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Kinetics and mechanism of dissociation of metal chelates—IV
J. tnorg, nucl. Chem., 1967, Vol. 29, pp. 2387 to 2390. Pergamon Press Ltd. Printed in Northern Ireland
KINETICS A N D MECHANISM OF DISSOCIATION OF METAL CHELATES--IV* DISSOCIATION OF
CIS-DIAQUO-BIS-MALONATO.CHROMATE(III)
D. BANERJEAand C. CHATTEILIEE Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-32 (India) (Received 27 February 1967) Abstract--The rate of dissociation of cis-diaquo.bis-malonato-chromate(III) to the mono-malonato complex in perchloric acid--sodium perchlorate media shows a first-order dependence on both the complex and H + ion concentrations under otherwise identical conditions. Values of the activation parameters AH~ (20.2 kcal/mole) and AS~ (--7.7 e.u.) have also been evaluated. An increase in ionic strength from 1 to 2 had no significant effect on the rate-constant. The behaviour of the bismalonato complex is very similar to that of the bis-oxalato complex in all respects; however, the former is much more labile (,~70 times at 45°C) with a lower value of AH~. All these observations are in accord with a solvent-assisted dissociation of the protonated form (conjugate acid) of the complex.
THE WORKreported here extends the investigations (x-a) on the kinetics and mechanisms
of dissociation of some Co(III) and Cr(III) chelates carried out earlier in this laboratory. As in the case of the corresponding oxalato complex, (1) spectral observations have shown that even at an acid (HC104) concentration as high as 0.5 M the bis-malonato complex (0.005 M) is quantitatively transformed to the mono-malonato complex and further dissociation to form Cr(H~O)e ~- occurs at higher acid concentrations. The rate of dissociation of the bis-complex into the corresponding mono-complex has been followed spectrophotometrically at 560 m~, at which the extinction coefficients of the two species differ widely (see Fig. 1). EXPERIMENTAL Potassium cis-diaquo-bis-malonato-chromate(III), K[Cr(OOCCH2COO),(HIO)d, was prepared by the method of HAMMet al. (4) and its purity was checked by analysis. A solution of tetra-aquo-mono-malonato-chromium(III)perchlorate, [Cr(OOCCHICOO)(H20)4]CIO,, was prepared for the first time in a manner similar to that used for the preparation of the monooxalato complex (see Ref. 1) using Dowex 50W-X8 cation exchanger and 0.075 M perchloric acid to elute the mono-malonato complex. The purity of the sample solution was checked by analysis (Found: Cr, 3.24 m-mole; Malonate, 3.23 m-mole). The apparatus, experimental procedures and techniques were as described previously. (a) * Refs. 1, 2 and 3 are to be considered as Parts I, II and III respectively of this series. u) D. BA~,rE~r~Aand M. S. MOHAN,J. inorg, nucl. Chem. 26, 613 (1964). (~) D. BANmUEAand B. CHArd~AVARTY,2. inorg, nucl. Chem. 26, 1233 (1964). (s) D. BANe~JEAand M. S. MOHAN,J. inorg, nucL Chem. 27, 1643 (1965). ~) R. E. HAMMand R. H. P~Rrdr~8, J. Am. chem. Soc. 77, 2083 (1955). 2387
2388
D. BANERJEA and C. CHATTERJEE
A 0-22
0-18
(M
0-14
o
0.I0
0'06
0.02
--
I
380
420
I
460
1
1
500
540
Wovelencjth,
t
580
I
620
m/z
Fxo. 1.--Absorption spectra: (A) K[Cr(Malonate),(HjO)s], 0.0025 M aqueous solution; (B) [Cr(Malonate)(HsO)~]CIO4, 0.0025 M in ,,--0.06 M HCIO,.
1.20
I" 04
0-88
~2
0-72
0"56
0.40
0-24
0.08 0
I0
I
20
I
30
I
I
40 Time (t),
50
1
60
I
70
I
80
rain
FIG. 2.--Graphical evaluation of kobs values under different conditions. (Complex, 0.005 M; /a, 2; Acid concentration given within parentheses.)
Kinetics and mechanism of dissociation of metal chelates
2389
RESULTS AND DISCUSSION Some of the actual first-order rate plots are shown in Fig. 2, while the effect of acid concentration on the pseudo-first-order rate constant, kobs, is shown in Fig. 3. These results, and the observation that under otherwise identical conditions kobs is independent of complex concentration (2-5 m-mole), suggest that the rate is first-order
//
7-
x
0.1
0.2
I
0"4
0-3
HCt04,
I
0'.5
M
F1~.~3.--Effectof acid concentration on kob~ (Complex, 0.005 M; p, 2; Temp., 45° C).
16.8
16.6
.~ ii- 16'4
~ L6.2
16.0
15.8
I
31
L
I
3~-5
32
I
32-5
_1 X I0 4 T
Fro. 4.--Eyring plot of rate data (using kob8 values obtained from Fig. 2).
Complex, 0.005 M; HCIO~,0"2 M; /~, 2. with respect to both the complex and H + ions. Under otherwise identical conditions a change in ionic strength, #, of the medium from 1 to 2 has little effect on the rateconstant (In 0-2 M HC104 at 45°C, kobs × l0 S, min -t is 2"8 at # = 1 and 2.76 a t # = 2). AH~ (20.2 kcal/mole) and AS:~ (--7.7 e.u.) values have been evaluated in the usual way (see Fig. 4), using the Eyring equation. The corresponding values for the bisoxalato complex (1) are AH~, 23.9 kcal/mole and AS~, --4.5 e.u. The behaviour of the bis-malonato complex is thus very similar in all respects to that of the bis-oxalato complex, but the former is much more labile (At 45°C and # = 2 the values of the H +
2390
D. BANERJEAand C. CHATrEamE
ion dependent second-order rate-constant, ka+, are 1.9 × 10-z M -1 min - t for the oxalato complex and 1.4 × 10-1 M -1 min -1 for the malonato complex). The mechanism of dissociation of the malonato complex is thus likely to be analogous to that o f the oxalato complex: Cr(=Mal)2(H~O) 2- + HaO + ~ Cr(=Mal)(--MalH)(H20)a,
(1)
(CA) C r ( = M a l ) ( - - M a l H ) ( H 2 0 ) a + H~O Slo~ Cr(=Mal)(n20)4+ + MalH-.
(2)
For both the oxalato and the malonato complexes the values of the entropy of activation, AS:~, are compatible with processes in which participation of a molecule o f water in the rate-determining steps is important c5~and this is likely to involve solventassisted dissociation: H
H
OH ..... O
Jr
OH ........ O
R--O---C--AH
/"
Ir
>. 1~........ O--C--AH (Transition state) [R--OHm]+ + -OOC--AH
where, R --
Cr
(H~O)
(3)
and A - = C O O - for the oxalato complex and
0 H 2 C C O 0 - for the malonato complex. The greater lability of the malonato complex may be due to a greater basicity o f the malonate ion, thus favouring the protonation pre-equilibrium, as presented by Equation (1), to form the conjugate acid (CA) with a simultaneous dissociation o f one of the chelate rings, the latter being again favoured by a lower thermodynamic stability of the malonato complex.
Acknowledgements--The investigations reported here have been carried out with financial assistance from the Council of Scientificand Industrial Research, India; the award of a Research Fellowship to one of us (C. C.) by the said Council is also gratefully acknowledged. c~ R. E. POWELL,J. phys. Chem. 58, 528 (1954).
KINETICS A N D MECHANISM OF DISSOCIATION OF METAL CHELATES--IV* DISSOCIATION OF
CIS-DIAQUO-BIS-MALONATO.CHROMATE(III)
D. BANERJEAand C. CHATTEILIEE Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-32 (India) (Received 27 February 1967) Abstract--The rate of dissociation of cis-diaquo.bis-malonato-chromate(III) to the mono-malonato complex in perchloric acid--sodium perchlorate media shows a first-order dependence on both the complex and H + ion concentrations under otherwise identical conditions. Values of the activation parameters AH~ (20.2 kcal/mole) and AS~ (--7.7 e.u.) have also been evaluated. An increase in ionic strength from 1 to 2 had no significant effect on the rate-constant. The behaviour of the bismalonato complex is very similar to that of the bis-oxalato complex in all respects; however, the former is much more labile (,~70 times at 45°C) with a lower value of AH~. All these observations are in accord with a solvent-assisted dissociation of the protonated form (conjugate acid) of the complex.
THE WORKreported here extends the investigations (x-a) on the kinetics and mechanisms
of dissociation of some Co(III) and Cr(III) chelates carried out earlier in this laboratory. As in the case of the corresponding oxalato complex, (1) spectral observations have shown that even at an acid (HC104) concentration as high as 0.5 M the bis-malonato complex (0.005 M) is quantitatively transformed to the mono-malonato complex and further dissociation to form Cr(H~O)e ~- occurs at higher acid concentrations. The rate of dissociation of the bis-complex into the corresponding mono-complex has been followed spectrophotometrically at 560 m~, at which the extinction coefficients of the two species differ widely (see Fig. 1). EXPERIMENTAL Potassium cis-diaquo-bis-malonato-chromate(III), K[Cr(OOCCH2COO),(HIO)d, was prepared by the method of HAMMet al. (4) and its purity was checked by analysis. A solution of tetra-aquo-mono-malonato-chromium(III)perchlorate, [Cr(OOCCHICOO)(H20)4]CIO,, was prepared for the first time in a manner similar to that used for the preparation of the monooxalato complex (see Ref. 1) using Dowex 50W-X8 cation exchanger and 0.075 M perchloric acid to elute the mono-malonato complex. The purity of the sample solution was checked by analysis (Found: Cr, 3.24 m-mole; Malonate, 3.23 m-mole). The apparatus, experimental procedures and techniques were as described previously. (a) * Refs. 1, 2 and 3 are to be considered as Parts I, II and III respectively of this series. u) D. BA~,rE~r~Aand M. S. MOHAN,J. inorg, nucl. Chem. 26, 613 (1964). (~) D. BANmUEAand B. CHArd~AVARTY,2. inorg, nucl. Chem. 26, 1233 (1964). (s) D. BANe~JEAand M. S. MOHAN,J. inorg, nucL Chem. 27, 1643 (1965). ~) R. E. HAMMand R. H. P~Rrdr~8, J. Am. chem. Soc. 77, 2083 (1955). 2387
2388
D. BANERJEA and C. CHATTERJEE
A 0-22
0-18
(M
0-14
o
0.I0
0'06
0.02
--
I
380
420
I
460
1
1
500
540
Wovelencjth,
t
580
I
620
m/z
Fxo. 1.--Absorption spectra: (A) K[Cr(Malonate),(HjO)s], 0.0025 M aqueous solution; (B) [Cr(Malonate)(HsO)~]CIO4, 0.0025 M in ,,--0.06 M HCIO,.
1.20
I" 04
0-88
~2
0-72
0"56
0.40
0-24
0.08 0
I0
I
20
I
30
I
I
40 Time (t),
50
1
60
I
70
I
80
rain
FIG. 2.--Graphical evaluation of kobs values under different conditions. (Complex, 0.005 M; /a, 2; Acid concentration given within parentheses.)
Kinetics and mechanism of dissociation of metal chelates
2389
RESULTS AND DISCUSSION Some of the actual first-order rate plots are shown in Fig. 2, while the effect of acid concentration on the pseudo-first-order rate constant, kobs, is shown in Fig. 3. These results, and the observation that under otherwise identical conditions kobs is independent of complex concentration (2-5 m-mole), suggest that the rate is first-order
//
7-
x
0.1
0.2
I
0"4
0-3
HCt04,
I
0'.5
M
F1~.~3.--Effectof acid concentration on kob~ (Complex, 0.005 M; p, 2; Temp., 45° C).
16.8
16.6
.~ ii- 16'4
~ L6.2
16.0
15.8
I
31
L
I
3~-5
32
I
32-5
_1 X I0 4 T
Fro. 4.--Eyring plot of rate data (using kob8 values obtained from Fig. 2).
Complex, 0.005 M; HCIO~,0"2 M; /~, 2. with respect to both the complex and H + ions. Under otherwise identical conditions a change in ionic strength, #, of the medium from 1 to 2 has little effect on the rateconstant (In 0-2 M HC104 at 45°C, kobs × l0 S, min -t is 2"8 at # = 1 and 2.76 a t # = 2). AH~ (20.2 kcal/mole) and AS:~ (--7.7 e.u.) values have been evaluated in the usual way (see Fig. 4), using the Eyring equation. The corresponding values for the bisoxalato complex (1) are AH~, 23.9 kcal/mole and AS~, --4.5 e.u. The behaviour of the bis-malonato complex is thus very similar in all respects to that of the bis-oxalato complex, but the former is much more labile (At 45°C and # = 2 the values of the H +
2390
D. BANERJEAand C. CHATrEamE
ion dependent second-order rate-constant, ka+, are 1.9 × 10-z M -1 min - t for the oxalato complex and 1.4 × 10-1 M -1 min -1 for the malonato complex). The mechanism of dissociation of the malonato complex is thus likely to be analogous to that o f the oxalato complex: Cr(=Mal)2(H~O) 2- + HaO + ~ Cr(=Mal)(--MalH)(H20)a,
(1)
(CA) C r ( = M a l ) ( - - M a l H ) ( H 2 0 ) a + H~O Slo~ Cr(=Mal)(n20)4+ + MalH-.
(2)
For both the oxalato and the malonato complexes the values of the entropy of activation, AS:~, are compatible with processes in which participation of a molecule o f water in the rate-determining steps is important c5~and this is likely to involve solventassisted dissociation: H
H
OH ..... O
Jr
OH ........ O
R--O---C--AH
/"
Ir
>. 1~........ O--C--AH (Transition state) [R--OHm]+ + -OOC--AH
where, R --
Cr
(H~O)
(3)
and A - = C O O - for the oxalato complex and
0 H 2 C C O 0 - for the malonato complex. The greater lability of the malonato complex may be due to a greater basicity o f the malonate ion, thus favouring the protonation pre-equilibrium, as presented by Equation (1), to form the conjugate acid (CA) with a simultaneous dissociation o f one of the chelate rings, the latter being again favoured by a lower thermodynamic stability of the malonato complex.
Acknowledgements--The investigations reported here have been carried out with financial assistance from the Council of Scientificand Industrial Research, India; the award of a Research Fellowship to one of us (C. C.) by the said Council is also gratefully acknowledged. c~ R. E. POWELL,J. phys. Chem. 58, 528 (1954).