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A note on the aquation of chromium (III)-malonate complexes
Notes
1213
A rigorous treatment, in which this approximation is not introduced, leads to So = { ( - 8 , 2 ( V t ° + 172)+ 21728~8~)/ART} - (I.~2- V~o)/V~°
(15a)
where the symbols have the meanings assigned in the original paper. Although the additional term makes only a relatively small change in the value of So for the systems studied in this paper, values of 8: calculated from experimental values of So are somewhat altered, and the figures given in Ref. [1] should be modified as follows: Calculated values of 82 for solutions of Cr(acac)3 Solvent Toluene CCIa Benzene CHCIa Acetone Ethanol Methanol
~ (Ref. [1])
6~ (revised value)
9.10 8-71 9-04 8.98 9.08 12-06 13.54
9.63 9.22 9-66 9.67 10.01 13.04 15.04
The changes in numerical value affect none of the conclusions reached in Ref. [1].
Department of Inorganic and Structural Chemistry. University of Leeds Leeds 2, England
H. M. N. H. I R V I N G J. S. S M I T H
J. inorg,nucl.Chem., 1969,Vol.31, pp. 1213to 1214. PergamonPress. Printedin Great Britain
A note on the aquation of chromium (llI)-malonate complexes (Received 31 July 1968)
RECENTLY, Chang[l] has investigated the aquation of trismalonatochromate (III), and has reported values of kinetic parameters for the aquation of [Crmala] 3- to cis-[Crmalzaq2]-, deduced from experimental data on the assumption that cis-[Crmal2aq2]- is the final product of the aquation reaction under the employed experimental conditions (room or slightly higher temperature, 0.10-1.00 total [H+]). At our Laboratory we had investigated previously the stability and kinetic behaviour on aquation of the Cr(llI)-malonate system including all possible chromium(III)-malonate species, and found that the course of the aquation reaction of [Crmal3] 3- can hardly be interpreted in terms of a single aquation step leading to cis-[Crmal2aq2]-: only at relatively high pH (> - 2"5) is cis-[Crmal2aqd- stable toward aquation, and at lower pH, the cis-diaquo complex becomes thermodynamically unstable, so at pH < 0.8 aquation is practically complete, [Craqd 3+ being the predominant species at equilibrium; e.g. at 50°C the successive conditional formation constants are approximately (2):
Kte = [Crmalaq4+] [H+]2 = !.4 [Craqe a+] [real H2] [cis-(Crmal2aq2)-] [H+] 2 K2c = [Crmalaq4+][mal H2] = 0-8 × 10-2 [Crmal 3-] [H +] K ~ = [ cis-( Crmal2aq2) - ] [real H - ] - 0.2 i. J. C. Chang,,/. inorg, nucl. Chem. 30, 945 (1968).
1214
Notes
That aquation proceeds actually further than cis-[Crmal2aq~]- is easily shown by the final molar extinction coefficient of the 18 kK maximum being well below 49 (the value corresponding to pure cis-[Crmal2aq~]-) in all solutions we measured at pH < 3 in the range 30°-70"C [2]. A preliminary evaluation of kinetic data measured in the pH region 1.5-3.0 at/z = 2, indicated k = 9.5 × 10-8[H +] moles-l/sec at 50°C with AE = 23.03 kcal/mole for the aquation of [Crmal3]a- to cis-[Crmal2aq2]and k = 3.2 × 10-4+9.5 × 10-a [H +] moles-1/sec at 50°C with AE ~ 24"8 kcal/mole for the aquation of cis-[Crmal2aq2]- to [Crmalaq4]+, whereas more accurate data could be obtained for the aquation of [Crmalaqd + to [Craq,]÷a: k = 8.3 × 10-5+2.0× 10-4 [H+]moles-l/sec at 50°C with AE = 19.4 kcal/mole for the [H+]-dependent path [2]. Thus it appears that, although at sufficiently low temperature aquation of cis-[Crmal2aq2]- in acid medium can proceed so slowly that it can be practically disregarded (as a matter of fact our estimate of the rate constant for the aquation of [Crmala]-3 would amount to 1.8 × 10-4 moles-l/sec at 25°C, hence not extremely different from 3.60 × 10-4 mole-l/sec as given in [1]), the rate constants of the successive aquation steps are nevertheless so close that a complete treatment including all three consecutive pseudo-first order aquation reactions should be in principle applied in the interpretation of kinetic data of aquation of tris-malonato chromate(Ill) in the low pH region where cis-[Crmal~aqd- is thermodynamically unstable.
Institute of General and Inorganic Chemistry University of Perugia Italy 2. E. Mantovani and C. Furlani, Z. anorg, allg. Chem. 359, (1968).
C. F U R L A N I E. MANTOVAN1
1213
A rigorous treatment, in which this approximation is not introduced, leads to So = { ( - 8 , 2 ( V t ° + 172)+ 21728~8~)/ART} - (I.~2- V~o)/V~°
(15a)
where the symbols have the meanings assigned in the original paper. Although the additional term makes only a relatively small change in the value of So for the systems studied in this paper, values of 8: calculated from experimental values of So are somewhat altered, and the figures given in Ref. [1] should be modified as follows: Calculated values of 82 for solutions of Cr(acac)3 Solvent Toluene CCIa Benzene CHCIa Acetone Ethanol Methanol
~ (Ref. [1])
6~ (revised value)
9.10 8-71 9-04 8.98 9.08 12-06 13.54
9.63 9.22 9-66 9.67 10.01 13.04 15.04
The changes in numerical value affect none of the conclusions reached in Ref. [1].
Department of Inorganic and Structural Chemistry. University of Leeds Leeds 2, England
H. M. N. H. I R V I N G J. S. S M I T H
J. inorg,nucl.Chem., 1969,Vol.31, pp. 1213to 1214. PergamonPress. Printedin Great Britain
A note on the aquation of chromium (llI)-malonate complexes (Received 31 July 1968)
RECENTLY, Chang[l] has investigated the aquation of trismalonatochromate (III), and has reported values of kinetic parameters for the aquation of [Crmala] 3- to cis-[Crmalzaq2]-, deduced from experimental data on the assumption that cis-[Crmal2aq2]- is the final product of the aquation reaction under the employed experimental conditions (room or slightly higher temperature, 0.10-1.00 total [H+]). At our Laboratory we had investigated previously the stability and kinetic behaviour on aquation of the Cr(llI)-malonate system including all possible chromium(III)-malonate species, and found that the course of the aquation reaction of [Crmal3] 3- can hardly be interpreted in terms of a single aquation step leading to cis-[Crmal2aq2]-: only at relatively high pH (> - 2"5) is cis-[Crmal2aqd- stable toward aquation, and at lower pH, the cis-diaquo complex becomes thermodynamically unstable, so at pH < 0.8 aquation is practically complete, [Craqd 3+ being the predominant species at equilibrium; e.g. at 50°C the successive conditional formation constants are approximately (2):
Kte = [Crmalaq4+] [H+]2 = !.4 [Craqe a+] [real H2] [cis-(Crmal2aq2)-] [H+] 2 K2c = [Crmalaq4+][mal H2] = 0-8 × 10-2 [Crmal 3-] [H +] K ~ = [ cis-( Crmal2aq2) - ] [real H - ] - 0.2 i. J. C. Chang,,/. inorg, nucl. Chem. 30, 945 (1968).
1214
Notes
That aquation proceeds actually further than cis-[Crmal2aq~]- is easily shown by the final molar extinction coefficient of the 18 kK maximum being well below 49 (the value corresponding to pure cis-[Crmal2aq~]-) in all solutions we measured at pH < 3 in the range 30°-70"C [2]. A preliminary evaluation of kinetic data measured in the pH region 1.5-3.0 at/z = 2, indicated k = 9.5 × 10-8[H +] moles-l/sec at 50°C with AE = 23.03 kcal/mole for the aquation of [Crmal3]a- to cis-[Crmal2aq2]and k = 3.2 × 10-4+9.5 × 10-a [H +] moles-1/sec at 50°C with AE ~ 24"8 kcal/mole for the aquation of cis-[Crmal2aq2]- to [Crmalaq4]+, whereas more accurate data could be obtained for the aquation of [Crmalaqd + to [Craq,]÷a: k = 8.3 × 10-5+2.0× 10-4 [H+]moles-l/sec at 50°C with AE = 19.4 kcal/mole for the [H+]-dependent path [2]. Thus it appears that, although at sufficiently low temperature aquation of cis-[Crmal2aq2]- in acid medium can proceed so slowly that it can be practically disregarded (as a matter of fact our estimate of the rate constant for the aquation of [Crmala]-3 would amount to 1.8 × 10-4 moles-l/sec at 25°C, hence not extremely different from 3.60 × 10-4 mole-l/sec as given in [1]), the rate constants of the successive aquation steps are nevertheless so close that a complete treatment including all three consecutive pseudo-first order aquation reactions should be in principle applied in the interpretation of kinetic data of aquation of tris-malonato chromate(Ill) in the low pH region where cis-[Crmal~aqd- is thermodynamically unstable.
Institute of General and Inorganic Chemistry University of Perugia Italy 2. E. Mantovani and C. Furlani, Z. anorg, allg. Chem. 359, (1968).
C. F U R L A N I E. MANTOVAN1