 c m - ~ [ 1 2 ] is absent indicating that ethylenediamine is not in the inner sphere. M - O stretching at 525 cm -~ is present. The absence of O - H stretching in catecholate confirms that the hydrogen of the - O H is-lost during chelation. T h e presence of it could not be detected in pyrogallolate also. This may be due to the lowering in the O - H stretching frequency as a result of hydrogen bonding between the - O H group of the two pyrogallol molecules and consequent merger with N - H stretching frequency or oxidation of the - O H group to quinonic forms. Acknowledgements--Authors' thanks are due to Professor S. M. Sethna. Head. Department of Chemistry for providing all the laboratory facilities. 12. D. B. Powell and N. Sheppard, J. chem. Soc. 1112 (1961).