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Effect of some added salts on the kinetics of dissociation of tris-2,2′-bipyridine-Fe(II) complex
.I. inorg, nucl. Chem. VoL 40, pp. 1073-1075 © Pergamon Press Ltd., 1978. Printedin Great Britain
0022-1902f78f0601-107~/$02.00/0
EFFECT OF SOME ADDED SALTS ON THE KINETICS OF DISSOCIATION OF TRIS-2,2'-BIPYRIDINE-Fe(II) COMPLEX SAROJA RAMAN Department of Chemistry, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology Pantnagar, Dist. Nainital, U.P., India (Received 18 December 1976; receivedfor publication I I November 1977) Al~-trtmt--Bothmonovalent cations and anions show a regular gradation, according to their sizes, in their effect on the rate and the activation parameters for the dissociation of tris(2,2'-bipyridine)-Fe(II)complex in acid. AS and AH of activation in M, 0.1 M and 0.01 M acid decrease in the order Cl- > Br- > I- for anions and Li* > Na+ > Cs+ for cations. The effect of the bulky tetra-alkyl ammonium ions is anomolous and does not depend on their sizes. The effect of the anions can be rationalized in terms of their involvement with the vacant coordination site of the partially bonded intermediate. The effect of cations is probably due to their altering the water structure at high concentration. The anomalous of the tetra-alkyl ammonium ions supports this view. INTRODUCTION
The effect of the addition of large quantities of sodium chloride, nitrate and sulphate (1 to 2 M) on the kinetics of dissociation of 2,2'-tris-bipyridine-Fe(II) in HCIO4, HCI and H2SO4 was reported earlier[l]. The rates were variously affected in the three main regions of H ÷ concentration studied. In I M acid, the rates decreased on adding these salts. In the range 0.1 to 0.2 M there was a significant increase and in 0.01 M acid a minimal increase. The main point of interest in this investigation was the acceleration brought about by anions at intermediate acid concentrations. Earlier results were indicative of the significant involvement of anions of suitable charge and size in the protonation of the partially bonded intermediate leading to complete dissociation step (k4 step). It was suggested that the anion may occupy the coordinating position left vacant by the dissociating bipyridine unit. This would facilitate the rotation of the pyridine moities around the 1-1' bond thus leading to a lower activation energy for the k4 step which appears in the rate constant under these conditions. The much smaller effects observed in 1 M and 10-2M acids where the kinetics is of zero order with respect to protons was attributed to secondary salt effects. The present investigation was undertaken in continuation of the earlier studies with the following objectives: (a) To analyse the effect of some related anions like CI-, Br- and I-, with regular gradation in properties, on the kinetics and activation parameters. (b) To study the effect of some similar monovalent cations from the same point of view, as it has been recently reported that cations have a unique role to play in the aquation of tris 5-nitro phenanthroline-Fe(II) complex [2]. (c) Another fact that prompted this investigation is the increasing awareness of the importance of the alteration of the water structure by salts and cosolvents, on some aquation and on some nucleophillic substitution and deprotonation reactions [2-7]. EXPERIMENTAL
All chemicals were of AR grade except n-tetrabutyl ammonium chloride which was of technical grade and used without
purification. Doubly distilled water (in glass) was used throughout, the kon complex was prepared by mixing stoichiometric amounts of ferrous ammonium sulphate and bipyridine in water. The kinetics was studied spectrophotometrically at least for two half lives in a Beclunan-DU spectrophotometer equipped with a thermostated cell compartment. The rates were calculated from first order plots of the concentration of the complex at varoius temperatures in the presence and absence of the salts. HCI had to be used in all cases except in some with 0.1 M acid due to sparing solubility of the large cation perchlorates, although HCIO4would have been the ideal acid for such comparative kinetics on account of the poor coordinating ability of the perchlorate ions. RESULTS
The rate constants in ! M HC1, 0.1 M (HCI or HCIO4) and 0.01 M' HCI were determined at different temperatures aad from these the activation parameters for the different cases were calculated using the absolute reaction rate theory. The results are reported in Tables ! and 2. They can be summarized as follows: 1. In 1 M HCI the addition of any of the salts reported, decreased the rate. The enthalpy and entropy of activation moreover show systematic trends. The changes in these parameters were always mutually compensating. The order of decrease in enthalpy and entropy of activation is (C4H9)4 NCI > LiCI > (C2H5)4 NI > NaCI NaBr > NaI > (CH3h NBr. The order of decrease in rates at all temperatures is 2 M HCi > I M HCI > LiCI > NaCi > (CH3h NMr > CsCi > NaBr > NaI > (C2H5)4 NI > (C~FIg),NCI. 2. The rates were determined in 0.1 M HC104, 0.1 M HCI and in mixtures of 1 M NaCI, NaBr, LiCl, and NaI in HCI04 and the rest in 0.1 M HCl because of the limited solubility of the perchlorates of these cations. All salts except (CzH~)4 NI and NaI increase the rates. The rates in the latter were even lower than in 0.1 M HCIO, Practically identical trends to the previous case were observed, although CsC1 falls in a different enthalpy and entropy and rate are as follows HCl > LiCI >(C2H~)4 NI > NaCI > NaBr > HCIO4 > NaI > (CH3)4NBr. The" decreasing order of rates is CsCI > LiCI ~ NaC1 > HCI > (CH3h NBr > NaBr > HCIO4 > (C2Hsh NI > NaI.
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SAROJA RAMAN Table 1. Effect of added salts on the rate of dissociation of tris-2,2'-bipyridine-Fe(II) complex in acids 1M HCI (sec-I x 103) Salt(lM) Free Acid LiCI NaCI CsCI NaBr Nal (CHs)4NBr (C2H5)4NI (C4H9)4 NCI
29-+0.2* 1.53t 1.60~: 1.46 1.47 1.36 1.27 1.12 1.40 1.13
o.4811
0.1M HCI/HCIO4(sec-I x 104)
32-+0.2* 35-+0.2° 29-+0.2° 32-+0.2* 35-+0.2* 2.23* 3.08 4.88§ 7.42§ 11.4§ 2.35;t 3.60~ 4.55¶ 6.795 10.1¶ 2.16 3.34 5.22 8.46 12.3 1.94 3.25 5.42 8.36 12.3 1.81 2.59 7.65 11.7 17.1 2.01 2.91 4.77 7.42 10.7 1.65 2.44 3.95 6.09 8.61 2.11 2.89 5.40 7.89 11.30 1.60 2.53 4.22 6.09 8.70 0.983** 1.66~:~ . . . .
0.01M HCI (sec-I x 104) 32-+0.2° 3.53 3.81 3.84 4.16 3.50 2.32§§ 3.62 3.04 . .
36x0.2 ° 6.13
42x0.2 ° 13.2
6.54 6.92 7.02 5.72 4.11'* 6.24 5.25
14.5 14.4 15.0 12.05 6.32§§ 14.0 11.6
tl M HCI, ;t2 M HCI, §0.1 M HCIO4, ¶0.1 M HCI, 1125"(2,**29.5°, ~:~:32.5°C,§§The rate in NaI fall rapidly and reaches an equilibrium. Hence, rates were calculated from data of first few minutes. Rates reported are an average of at least three runs. The variation in rate between individual runs was -+0.1x 10-3 see -~.
Table 2. Thermodynamic activation parameters for the dissociation of tris-(2,2'-bipyridine)-Fe(II) complex in the presence of added salts I M Acid Salt (1 M) Freeacid (no salt) LiCI NaCI CsCI NaBr NaI (CH3)4NBr (C2HshNI (C4Hg)4NCI*÷
AH + AS+ kcal/mole e.u./mole 25.7-t_+1.0 13.6,_+1.5 26.0~-+0.9 14.7:~-+1.2 24.8 -+0.5 10.7 -+ 1.0 23.7-+0.2 7.1 -+0.4 23.1 -+0.6 4.9-+ 1.0 23.1 -+0.6 6.1 -+0.8 23.3-+0.4 5.2+0.6 21.7-+0.8 0.3-+1.0 24.2-+ 1.0 7.9-+ 1.8 29.2 -+0.5 24.3 -+ 1.0
0.1M Acid AH + kcal/mole 25.5§-+0.4 23.9~-+0.6 25.7 -+ 1.0 24.6-+0.7 23.5 -+0.5 24.2±0.8 23.3-+0.9 22.1-+0.2 24.9-+1.0 . .
AS+ e.u./mole 10.7§-+0.7 5.3¶-+0.8 11.7 -+ 1.5 7.9-+ 1.2 5.2-+0.8 6.5-+ 1.2 3.1-+1.5 2.9-+0.5 8.7-+ 1.5 .
0.01 M Acid AH* AS+ kcal/mole e.u./mole 25.6-+0.4 6.2-+0.5 25.6-+0.6 6.2-+0.8 24.7 -+0.2 6.9 -+0.2 24.5-+0.5 6.2-+0.6 23.9-+0.3 4.4+0.4 22.9-+0.4 0.9-+0.8 18.111-+1.7 -15.5]1-+2.3 24.5-+0.6 6.1-+0.8 24.9-+0.3 7.2-+0.7 .
tl M HCI, ~2 M HCI, §0.1 M HCI, ¶0.1 M HCIO4, HSubjectto large errors because the first order plot deviated very early in the reaction probably due to reverse or side reactions. ~*Conc. 0.25 M. 3. In 10-2M HCI, all except NaBr, NaI and (C2H5)4 NI increase the rate. The following order is observed with respect to the activation parameters and rates. A ' S and A H " : HCI, (C2H~h NI, (CH3h NBr, LiCI, NaC1, NaBr, NaI. Rate: CsC! > LiCi --- NaCI > (CH3h NBr > • NaBr > (C2H5)4 NI > NaI. ' DISCUSSION AND CONCLUSIONS
Since the aquation of tris bipyridine Fe(II) complex proceeds via three distinct paths in the three levels of acid concentration used in this study, the effects that are observed are necessarily to be inte~reted separately for each. In 1 M acid and above the aquation rate is zero order in protons. The rate determining step under these conditions is the initial breaking of an Fe-N bond. The partially bonded intermediate is then protonated and the ligand removed by fast steps[8-10]. The transition state for this step obviously has to be a partially broken or weakened F e - N bond, resulting in positive entropy. This is corroborated by the results. The effect of all the added salts is to decrease both the enthalpy and entropy of activation compared to the free acid. This shows clearly that in spite of the high concentration of HCI these salts produce a definite, regular and discernible effect. The effect in 1 M HCI solution in all cases is to produce favourable enthalpy and unfavourable entropy of activa-
tion changes. The resultant Gibb's free energy of activation is, however, higher in all cases in the presence of the salts. Hence, a decrease in overall rate is observed. The activation parameters thus suggest a destablized and more ordered transition state in the presence of the salts. It is further evident that the effect cannot be rationalized as an effect of the cation or anion alone. But a regular gradation in the effect of both anions and cations is evident. Thus the enthalpy and entropy of activation vary as C I - > B r - > I - for anions when used as the corresponding sodium salts. Similarly the effect decreases in the order Li ÷ > Na + > Cs ÷ for cations, which is the order of their size also. There is, however a shift in the order for (C2Hsh N ÷ and (C4H9)4 N + for both entropy and enthalpy of activation (C4H9) N + and (C2H5)4 N 4 fall at the beginning o f the series. The mechanism in 0.1M acid is markedly different from that in 1 M acid. Here the rate is a function of H ÷ ion concentration as given by scheme I. kob = k i {k3 + k4 [H+]}/k2 + ka + k4 [H +]
Where k~ and k2 are the initial bond breaking remaking constants, k3 is the rate constant for sociation of the half bonded intermediate, and k, constant for the protonation and dissociation of the bonded intermediate.
I and disthe half
Dissociation of tris-2,2'-bipyridine-Fe(II)complex The effect of CI-, NO3 and SO24- ions on this step was reported earlier. It was suggested that the attachment or proximity of an anion of favourable size and charge, at the free coordination site left vacant by the dissociating Fe-N bond will facilitate the rotation of the pyridine ring around the l - l ' axis and hence, lower the energy of activation for k4 step. The present data on activation parameters and rates has corroborated this statement. C1 and Br ions affect the enthalpy and entropy, with a net favourable free energy of activation leading to an acceleration. The effect of the bulky and highly polarizable I is less favourable, even compared to the CIO~ ion. The rate favouring term appears to be the enthalpy rather than entropy of activation. However, in most cases, the transition state is affected in a mutually compensatory manner in terms of entropy and enthalpy, thus attenuating the change in AG ~. Although the calculated parameters are for the combination of the various steps occurring in this concentration of acid, the observed increase in the rate in 0.1 M HCI in spite of the decrease in k, in the presence of added salts lends credence to the suggestion that the anions in effect are leading to a more stable and ordered transition state. The decreasing order for AH ~ and AS ~ is C l - > B r - > I . It is interesting that cations also influence the rate in 0.1 M HC1. The order with respect to AH ~ and AS ~ is as before Li ~ > Na ÷ > Cs ÷. The bulky quarternary ammonium ions do not fit into this order in respect of size. The rate in 0.01 M acid and below is zero order in hydrogen ions and involves the initial bond breaking step and the subsequent spontaneous dissociation of this intermediate, k = k,k3/k2 + k3. The trend in the effect of the added salts is the same as that in 0.1 M acid, the quarternary ammonium ions being exceptional. The effect of anions can again be visualized as at least partially due to the stabilized transition state as in 0.1 M acid. The increase in rates as opposed to the decrease in k, supports this view again. But for this unfavourable effect on k,, it might have been possible to observe more marked accelerating effects on the latter step of dissociation. The effect of cations, especially the effect of the quarternary ammonium ions, and the destabilization of the transition state for the k, step are more difficult to interpret. The enthalpy of activation is not very different but the entropy varies considerably. This may be due to the different environment around the complex ion in the initial state and in the transition state in the presence and absence of these salts. The transition state in the present case is dipositive and there is no change in charge as
]]NCVo].~), No,6--J
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compared to the initial state. Under these circumstances, it seems reasonable to regard the lowering of the entropy of activation to the change in water structure around the transition state in the presence of these salts. The rupture of one Fe-N bond would expose a tertiary nitrogen of bipyridine with its lone pair to the surrounding water. This may facilitate hydrogen bond formation, resulting in more H bonds around this ion through a cooperative effect[ll]. In the presence of HCI or LiCI. where there is already a change in the water structure as they are structure makers, this effect may not be visible. On the contrary with salts like NaCI, NaBr, Nal, (CH3)4, N ÷ Br, CsCI etc. which are known to be structure breakers, structure remaking may be more possible[I l, 12]. One reason for the enhanced hydrogen bonded structure around the transition intermediate may be the interaction of solvent cospheres of the transition intermediate and the co-ions. This is in keeping with the observed order of entropy in the presence of these salts. The seemingly anamolous effect of the quarternary ammonium halides which cannot be otherwise interpreted also fits into this scheme. (C4H9)4 NCI is a structure former. (C2H5)4 NBr is classified as more or less borderline between structure breakers and structure formers, and would therefore differ from the other cations with regard to their effect on the initial and transition states[13]. Hence it is reasonable that they do not fall in line with the other cations. Such trends have been found for thermodynamic parameters of transfer from H20 to other solvents and solutions [ 12, 13]. REFERENCES
I. S. Raman, J. Inorg. Nucl. Chem. 38, 781 (t976). 2. M. J. Blandamer, J. Burgess and S. H. Morris. d. Chem. Solids (Dalton Trans.)16, 1717 (1974). 3. F. Hibbert and F. A. Long, J. Am. Chem. Soc. 94, 7637 (1972). 4. H. S. Golinkin and J. B. Hyne, Can. J. Chem. 46, 125 (1968). 5. R. E. Robertson and S. E. Sugamori, Can. J. Chem. 50, 1353 (1972). 6. L. Mennings and J. B. F. N. Engberts, J. Phs~. Chem. 77, 1271 (1973). 7. M. J. Blandamer and J. Burgess. Chem. Soc~ Rer. 4, 55. (1975). 8. F. Basolo, J. C. Hayes and H. M. Neumann. J. Am. ('hem. Soc. 76, 3807, (1954). 9. F. Basolo and R. G. Pearson, Mechanism of Inorganic Reactions. Wiley, New York (1%7). 10. S. Raman, J. Inorg. Nucl. Chem. 37, 1747!1975). 11. M. J. Blandamer, Quart. Rev. ~, 169 !1970). 12. Water, A Comprehensive Treatise (Edited by F. Franks), Vol. 3. Plenum Press, London (1973). 13. T. S. Sarma and J. C. Ahluwalia, Chem. Soc. Rer. 2, 203 (1973).
0022-1902f78f0601-107~/$02.00/0
EFFECT OF SOME ADDED SALTS ON THE KINETICS OF DISSOCIATION OF TRIS-2,2'-BIPYRIDINE-Fe(II) COMPLEX SAROJA RAMAN Department of Chemistry, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology Pantnagar, Dist. Nainital, U.P., India (Received 18 December 1976; receivedfor publication I I November 1977) Al~-trtmt--Bothmonovalent cations and anions show a regular gradation, according to their sizes, in their effect on the rate and the activation parameters for the dissociation of tris(2,2'-bipyridine)-Fe(II)complex in acid. AS and AH of activation in M, 0.1 M and 0.01 M acid decrease in the order Cl- > Br- > I- for anions and Li* > Na+ > Cs+ for cations. The effect of the bulky tetra-alkyl ammonium ions is anomolous and does not depend on their sizes. The effect of the anions can be rationalized in terms of their involvement with the vacant coordination site of the partially bonded intermediate. The effect of cations is probably due to their altering the water structure at high concentration. The anomalous of the tetra-alkyl ammonium ions supports this view. INTRODUCTION
The effect of the addition of large quantities of sodium chloride, nitrate and sulphate (1 to 2 M) on the kinetics of dissociation of 2,2'-tris-bipyridine-Fe(II) in HCIO4, HCI and H2SO4 was reported earlier[l]. The rates were variously affected in the three main regions of H ÷ concentration studied. In I M acid, the rates decreased on adding these salts. In the range 0.1 to 0.2 M there was a significant increase and in 0.01 M acid a minimal increase. The main point of interest in this investigation was the acceleration brought about by anions at intermediate acid concentrations. Earlier results were indicative of the significant involvement of anions of suitable charge and size in the protonation of the partially bonded intermediate leading to complete dissociation step (k4 step). It was suggested that the anion may occupy the coordinating position left vacant by the dissociating bipyridine unit. This would facilitate the rotation of the pyridine moities around the 1-1' bond thus leading to a lower activation energy for the k4 step which appears in the rate constant under these conditions. The much smaller effects observed in 1 M and 10-2M acids where the kinetics is of zero order with respect to protons was attributed to secondary salt effects. The present investigation was undertaken in continuation of the earlier studies with the following objectives: (a) To analyse the effect of some related anions like CI-, Br- and I-, with regular gradation in properties, on the kinetics and activation parameters. (b) To study the effect of some similar monovalent cations from the same point of view, as it has been recently reported that cations have a unique role to play in the aquation of tris 5-nitro phenanthroline-Fe(II) complex [2]. (c) Another fact that prompted this investigation is the increasing awareness of the importance of the alteration of the water structure by salts and cosolvents, on some aquation and on some nucleophillic substitution and deprotonation reactions [2-7]. EXPERIMENTAL
All chemicals were of AR grade except n-tetrabutyl ammonium chloride which was of technical grade and used without
purification. Doubly distilled water (in glass) was used throughout, the kon complex was prepared by mixing stoichiometric amounts of ferrous ammonium sulphate and bipyridine in water. The kinetics was studied spectrophotometrically at least for two half lives in a Beclunan-DU spectrophotometer equipped with a thermostated cell compartment. The rates were calculated from first order plots of the concentration of the complex at varoius temperatures in the presence and absence of the salts. HCI had to be used in all cases except in some with 0.1 M acid due to sparing solubility of the large cation perchlorates, although HCIO4would have been the ideal acid for such comparative kinetics on account of the poor coordinating ability of the perchlorate ions. RESULTS
The rate constants in ! M HC1, 0.1 M (HCI or HCIO4) and 0.01 M' HCI were determined at different temperatures aad from these the activation parameters for the different cases were calculated using the absolute reaction rate theory. The results are reported in Tables ! and 2. They can be summarized as follows: 1. In 1 M HCI the addition of any of the salts reported, decreased the rate. The enthalpy and entropy of activation moreover show systematic trends. The changes in these parameters were always mutually compensating. The order of decrease in enthalpy and entropy of activation is (C4H9)4 NCI > LiCI > (C2H5)4 NI > NaCI NaBr > NaI > (CH3h NBr. The order of decrease in rates at all temperatures is 2 M HCi > I M HCI > LiCI > NaCi > (CH3h NMr > CsCi > NaBr > NaI > (C2H5)4 NI > (C~FIg),NCI. 2. The rates were determined in 0.1 M HC104, 0.1 M HCI and in mixtures of 1 M NaCI, NaBr, LiCl, and NaI in HCI04 and the rest in 0.1 M HCl because of the limited solubility of the perchlorates of these cations. All salts except (CzH~)4 NI and NaI increase the rates. The rates in the latter were even lower than in 0.1 M HCIO, Practically identical trends to the previous case were observed, although CsC1 falls in a different enthalpy and entropy and rate are as follows HCl > LiCI >(C2H~)4 NI > NaCI > NaBr > HCIO4 > NaI > (CH3)4NBr. The" decreasing order of rates is CsCI > LiCI ~ NaC1 > HCI > (CH3h NBr > NaBr > HCIO4 > (C2Hsh NI > NaI.
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SAROJA RAMAN Table 1. Effect of added salts on the rate of dissociation of tris-2,2'-bipyridine-Fe(II) complex in acids 1M HCI (sec-I x 103) Salt(lM) Free Acid LiCI NaCI CsCI NaBr Nal (CHs)4NBr (C2H5)4NI (C4H9)4 NCI
29-+0.2* 1.53t 1.60~: 1.46 1.47 1.36 1.27 1.12 1.40 1.13
o.4811
0.1M HCI/HCIO4(sec-I x 104)
32-+0.2* 35-+0.2° 29-+0.2° 32-+0.2* 35-+0.2* 2.23* 3.08 4.88§ 7.42§ 11.4§ 2.35;t 3.60~ 4.55¶ 6.795 10.1¶ 2.16 3.34 5.22 8.46 12.3 1.94 3.25 5.42 8.36 12.3 1.81 2.59 7.65 11.7 17.1 2.01 2.91 4.77 7.42 10.7 1.65 2.44 3.95 6.09 8.61 2.11 2.89 5.40 7.89 11.30 1.60 2.53 4.22 6.09 8.70 0.983** 1.66~:~ . . . .
0.01M HCI (sec-I x 104) 32-+0.2° 3.53 3.81 3.84 4.16 3.50 2.32§§ 3.62 3.04 . .
36x0.2 ° 6.13
42x0.2 ° 13.2
6.54 6.92 7.02 5.72 4.11'* 6.24 5.25
14.5 14.4 15.0 12.05 6.32§§ 14.0 11.6
tl M HCI, ;t2 M HCI, §0.1 M HCIO4, ¶0.1 M HCI, 1125"(2,**29.5°, ~:~:32.5°C,§§The rate in NaI fall rapidly and reaches an equilibrium. Hence, rates were calculated from data of first few minutes. Rates reported are an average of at least three runs. The variation in rate between individual runs was -+0.1x 10-3 see -~.
Table 2. Thermodynamic activation parameters for the dissociation of tris-(2,2'-bipyridine)-Fe(II) complex in the presence of added salts I M Acid Salt (1 M) Freeacid (no salt) LiCI NaCI CsCI NaBr NaI (CH3)4NBr (C2HshNI (C4Hg)4NCI*÷
AH + AS+ kcal/mole e.u./mole 25.7-t_+1.0 13.6,_+1.5 26.0~-+0.9 14.7:~-+1.2 24.8 -+0.5 10.7 -+ 1.0 23.7-+0.2 7.1 -+0.4 23.1 -+0.6 4.9-+ 1.0 23.1 -+0.6 6.1 -+0.8 23.3-+0.4 5.2+0.6 21.7-+0.8 0.3-+1.0 24.2-+ 1.0 7.9-+ 1.8 29.2 -+0.5 24.3 -+ 1.0
0.1M Acid AH + kcal/mole 25.5§-+0.4 23.9~-+0.6 25.7 -+ 1.0 24.6-+0.7 23.5 -+0.5 24.2±0.8 23.3-+0.9 22.1-+0.2 24.9-+1.0 . .
AS+ e.u./mole 10.7§-+0.7 5.3¶-+0.8 11.7 -+ 1.5 7.9-+ 1.2 5.2-+0.8 6.5-+ 1.2 3.1-+1.5 2.9-+0.5 8.7-+ 1.5 .
0.01 M Acid AH* AS+ kcal/mole e.u./mole 25.6-+0.4 6.2-+0.5 25.6-+0.6 6.2-+0.8 24.7 -+0.2 6.9 -+0.2 24.5-+0.5 6.2-+0.6 23.9-+0.3 4.4+0.4 22.9-+0.4 0.9-+0.8 18.111-+1.7 -15.5]1-+2.3 24.5-+0.6 6.1-+0.8 24.9-+0.3 7.2-+0.7 .
tl M HCI, ~2 M HCI, §0.1 M HCI, ¶0.1 M HCIO4, HSubjectto large errors because the first order plot deviated very early in the reaction probably due to reverse or side reactions. ~*Conc. 0.25 M. 3. In 10-2M HCI, all except NaBr, NaI and (C2H5)4 NI increase the rate. The following order is observed with respect to the activation parameters and rates. A ' S and A H " : HCI, (C2H~h NI, (CH3h NBr, LiCI, NaC1, NaBr, NaI. Rate: CsC! > LiCi --- NaCI > (CH3h NBr > • NaBr > (C2H5)4 NI > NaI. ' DISCUSSION AND CONCLUSIONS
Since the aquation of tris bipyridine Fe(II) complex proceeds via three distinct paths in the three levels of acid concentration used in this study, the effects that are observed are necessarily to be inte~reted separately for each. In 1 M acid and above the aquation rate is zero order in protons. The rate determining step under these conditions is the initial breaking of an Fe-N bond. The partially bonded intermediate is then protonated and the ligand removed by fast steps[8-10]. The transition state for this step obviously has to be a partially broken or weakened F e - N bond, resulting in positive entropy. This is corroborated by the results. The effect of all the added salts is to decrease both the enthalpy and entropy of activation compared to the free acid. This shows clearly that in spite of the high concentration of HCI these salts produce a definite, regular and discernible effect. The effect in 1 M HCI solution in all cases is to produce favourable enthalpy and unfavourable entropy of activa-
tion changes. The resultant Gibb's free energy of activation is, however, higher in all cases in the presence of the salts. Hence, a decrease in overall rate is observed. The activation parameters thus suggest a destablized and more ordered transition state in the presence of the salts. It is further evident that the effect cannot be rationalized as an effect of the cation or anion alone. But a regular gradation in the effect of both anions and cations is evident. Thus the enthalpy and entropy of activation vary as C I - > B r - > I - for anions when used as the corresponding sodium salts. Similarly the effect decreases in the order Li ÷ > Na + > Cs ÷ for cations, which is the order of their size also. There is, however a shift in the order for (C2Hsh N ÷ and (C4H9)4 N + for both entropy and enthalpy of activation (C4H9) N + and (C2H5)4 N 4 fall at the beginning o f the series. The mechanism in 0.1M acid is markedly different from that in 1 M acid. Here the rate is a function of H ÷ ion concentration as given by scheme I. kob = k i {k3 + k4 [H+]}/k2 + ka + k4 [H +]
Where k~ and k2 are the initial bond breaking remaking constants, k3 is the rate constant for sociation of the half bonded intermediate, and k, constant for the protonation and dissociation of the bonded intermediate.
I and disthe half
Dissociation of tris-2,2'-bipyridine-Fe(II)complex The effect of CI-, NO3 and SO24- ions on this step was reported earlier. It was suggested that the attachment or proximity of an anion of favourable size and charge, at the free coordination site left vacant by the dissociating Fe-N bond will facilitate the rotation of the pyridine ring around the l - l ' axis and hence, lower the energy of activation for k4 step. The present data on activation parameters and rates has corroborated this statement. C1 and Br ions affect the enthalpy and entropy, with a net favourable free energy of activation leading to an acceleration. The effect of the bulky and highly polarizable I is less favourable, even compared to the CIO~ ion. The rate favouring term appears to be the enthalpy rather than entropy of activation. However, in most cases, the transition state is affected in a mutually compensatory manner in terms of entropy and enthalpy, thus attenuating the change in AG ~. Although the calculated parameters are for the combination of the various steps occurring in this concentration of acid, the observed increase in the rate in 0.1 M HCI in spite of the decrease in k, in the presence of added salts lends credence to the suggestion that the anions in effect are leading to a more stable and ordered transition state. The decreasing order for AH ~ and AS ~ is C l - > B r - > I . It is interesting that cations also influence the rate in 0.1 M HC1. The order with respect to AH ~ and AS ~ is as before Li ~ > Na ÷ > Cs ÷. The bulky quarternary ammonium ions do not fit into this order in respect of size. The rate in 0.01 M acid and below is zero order in hydrogen ions and involves the initial bond breaking step and the subsequent spontaneous dissociation of this intermediate, k = k,k3/k2 + k3. The trend in the effect of the added salts is the same as that in 0.1 M acid, the quarternary ammonium ions being exceptional. The effect of anions can again be visualized as at least partially due to the stabilized transition state as in 0.1 M acid. The increase in rates as opposed to the decrease in k, supports this view again. But for this unfavourable effect on k,, it might have been possible to observe more marked accelerating effects on the latter step of dissociation. The effect of cations, especially the effect of the quarternary ammonium ions, and the destabilization of the transition state for the k, step are more difficult to interpret. The enthalpy of activation is not very different but the entropy varies considerably. This may be due to the different environment around the complex ion in the initial state and in the transition state in the presence and absence of these salts. The transition state in the present case is dipositive and there is no change in charge as
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compared to the initial state. Under these circumstances, it seems reasonable to regard the lowering of the entropy of activation to the change in water structure around the transition state in the presence of these salts. The rupture of one Fe-N bond would expose a tertiary nitrogen of bipyridine with its lone pair to the surrounding water. This may facilitate hydrogen bond formation, resulting in more H bonds around this ion through a cooperative effect[ll]. In the presence of HCI or LiCI. where there is already a change in the water structure as they are structure makers, this effect may not be visible. On the contrary with salts like NaCI, NaBr, Nal, (CH3)4, N ÷ Br, CsCI etc. which are known to be structure breakers, structure remaking may be more possible[I l, 12]. One reason for the enhanced hydrogen bonded structure around the transition intermediate may be the interaction of solvent cospheres of the transition intermediate and the co-ions. This is in keeping with the observed order of entropy in the presence of these salts. The seemingly anamolous effect of the quarternary ammonium halides which cannot be otherwise interpreted also fits into this scheme. (C4H9)4 NCI is a structure former. (C2H5)4 NBr is classified as more or less borderline between structure breakers and structure formers, and would therefore differ from the other cations with regard to their effect on the initial and transition states[13]. Hence it is reasonable that they do not fall in line with the other cations. Such trends have been found for thermodynamic parameters of transfer from H20 to other solvents and solutions [ 12, 13]. REFERENCES
I. S. Raman, J. Inorg. Nucl. Chem. 38, 781 (t976). 2. M. J. Blandamer, J. Burgess and S. H. Morris. d. Chem. Solids (Dalton Trans.)16, 1717 (1974). 3. F. Hibbert and F. A. Long, J. Am. Chem. Soc. 94, 7637 (1972). 4. H. S. Golinkin and J. B. Hyne, Can. J. Chem. 46, 125 (1968). 5. R. E. Robertson and S. E. Sugamori, Can. J. Chem. 50, 1353 (1972). 6. L. Mennings and J. B. F. N. Engberts, J. Phs~. Chem. 77, 1271 (1973). 7. M. J. Blandamer and J. Burgess. Chem. Soc~ Rer. 4, 55. (1975). 8. F. Basolo, J. C. Hayes and H. M. Neumann. J. Am. ('hem. Soc. 76, 3807, (1954). 9. F. Basolo and R. G. Pearson, Mechanism of Inorganic Reactions. Wiley, New York (1%7). 10. S. Raman, J. Inorg. Nucl. Chem. 37, 1747!1975). 11. M. J. Blandamer, Quart. Rev. ~, 169 !1970). 12. Water, A Comprehensive Treatise (Edited by F. Franks), Vol. 3. Plenum Press, London (1973). 13. T. S. Sarma and J. C. Ahluwalia, Chem. Soc. Rer. 2, 203 (1973).