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Mechanism of acid catalysed decomposition of tris(acetylacetonato)cobalt(III) in aqueous solution
J. inorg, nucl.Chem.. 1971,Vol. 33, pp. 515 to 520. PergamonPress. Printedin GreatBritain
MECHANISM OF ACID CATALYSED DECOMPOSITION OF TRIS(ACETYLACETONATO)COBALT(III) IN AQUEOUS SOLUTION D. B A N E R J E A and S. D U T T A C H A U D H U R 1
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-32, India (Received 16 April 1970)
Abstract-In aqueous solution tris(acetylacetonato)cobalt(lll) undergoes an acid catalysed decomposition forming cobalt(II), oxidation products of acetylacetone and free acetylacetone. From detailed kinetic studies of this reaction carried out spectrophotometrically an intramolecular electron transfer mechanism has been established. Formation of free radicals as intermediates has been demonstrated and the oxidation products of acetylacetone have been characterized. INTRODUCTION
THE MEC~ANISTIC aspects of dissociation of tris(acetylacetonato)chromium(l I 1) in acid media have been investigated earlier[l]. In an attempt to study the corresponding reaction of tris(acetylacetonato)cobalt(lll) it was found that under the experimental conditions where a reaction occurs fast enough to be followed conveniently the net change is the formation of cobalt(II), oxidation products of acetylacetone and free acetylacetone corresponding to two-thirds of the total amount of acetylacetone bound to cobalt(Ill) in the original complex. Generation of free radicals as intermediates in this process has also been demonstrated by the rapid polymerization of monomers like methylmethacrylate(MMA) and acrylonitrile(ACN) which occurs in the reacting solution. This behaviour is typical of a cobalt(Ill) complex having oxidisable ligands, such as Co(NH:05I '+ [2], Co(NH3)sNO22+ [3], Co(C204)~ :~- [4], etc. all of which undergo similar redox reactions [5]. In favourable cases the photochemical decomposition of complexes often involves intramolecular oxidation-reduction reactions between the higher valent metal ion and an oxidisable ligand. A similar intramolecular mechanism has been established for the decomposition of Co(NH3)sNOJ + in acidic aqueous solutions[3]. Detailed studies on the mechanistic aspects of the decomposition of tris(acetylacetonato)cobalt(I I I) are reported below. EXPERIMENTAL Materials and reagents. Tris(acetylacetonato)cobalt(lli), Co(C,~H702):~, was prepared by the method described in literature[6] and its purity was ascertained by analysis for cobalt and its melting 1. 2. 3. 4. 5. 6.
D. Banerjea, Z. anorg, allg. Chem. 359, 31)5 (1968). R. G. Yalman, lnorg. Chem. 1, 16(1962). D. Banerjea, J. inorg, nucl. Chem. 29, 2795 (1967). C. A. Bunton, J. H. Carter, D. R. Llewellyn, A. L. Odell and S. Y. Yih, J, chem. Soc. 4622 (1964). V. Balzani, L. Moggi, F. Scandala and V. Carassiti, Inorg. chim. A cta 1, 7 (1967). B. E. Bryant and W. C. Fernelius, In Inorzanic Syntheses, (Edited by T. Moeller, Vol. 5, p. 188. M c G r a w - H i l l , N e w Y o r k (1957). 515
516
D. B A N E R J E A and S. D U T T A C H A U D H U R I
point (Found; Co, 16.7 per cent; m.p. 198°C. Required: Co, 16.56 percent; m.p., 196°-198.5°C[7,8]). Methylmethacrylate, (MMA), obtained from Rohm and Haas Co. (Philadelphia, USA), was purified by successive washings with 10% N a O H solution and distilled water and dried by storing over anhydrous CaCI~. It was distilled under reduced pressure before use. Acrylonitrile, (ACN), from Fluka (Switzerland), was purified by successive washings with 10% N a O H solution and 10% H2SO4 and was then washed several times with NaHCO3 solution and finally distilled water. It was then dried over anhydrous CaClz and distilled under reduced pressure before USe. o-phenylenediamine. Marketed by Fluka (Switzerland) was used as such. Other chemicals were of reagent quality or were purified before use by standard methods. Apparatus and procedure. Preliminary observations revealed that aqueous solutions of the complex are quite stable (no observable spectral change in two weeks at ca. 30°C). In 2M perchloric acid, however, the decomposition is complete within ca. 48 hr (at 30°C). The rate of decomposition was followed spectrophotometrically at 600m/z where the original cobalt(Ill) complex absorbs appreciably (ei = 130); the reaction products do not absorb in this region. The rate of decomposition has been conviently followed in 0.1-0.5M perchloric acid solution (with ionic strength adjusted with NaCIO4) at 50°-65°C. A Hilger Uvispek Spectrophotometer and the sample quenching procedure described previously [1] were used. The pseudo-first-order rate-constant, ko~s, was evaluated graphically by plotting log(Ao/At) vs. time (t) (since in this caseA= = 0, loc. tit.). RESULTS AND DISCUSSION
The observed dependence of the rate of decomposition on acid concentration (see Table 1 and Fig. 1) indicates that the reaction goes essentially by the acid catalysed path with virtually no contribution by an acid independent path. In order to ascertain the stoichiometry of the reaction the total concentration of acetylacetone in the reaction mixture, alter it has been maintained long enough for the reaction to be essentially complete, was estimated spectrophotometrically with o-phenylenediamine in acid media[9]. The results indicated that the decomposed solution contained about 2 moles (1.94 × 10-3M)of free acetylacetone per mole of the original complex (10-3M), the rest having been destroyed (oxidised) in the reaction simultaneously with the reduction of all the cobalt(Ill) to cobalt(II). Formation of free radicals in the process has been demonstrated by the rapid polymerisation of monomers like methylmethacrylate(MMA) and Table 1. Values of kobs under different conditions. Co(acac)3, 0.001M Temp. (°C) & Ionic strength(/z)
50 (/z, 1) 55 (/z, 1) 60 (/z, 1) 60 (/z, 0-5) 60 (/z, O"1) 65 (/z, 1)
103kobs(min-~)
HCIO.~(M) = 0.1 0.41 1.29 2.53 1.47 1'06 5.04
0.2 0.92 2.35 5.33
10.0
0.3 1.4 3.49 7.13
15.0
10akH* (min -j) 0-4 1-65 3.95 9.33
0.5 2.5 5.43 11.5
5.0 11.0 23.6
20.05
24.6
49.2
*Evaluated graphically (see Fig. 1): kobs = kn+[H+]. 7. B. West, J. chem. Soc. 3115 (1952). 8. R. O. Whipple, R. West and K. Emerson, J. chem. Soc. 3715 (1953). 9. J. Aggett and A. L. Odell, J. chem. Soc. A, 1820 (1966).
517
Mechanism of acid catalysed
acrylonitrile (ACN) in the reacting solution. Thus, an aqueous solution (50 ml) of tris(acetylacetonato)cobalt(llI) (10-3M) in 0.1M perchloric acid was treated with 1 ml of the pure monomer (MMA, ACN) and after the mixture was thoroughly flushed with pure nitrogen it was maintained at room temperature (ca. 30°C) when polymerisation was noticable after being allowed to stand overnight. In 1M perchloric acid, polymerisation was detected within 30-40 rain at room temperature, but in the absence of free acid no polymerisation was observed even after 24 hr at 50°C. From studies on the dissociation of tris(acetylacetonato)chromium(lll) it has been postulated [1] that protonation of the bound acetylacetonato ligand at the carbonyi oxygen site facilitates the opening of the chelate ring which leads to a loss in the quasi-aromatic character of the ring and hence leads to a faster rate for the rupture of the remaining metal-ligand bond resulting in loss of the ligand. On this basis, and taking into co,nsideration the other observed facts, the following mechanism appears reasonable in the present system:
/
CH:~
/
O--C
.4-'- -
O--C
- ",~
(acac)2Co ~\• . . . .
f~st
"C , H
O---C
A "~
+ H+ "Equil."(acac)2Co const.
K~,
\
-
-~
_ _ @' C H O---C H*
/ /
+
/
O~C
'acacH ~ (acac)2 Co*
Oxidation products of acetylacetone
CH3
\
/ O~C
Co 2++ 2 acacH
\
CH:~
CH:~
(acac)zCo
CH:~
CH2
\ CH3
Generation of the O H radical has been confirmed by the detection of - O H as the end-group in the polymer using the technique of end-group analysis[1 1] on the polymer formed by MMA in the system. The presence of free acetone in the decomposed solution was also detected as follows: An aliquot of the decomposed solution was treated with a slight excess of copper(II) acetate solution, its pH was adjusted to ca. 5 and this was then extracted with chloroform to remove all the acetylacetone as Cu(acac)2 [121. The aqueous phase was then passed through a cation exchange resin (Dowex 50WX8) in the H + form to remove the excess Cu(II) and COOI) from the solution. Aliquots of the effluent were tested for the absence of acetylacetone by treatment 1 la. P. Ghosh, A. R. Mukherjee and S. R. Palit, J. poly. Sci. 2A, 2807 (1964); b. P. Ghosh. P. K. Sengupta and A. Pramanik, J. poly. Sci. 3A, 1725 (1965). 12. W. Seaman, J. T. Woods and E. A. Massad, Analyt. Chem. 19,250 (1947).
518
D. BANERJEA and S. DUTTA CHAUDHURI 'I"
"~
"I
"
I
24
D f
20
•
7 E 16
8
4
,
I
o,~
o.~
0-3 [Hc~o43
0.4
o-~
,M
Fig, 1. Effectof acid concentrationon ko~s.Co(acac)~,0.001M; Ionic strength, l (HCIO4+ NaC104), Temp,: A, 50°C; B, 55°C; C, 60°C; D, 65°C, with o-phenylenediamine (loc, cir.) and copper(II) by H~S. In another aliquot the presence of acetone was detected spectrophotometrically by treatment with salicylaldehyde in alkaline media[13] when an orange-yellow color developed (kraal, 475 m/x). On this basis it is reasonable to suggest that the degradation of the a c a c H radical occurs as follows:
H3C---C--O" CHz
fast
~ H3C.CO +
+ H2(~'CO'CH3
H3C--C=O H3C'COOH + H +
H a C ' C O ' C H ~ ÷ "OH! ½H20 +¼02 ~f~l [14]
On the basis of the assigned mechanism the rate of the reaction should be expressed as Rate =
{K+klk2/(k--1 + k~)}[Co(acac)z][H +] = kH+[Co(acac) z] [H +]
l f i t is reasonably assumed that k-1 >> ks, then kn+ ~
K~+klk~/k-1 = Kn+Kk2
13. S. Beratsson, AnalyL Chem. 28, 1337 (1956). 14. W. E. Wilson and J. T. O ' D o n o v a n , J. chem. Phys. 47, 5455 (t 967).
Mechanism of acid catalysed
519
where K is the equilibrium constant for the one-ended dissociation of the protonated complex (loc. cit.). From the kH+ values at the different temperatures the enthalpy (AH$) and entropy (AS~) of activation for the system were evaluated (see Fig. 2) using the Eyring equation (see Ref.[l]), as AH$, 33 kcal/mole and ASS, + 24.3 e.u. These values are comparable to those in several analogous systems (Table 2). l
I
16.8
16"6
16.4 I
16.2
16.0
29-5
I
I
30.0
305
3tO
liT x 104
Fig. 2. Eyring plot (Ionic strength, 1). h = Planck's constant; K = Boltzmann's constant. Table 2. Activation parameters for some electron transfer reactions of cobalt(l I 1) complexes
System
(kcal/mole)
AH$
ASS (e.u.)
Co(acac)3 Co(acac)3-Cyclohexanol
33.0 30.4 29.8 29-9 26.3 21.1 22.6
24.3 15.2 21.9 43.0 21.1 22-6 20.0
Co(C204)~Co3÷(aq)-Cyclohexanol Co:~+(aq)-HCOOH Co3+(aq)-HCOOCo3+(aq)-HCHO
Ref.
This work [151 14] [ 16] [ 17] [171 [171
The value of kobs increases with increasing ionic strength of the medium (see Table 1). This is in keeping with the earlier observations on the effect of ionic strength on the rate of acid catalysed dissociation of Cr(acac)~[l] and the intramolecular redox transformation of Co(NH:05NO22+ into cobalt(lI) and NO2 (primary product)[3] in acid media. A similar intramolecular electron transfer 15. V. S. Materm'yanov and E. T. Denisov, Russ. J. phys. Chem. 40, 1243 (1966). 16. D . G . Hoare and W. A. Waters,J. chem. Soc. 2560 (1964). 17. C. E. H. Bawn and A. G. White,J. chem. Soc. 331,343 (1951).
520
D. B A N E R J E A and S. D U T T A C H A U D H U R I
process is believed to operate in the photochemical decomposition of Co(NH3)5NO~ + [18], tris(glycinato)cobalt(III) [19] and tris(alaninato)cobalt(III) [5] and also in the photochemical[5] and thermal[4] (acid catalysed) decomposition of tris(oxalato)cobaltate(III). 18. V. Balzani, R. Ballardini, N. Sabbatini and L. Moggi, lnorg. Chem. 7, 1398 (I 968). 19. V. Balzani, V. Carassiti, L. Moggi and N. Sabbatini, lnorg. Chem. 4, 1247 (1965).
MECHANISM OF ACID CATALYSED DECOMPOSITION OF TRIS(ACETYLACETONATO)COBALT(III) IN AQUEOUS SOLUTION D. B A N E R J E A and S. D U T T A C H A U D H U R 1
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-32, India (Received 16 April 1970)
Abstract-In aqueous solution tris(acetylacetonato)cobalt(lll) undergoes an acid catalysed decomposition forming cobalt(II), oxidation products of acetylacetone and free acetylacetone. From detailed kinetic studies of this reaction carried out spectrophotometrically an intramolecular electron transfer mechanism has been established. Formation of free radicals as intermediates has been demonstrated and the oxidation products of acetylacetone have been characterized. INTRODUCTION
THE MEC~ANISTIC aspects of dissociation of tris(acetylacetonato)chromium(l I 1) in acid media have been investigated earlier[l]. In an attempt to study the corresponding reaction of tris(acetylacetonato)cobalt(lll) it was found that under the experimental conditions where a reaction occurs fast enough to be followed conveniently the net change is the formation of cobalt(II), oxidation products of acetylacetone and free acetylacetone corresponding to two-thirds of the total amount of acetylacetone bound to cobalt(Ill) in the original complex. Generation of free radicals as intermediates in this process has also been demonstrated by the rapid polymerization of monomers like methylmethacrylate(MMA) and acrylonitrile(ACN) which occurs in the reacting solution. This behaviour is typical of a cobalt(Ill) complex having oxidisable ligands, such as Co(NH:05I '+ [2], Co(NH3)sNO22+ [3], Co(C204)~ :~- [4], etc. all of which undergo similar redox reactions [5]. In favourable cases the photochemical decomposition of complexes often involves intramolecular oxidation-reduction reactions between the higher valent metal ion and an oxidisable ligand. A similar intramolecular mechanism has been established for the decomposition of Co(NH3)sNOJ + in acidic aqueous solutions[3]. Detailed studies on the mechanistic aspects of the decomposition of tris(acetylacetonato)cobalt(I I I) are reported below. EXPERIMENTAL Materials and reagents. Tris(acetylacetonato)cobalt(lli), Co(C,~H702):~, was prepared by the method described in literature[6] and its purity was ascertained by analysis for cobalt and its melting 1. 2. 3. 4. 5. 6.
D. Banerjea, Z. anorg, allg. Chem. 359, 31)5 (1968). R. G. Yalman, lnorg. Chem. 1, 16(1962). D. Banerjea, J. inorg, nucl. Chem. 29, 2795 (1967). C. A. Bunton, J. H. Carter, D. R. Llewellyn, A. L. Odell and S. Y. Yih, J, chem. Soc. 4622 (1964). V. Balzani, L. Moggi, F. Scandala and V. Carassiti, Inorg. chim. A cta 1, 7 (1967). B. E. Bryant and W. C. Fernelius, In Inorzanic Syntheses, (Edited by T. Moeller, Vol. 5, p. 188. M c G r a w - H i l l , N e w Y o r k (1957). 515
516
D. B A N E R J E A and S. D U T T A C H A U D H U R I
point (Found; Co, 16.7 per cent; m.p. 198°C. Required: Co, 16.56 percent; m.p., 196°-198.5°C[7,8]). Methylmethacrylate, (MMA), obtained from Rohm and Haas Co. (Philadelphia, USA), was purified by successive washings with 10% N a O H solution and distilled water and dried by storing over anhydrous CaCI~. It was distilled under reduced pressure before use. Acrylonitrile, (ACN), from Fluka (Switzerland), was purified by successive washings with 10% N a O H solution and 10% H2SO4 and was then washed several times with NaHCO3 solution and finally distilled water. It was then dried over anhydrous CaClz and distilled under reduced pressure before USe. o-phenylenediamine. Marketed by Fluka (Switzerland) was used as such. Other chemicals were of reagent quality or were purified before use by standard methods. Apparatus and procedure. Preliminary observations revealed that aqueous solutions of the complex are quite stable (no observable spectral change in two weeks at ca. 30°C). In 2M perchloric acid, however, the decomposition is complete within ca. 48 hr (at 30°C). The rate of decomposition was followed spectrophotometrically at 600m/z where the original cobalt(Ill) complex absorbs appreciably (ei = 130); the reaction products do not absorb in this region. The rate of decomposition has been conviently followed in 0.1-0.5M perchloric acid solution (with ionic strength adjusted with NaCIO4) at 50°-65°C. A Hilger Uvispek Spectrophotometer and the sample quenching procedure described previously [1] were used. The pseudo-first-order rate-constant, ko~s, was evaluated graphically by plotting log(Ao/At) vs. time (t) (since in this caseA= = 0, loc. tit.). RESULTS AND DISCUSSION
The observed dependence of the rate of decomposition on acid concentration (see Table 1 and Fig. 1) indicates that the reaction goes essentially by the acid catalysed path with virtually no contribution by an acid independent path. In order to ascertain the stoichiometry of the reaction the total concentration of acetylacetone in the reaction mixture, alter it has been maintained long enough for the reaction to be essentially complete, was estimated spectrophotometrically with o-phenylenediamine in acid media[9]. The results indicated that the decomposed solution contained about 2 moles (1.94 × 10-3M)of free acetylacetone per mole of the original complex (10-3M), the rest having been destroyed (oxidised) in the reaction simultaneously with the reduction of all the cobalt(Ill) to cobalt(II). Formation of free radicals in the process has been demonstrated by the rapid polymerisation of monomers like methylmethacrylate(MMA) and Table 1. Values of kobs under different conditions. Co(acac)3, 0.001M Temp. (°C) & Ionic strength(/z)
50 (/z, 1) 55 (/z, 1) 60 (/z, 1) 60 (/z, 0-5) 60 (/z, O"1) 65 (/z, 1)
103kobs(min-~)
HCIO.~(M) = 0.1 0.41 1.29 2.53 1.47 1'06 5.04
0.2 0.92 2.35 5.33
10.0
0.3 1.4 3.49 7.13
15.0
10akH* (min -j) 0-4 1-65 3.95 9.33
0.5 2.5 5.43 11.5
5.0 11.0 23.6
20.05
24.6
49.2
*Evaluated graphically (see Fig. 1): kobs = kn+[H+]. 7. B. West, J. chem. Soc. 3115 (1952). 8. R. O. Whipple, R. West and K. Emerson, J. chem. Soc. 3715 (1953). 9. J. Aggett and A. L. Odell, J. chem. Soc. A, 1820 (1966).
517
Mechanism of acid catalysed
acrylonitrile (ACN) in the reacting solution. Thus, an aqueous solution (50 ml) of tris(acetylacetonato)cobalt(llI) (10-3M) in 0.1M perchloric acid was treated with 1 ml of the pure monomer (MMA, ACN) and after the mixture was thoroughly flushed with pure nitrogen it was maintained at room temperature (ca. 30°C) when polymerisation was noticable after being allowed to stand overnight. In 1M perchloric acid, polymerisation was detected within 30-40 rain at room temperature, but in the absence of free acid no polymerisation was observed even after 24 hr at 50°C. From studies on the dissociation of tris(acetylacetonato)chromium(lll) it has been postulated [1] that protonation of the bound acetylacetonato ligand at the carbonyi oxygen site facilitates the opening of the chelate ring which leads to a loss in the quasi-aromatic character of the ring and hence leads to a faster rate for the rupture of the remaining metal-ligand bond resulting in loss of the ligand. On this basis, and taking into co,nsideration the other observed facts, the following mechanism appears reasonable in the present system:
/
CH:~
/
O--C
.4-'- -
O--C
- ",~
(acac)2Co ~\• . . . .
f~st
"C , H
O---C
A "~
+ H+ "Equil."(acac)2Co const.
K~,
\
-
-~
_ _ @' C H O---C H*
/ /
+
/
O~C
'acacH ~ (acac)2 Co*
Oxidation products of acetylacetone
CH3
\
/ O~C
Co 2++ 2 acacH
\
CH:~
CH:~
(acac)zCo
CH:~
CH2
\ CH3
Generation of the O H radical has been confirmed by the detection of - O H as the end-group in the polymer using the technique of end-group analysis[1 1] on the polymer formed by MMA in the system. The presence of free acetone in the decomposed solution was also detected as follows: An aliquot of the decomposed solution was treated with a slight excess of copper(II) acetate solution, its pH was adjusted to ca. 5 and this was then extracted with chloroform to remove all the acetylacetone as Cu(acac)2 [121. The aqueous phase was then passed through a cation exchange resin (Dowex 50WX8) in the H + form to remove the excess Cu(II) and COOI) from the solution. Aliquots of the effluent were tested for the absence of acetylacetone by treatment 1 la. P. Ghosh, A. R. Mukherjee and S. R. Palit, J. poly. Sci. 2A, 2807 (1964); b. P. Ghosh. P. K. Sengupta and A. Pramanik, J. poly. Sci. 3A, 1725 (1965). 12. W. Seaman, J. T. Woods and E. A. Massad, Analyt. Chem. 19,250 (1947).
518
D. BANERJEA and S. DUTTA CHAUDHURI 'I"
"~
"I
"
I
24
D f
20
•
7 E 16
8
4
,
I
o,~
o.~
0-3 [Hc~o43
0.4
o-~
,M
Fig, 1. Effectof acid concentrationon ko~s.Co(acac)~,0.001M; Ionic strength, l (HCIO4+ NaC104), Temp,: A, 50°C; B, 55°C; C, 60°C; D, 65°C, with o-phenylenediamine (loc, cir.) and copper(II) by H~S. In another aliquot the presence of acetone was detected spectrophotometrically by treatment with salicylaldehyde in alkaline media[13] when an orange-yellow color developed (kraal, 475 m/x). On this basis it is reasonable to suggest that the degradation of the a c a c H radical occurs as follows:
H3C---C--O" CHz
fast
~ H3C.CO +
+ H2(~'CO'CH3
H3C--C=O H3C'COOH + H +
H a C ' C O ' C H ~ ÷ "OH! ½H20 +¼02 ~f~l [14]
On the basis of the assigned mechanism the rate of the reaction should be expressed as Rate =
{K+klk2/(k--1 + k~)}[Co(acac)z][H +] = kH+[Co(acac) z] [H +]
l f i t is reasonably assumed that k-1 >> ks, then kn+ ~
K~+klk~/k-1 = Kn+Kk2
13. S. Beratsson, AnalyL Chem. 28, 1337 (1956). 14. W. E. Wilson and J. T. O ' D o n o v a n , J. chem. Phys. 47, 5455 (t 967).
Mechanism of acid catalysed
519
where K is the equilibrium constant for the one-ended dissociation of the protonated complex (loc. cit.). From the kH+ values at the different temperatures the enthalpy (AH$) and entropy (AS~) of activation for the system were evaluated (see Fig. 2) using the Eyring equation (see Ref.[l]), as AH$, 33 kcal/mole and ASS, + 24.3 e.u. These values are comparable to those in several analogous systems (Table 2). l
I
16.8
16"6
16.4 I
16.2
16.0
29-5
I
I
30.0
305
3tO
liT x 104
Fig. 2. Eyring plot (Ionic strength, 1). h = Planck's constant; K = Boltzmann's constant. Table 2. Activation parameters for some electron transfer reactions of cobalt(l I 1) complexes
System
(kcal/mole)
AH$
ASS (e.u.)
Co(acac)3 Co(acac)3-Cyclohexanol
33.0 30.4 29.8 29-9 26.3 21.1 22.6
24.3 15.2 21.9 43.0 21.1 22-6 20.0
Co(C204)~Co3÷(aq)-Cyclohexanol Co:~+(aq)-HCOOH Co3+(aq)-HCOOCo3+(aq)-HCHO
Ref.
This work [151 14] [ 16] [ 17] [171 [171
The value of kobs increases with increasing ionic strength of the medium (see Table 1). This is in keeping with the earlier observations on the effect of ionic strength on the rate of acid catalysed dissociation of Cr(acac)~[l] and the intramolecular redox transformation of Co(NH:05NO22+ into cobalt(lI) and NO2 (primary product)[3] in acid media. A similar intramolecular electron transfer 15. V. S. Materm'yanov and E. T. Denisov, Russ. J. phys. Chem. 40, 1243 (1966). 16. D . G . Hoare and W. A. Waters,J. chem. Soc. 2560 (1964). 17. C. E. H. Bawn and A. G. White,J. chem. Soc. 331,343 (1951).
520
D. B A N E R J E A and S. D U T T A C H A U D H U R I
process is believed to operate in the photochemical decomposition of Co(NH3)5NO~ + [18], tris(glycinato)cobalt(III) [19] and tris(alaninato)cobalt(III) [5] and also in the photochemical[5] and thermal[4] (acid catalysed) decomposition of tris(oxalato)cobaltate(III). 18. V. Balzani, R. Ballardini, N. Sabbatini and L. Moggi, lnorg. Chem. 7, 1398 (I 968). 19. V. Balzani, V. Carassiti, L. Moggi and N. Sabbatini, lnorg. Chem. 4, 1247 (1965).