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A kinetic study of the reaction between thiourea and ethylenediaminetetraacetato ruthenate(III) complex by the stopped flow technique
Polyhedron Vol. 8, No. 4, pp. Printed in Great Britain
463461,
1989 0
0277-5387/89 $3.00+.00 1989 Pergamon Press plc
A KINETIC STUDY OF THE REACTION BETWEEN THIOUREA AND ETHYLENEDIAMINETETRAACETATO RUTHENATE(III) COMPLEX BY THE STOPPED FLOW TECHNIQUE M. M. TAQUI
KHAN* and R. M. NAIK
Discipline of Coordination Chemistry and Homogeneous Catalysis, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India (Received 19 April 1988 ; accepted after revision 15 August 1988) Abstract-The
kinetics of reaction between thiourea and [Ru”*(H-EDTA)(H20)] (1) were investigated in aqueous solution as a function of pH, temperature and metal ion concentration at 0.1 M ionic strength using the stopped flow technique. It was established that the monoanionic species of complex 1, [Ru”‘(EDTA)(H20)](2) is the major reacting species over the pH range 1.5-8.5. The substitution reaction follows first-order kinetics in [Ru(EDTA)(H,O)]as well as in thiourea concentrations over a wide concentration range of the two reactants. The rate constant (k,) was found to be (2.04f 0.17) x lo3 M- ’ s- I at 25°C and p = 0.1 M (KCl). From the activation parameters of the reaction it was concluded that the substitution reaction of [Ru(EDTA)H,O]follows an associative mechanism. The reactivities of different species of the thiourea complexes formed, have also been discussed.
The potentially hexadentate chelating agent EDTA forms complexes with many metal ions. The EDTA complexes of Ru”’ have been known for more than a decade. I-5 We have recently investigated the equilibrium and electrochemical and kinetic studies on the interaction of [Ru(H-EDTA)H,O] (1) with H202 and 02. 6,7 The potentiometric data on 1 shows the EDTA ligand to be pentadentate in aqueous solution at low pH values.6 The complex has a pK, of 7.86f0.02 at 30” (,u = 0.1 M KCl) corresponding to the dissociation of a proton from the coordinated H-EDTA moiety. Furthermore, the single titrable proton corresponding to the above pK, value is consistent with the presence of a water molecule bound to the metal ion. Thus, all the available experimental observations reveal that the dominant form of complex 1 contains pentadentate EDTA in aqueous solution with the sixth coordination site of the metal centre occupied by a water molecule (or by a hydroxide ion at higher pH values). In this paper, we report the kinetics of the reaction between thiourea and complex 1 at p = 0.1 M
KC1 and temperature (T) = 25°C in the pH range 1.5-8.5. The equilibrium constant and the stoichiometry of the complex formed, have also been determined by the mole ratio method.* The activation parameters for the reaction have been evaluated and discussed. The rate constants due to different reactive species were calculated. EXPERIMENTAL
*Author to whom correspondence should be addressed.
The complex’ K2[RuClS*H20] was used as the starting material for the synthesis” of the complex K[Ru(H-EDTA)H,O] (1). A. R. grade thiourea was used as such without any purification. The ionic strength of the reactants was maintained at 0.1 M using KCl. The temperature of the reactants was maintained at the desired value by a thermostat before mixing the reactants inside the reaction chamber of a stopped flow spectrophotometer HiTech coupled with an Apple data processor. The reaction was followed under pseudo-first-order conditions taking a large excess of thiourea by monitoring the peak at 468 nm which is the A,, of [Ru(EDTA)(thiourea)]complex formed.” The pseudo-first-order rate constants were calculated from plots of log (A, -A,) vs time.
463
M. M. TAQUI
464
RESULTS
KHAN
AND DISCUSSION
1. Stoichiometry of the reaction of thiourea with complex 1 The mole ratio method was used to determine the stoichiometry of the complex formed by the displacement of water by thiourea in complex 1. The absorbance at 468 (I,,, of [Ru(EDTA-H) (thiourea)]) was measured as a function of mole ratio of [Ru(H-EDTA)H,O] to [thiourea]. The plot of absorbance vs mole ration in Fig. 1 clearly indicates that a 1 : 1 complex is formed immediately after mixing of the reactants. The equilibrium constant for reaction (1) was determined from the ratio of intercept to slope of the plot of 1 mvs
and R. M. NAIK
Table 1. Thiourea dependence of the observed pseudofirst-order rate constant (kabs)for its reaction with [Ru(H= 5 x 1O-4 M, EDTA)H @I,, [Ru(H-EDTA)H,O], pH = 3.OkO.02, p = 0.1 M (KCl) and T = 25°C 10’ x [Thiourea] (M)
lo-‘xk,, (s- ‘)
0.6 1.1 2.1 4.0 5.0
1.21 2.58 5.43 7.30 9.70
lo-3xk, (M- ’ s- ‘) 2.11 2.34 2.01 1.82 1.94
k’;’ = (2.04kO.17) x lo3 M-’ s- ‘.
2. Kinetics of the substitution reaction
1 [Thiourea]
(Fig. 2). The value of equilibrium constant Kdetermined is 9.04 x 103.
1.21
I
The reaction between 1 and thiourea follows firstorder dependence in 1 as well as in thiourea concentration over a wide range of concentrations. The pseudo-first-order plots of log (A, -A,) vs time were linear for at least three half lives of the reaction. The results in Table 1 show that the reaction is firstorder in both the reactants over a wide range of concentration and PH. The values of pseudo-firstorder rate constants (kobs) obtained from the plot of log (A, -A,) vs time are plotted against thiourea concentration (Fig. 3). 3. Dependence of rate on pH
I
0
I
1
04
2.0 24 2.8 1.6 (CL/ [RdH-EDTA)H20])
0.8 1.2 Mole ratio
3.2
Fig. 1. Determination of stoichiometry by the mole ratio method. [Ru(H-EDTA)H,O] = 5 x lop4 M, p = 0.1 KCl, pH = 3.0 and T = 25°C.
The kinetic measurements over a wide pH range of 1.5-8.5 (Fig. 4) (Table 2) show the pH dependence of the reaction. The rate constant of the reaction was found to increase in the pH range 1.5 4.5, remain constant in the pH range 4.5-6.5 and decrease in the higher pH range (6.5-8.5) (Fig. 4).
I
2
L
1
6
[Thiourea]
1
,
a
10
,ti’
Fig. 2. Determination of the stability constant for IRu(H-EDTA)(thiourea)l at u = 0.1 M and T = 25°C. I_
.
[Thiourea]
x 102, M
Fig. 3. Thiourea dependence of observed pseudo-firstorder rate constant for the reaction of thiourea with [Ru(EDTA)H,O]. Reaction conditions as in Table 1.
465
Reaction of thiourea and [Ru”‘(H-EDTA)(H,O)]
The total concentration of Ru’u complex [Ru (EDTA)(H,O)], can be expressed by a sum of the concentration of the three species la, lb and lc. [Ru(H-EDTA)H,O],
= [Ru(H-EDTA)H,O] la
+[Ru(EDTA)H,O]0
2
L
6 - log[li’]
6
+[Ru(EDTA)OH]‘-.
10
Fig. 4. pH Dependence of the reaction of thiourea with [Ru(H-EDTA)H,O]. [Ru(H-EDTA)H,O], = 5 x 1O-4 M, mourea] = 5 x lo-* M, b = 0.1 M (KCl) and 7’ = 25°C.
Substituting the pK,, and pK+ values6 of [Ru(EDTA)H,O]and [Ru(EDTA)OH12-, eq. (3) can be finally expressed in the form of eq. (4) :
NW-EDTAW,Ol, = 1+ k [Ru(H-EDTA)H,O]
This interesting behaviour of the reaction over the entire pH range can be explained in terms of different reactivities of species 1 towards the nucleophile SC(NH&. Complex 1 can exist in three forms viz.
protonated [Ru(H-EDTA)H,O] la, neutral [Ru (EDTA)H,O]lb and hydroxo form [Ru (EDTA)OH12- lc. The species lb was found to be the most reactive species over the whole pH region. The rate constants due to these species can be calculated by an algebraic assignment of the specific rate constants outlined below. The rate of reaction (1) may be expressed by eq. (2). [Ru(H-EDTA)H,O]
+ [thiourea] G% k-1
[Ru(H-EDTA)(thiourea)]
+ H20.
Rate = kr [Ru(H-EDTA)H201T[thiourea].
(1) (2)
(3)
lc
lb
[H+l +
K, x K, [H+]2 )
where Ka, and Ka2 are expressed as follows,
K = [WEDTWWIW+l ” [Ru(H-EDTA)H,O] = 4.27 x lop3 M, K =
a2
(ref. 12)
[WEDWW-012-[H+l [Ru(EDTA)H,O] = 2.34 x lo-* M.
(ref. 12)
The pH dependence of the rate constant for reaction (1) can be analysed in terms of the following reaction pathways : [Ru(H-EDTA)H,O]
+ L& [Ru(H-EDTA)L] + H,O,
Table 2. pH Dependence of the rate constant (k,) for the [Ru(HEDTA)H20h = 5 x 10m4 M, [Thiourea] = 5 x lo-* M P= 0.1 M (KCl) and T = 25°C
[Ru(EDTA)H,O]-
[Ru(EDTA)L][Ru(EDTA)(OH)]‘-
lO-3x k, (M-’ s-‘)
1.50 2.02 2.50 3.00 4.02 4.70 5.30 6.02 6.52 7.02 7.30 7.60 8.02 8.62 9.30
0.55 0.94 1.66 1.95 2.30 2.65 2.64 2.70 2.65 2.30 1.70 1.10 0.76 0.35 0.30
(5)
+LA
reaction of thiourea with [Ru(H-EDTA)H,O],
PH
c4)
+ H20,
(6)
+ OH-,
(7)
+L& [Ru(EDTA)L]-
where L = thiourea. The rate constant (k,) given by eq. (2) can be expressed in terms of the specific rate constants k,, kb and k, of the three species in the form : [Ru(H-EDTA)H20h [Ru(H-EDTA)H,O]
) = k+k(&)
+kc
.
(8)
In order to simplify the evaluation of the rate parameters of eq. (8), one can consider the sections of the pH rate profile of Fig. 4. In the pH range 1.5-4.5, it is assumed that kb >>k,
466
M. M. TAQUI KHAN and R. M. NAIK
or k,. Thus, it is reasonable to neglect the contribution from path k, (eq. 8) in the above pH range. Similarly, in the pH range (4.5-8.5), the contributions from path k, (eq. 5) can be ignored. Applying the above approximations to eq. (S), the rate expression finally reduces to eqs (9) and (lo), respectively :
(atpH < 4.5)
(9)
k,(l+~) =k~+k~(~). (atpH >, 4.5)
(10)
The plot of the left hand side of eq. (9) vs 1,&l+], yields a straight line (Fig. 5) which gives the values of the rate constants (k, = 5 x 10’ M- ’ s- ‘) and (kb = 2.34 x lo3 M- * smi) from the intercept and slope of the above plot, respectively. Similarly, a plot of the left hand side of eq. (IO) vs fH+] gives a straight fine where the values of the constants (k, = 0 M- ’ s- ‘) and (kb = 2.6 x 10’ M- ’ s- ‘) were calculated from the intercept and slope of the plot, respectively (Fig. 6). The intercept of Fig. 6 is almost zero within experimental error. This dearly shows that the species [Ru(EDTA)(OH)J2- is the least reactive of the three species. The reactivities of different species of 1 over the entire pH range, thus follow the order [Ru(EDTA)~OH)12- < ~Ru(H-E~TA)H*O]
Pig. 6. Resolution of rate constant due to the reaction of [Ru(EDTA)H,OJ- and [Ru(EDTA)(OH)j2- with thiourea. Plot of ki
atp==O.l MIWIand
T-25%,
< ~Ru(EDTA)H~O~- . The substitution of the aqua group by thiourea, thus proceeds through the major path kb in the entire pH range studied. Similar rate vs pH profiles were observed in several other systems.i’$‘*
The reaction was studied over a wide temperature range. The activation energy E, was calculated from the plot of log (k,) vs l/T (Fig. 7 ; Table 3). The enthalpy of activation, AHt and entropy of activation, AIIsiwere also calculated. The values of the
3.8 3.7 36 g-3.5 k 3.4 3.3
Pig, 5. Resolution of rate constants due to the reaction of [Ru(H-EDTA)H*O] and [Ru(EDTA)H,O]- with thiourea. Plot of
3:l
3.2
3.3
3.4
3
(#x103
Fig. 7. Plot ‘of log (k,) vs l/T for the detemxination of activation parameters E,, A# and A&‘*. [Ru(HEDTA)H,O] = 5 x 10e4 M, vhiourea] = 5 x 10ee3M, p = 0.1 M KG, pH = 3.OkO.02.
Reaction of thiourea and [Ru”‘(H-EDTA)(H,O)] Table 3. Temperature dependence of the rate constant (k,) for the reaction of thiourea with [Ru(HM, [Ru(H-EDTA)H,O], = 5 x lO-4 EDTA)H,O], [Thiourea] = 5 x 10e3 M, pH = 3.OkO.02 and p = 0.1 M (KCl) lO-3x k, (M-Is-‘) 2.06 2.56 3.45 4.30 4.90
Temperature (K) 293 293 308 313 318
467
of complex 1 indicates that hydrogen bonding between the free carboxylate group, the coordinated water molecule and thiourea appears favourable. The highly negative value of A$ again supports steric crowding in the transition state and distortion in the EDTA ligand in complex 1.Other structurally and similar complexes’ 6*I 7 viz. [Fe(EDTA)H,O][Cr(EDTA)H,O]also follow an associative mechanism like complex 1 in their reaction with nucleophiles.
REFERENCES
parameters, E,, A@ and AS% were found to be 26.7 kJ mol- ‘, 24.4 kJ mol- ’ and -97.2 JK- ’ mall’, respectively. The high negative value of ASt favours an associative Si mechanism for the displacement of water by thiourea in complex 1. The values of enthalpies of activation for substitution reactions of Ru”‘-OH2 species with other ligands were found to be in the range 8&100 kJ mol- ‘. I3914These data clearly indicate a high energy barrier for cleavage of the M---OH2 bond and a dissociative nature of these reactions. In the displacement reaction of water by thiourea in complex 1, however, the enthalpy barrier is lowered to an appreciable extent possibly due to an associative mechanism. It is to be emphasized here that the associative mechanism is only indicative from the second-order mechanism which is always obtained by the Id replacement of water. The reactivity of complex 1 towards substitution reaction through an associative activation is possible because of the presence of free carboxylate group in complex 1 and availability of labile coordination sites on the complex. Recently, the single crystal X-ray analysis has confirmed that EDTA is five coordinate in complex 1 and in [Ru(EDTA-H)Cl,], EDTA is four coordinate. I5 This flexibility of coordinated EDTA in complex 1 thus allows the simultaneous coordination of thiourea and the H,O molecule in the transition state and supports a low energy associative pathway. The space filling model
1. M. M. Taqui Khan and G. Ramachandraiah, Inorg. Chem. 1982,21,2109. 2. M. M. Taqui Khan, A. Hussain, K. Venkatasubramanian, G. Ramachandraiah and V. Oommen, J. Mol. Catal. 1988,44, 117. M. Mikadia, H. Okuno and T. Ishimori, Nippon Kagaku Zasshi 1965,86, 589. N. A. Ezerskaya and T. P. Solovykh, Russ. J. Znorg. Chem. (Z&g. Transl.) 1966,11,991. J. Scherzer and L. B. Clapp, J. Znorg. Nucl. Chem. 1968,30, 1107. M. M. Taqui Khan, A. Hussain, G. Ramachandraiah and M. A. Moiz, Znorg. Chem. 1986, 25, 3023, and references therein. 7. M. M. Taqui Khan, H. C. Bajaj and M. R. H. Siddiqui, J. Mol. Catal. 1988,44, 279. 8. S. Brewer, A Text Book on Analytical Chemistry, pp. 289-294. Wiley (1980). 9. E. E. Mercer and R. R. Buckley, Znorg. Chem. 1965, 4, 1692. 10. A. A. Diamantis and J. V. Dubrawaski, Znorg. Chem. 1981,20, 1142. 11. Y. Yoshino, T. Vehiro and M. Saito, Bull. Chem. Sot. Japan 1979,52, 1060. 12. T. Matsubara and C. Creutz, Znorg. Chem. 1979,18, 1956, and references therein. 13. J. A. Broomhead and L. Kane Maguire, Znorg. Chem. 1971, 10, 85. 14. J. A. Broomhead, F. Basolo and R. G. Pearson, Znorg. Chem. 1964,3, 10. 15. M. M. Taqui Khan (unpublished work). 16. J. L. Hoard, M. Lind and J. V. Silverton, J. Am. Chem. Sot. 1961,83,2770. 17. Y. Sulfab, R. S. Taylor and A. G. Skyes, Znorg. Chem. 1976,15,2388.
463461,
1989 0
0277-5387/89 $3.00+.00 1989 Pergamon Press plc
A KINETIC STUDY OF THE REACTION BETWEEN THIOUREA AND ETHYLENEDIAMINETETRAACETATO RUTHENATE(III) COMPLEX BY THE STOPPED FLOW TECHNIQUE M. M. TAQUI
KHAN* and R. M. NAIK
Discipline of Coordination Chemistry and Homogeneous Catalysis, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India (Received 19 April 1988 ; accepted after revision 15 August 1988) Abstract-The
kinetics of reaction between thiourea and [Ru”*(H-EDTA)(H20)] (1) were investigated in aqueous solution as a function of pH, temperature and metal ion concentration at 0.1 M ionic strength using the stopped flow technique. It was established that the monoanionic species of complex 1, [Ru”‘(EDTA)(H20)](2) is the major reacting species over the pH range 1.5-8.5. The substitution reaction follows first-order kinetics in [Ru(EDTA)(H,O)]as well as in thiourea concentrations over a wide concentration range of the two reactants. The rate constant (k,) was found to be (2.04f 0.17) x lo3 M- ’ s- I at 25°C and p = 0.1 M (KCl). From the activation parameters of the reaction it was concluded that the substitution reaction of [Ru(EDTA)H,O]follows an associative mechanism. The reactivities of different species of the thiourea complexes formed, have also been discussed.
The potentially hexadentate chelating agent EDTA forms complexes with many metal ions. The EDTA complexes of Ru”’ have been known for more than a decade. I-5 We have recently investigated the equilibrium and electrochemical and kinetic studies on the interaction of [Ru(H-EDTA)H,O] (1) with H202 and 02. 6,7 The potentiometric data on 1 shows the EDTA ligand to be pentadentate in aqueous solution at low pH values.6 The complex has a pK, of 7.86f0.02 at 30” (,u = 0.1 M KCl) corresponding to the dissociation of a proton from the coordinated H-EDTA moiety. Furthermore, the single titrable proton corresponding to the above pK, value is consistent with the presence of a water molecule bound to the metal ion. Thus, all the available experimental observations reveal that the dominant form of complex 1 contains pentadentate EDTA in aqueous solution with the sixth coordination site of the metal centre occupied by a water molecule (or by a hydroxide ion at higher pH values). In this paper, we report the kinetics of the reaction between thiourea and complex 1 at p = 0.1 M
KC1 and temperature (T) = 25°C in the pH range 1.5-8.5. The equilibrium constant and the stoichiometry of the complex formed, have also been determined by the mole ratio method.* The activation parameters for the reaction have been evaluated and discussed. The rate constants due to different reactive species were calculated. EXPERIMENTAL
*Author to whom correspondence should be addressed.
The complex’ K2[RuClS*H20] was used as the starting material for the synthesis” of the complex K[Ru(H-EDTA)H,O] (1). A. R. grade thiourea was used as such without any purification. The ionic strength of the reactants was maintained at 0.1 M using KCl. The temperature of the reactants was maintained at the desired value by a thermostat before mixing the reactants inside the reaction chamber of a stopped flow spectrophotometer HiTech coupled with an Apple data processor. The reaction was followed under pseudo-first-order conditions taking a large excess of thiourea by monitoring the peak at 468 nm which is the A,, of [Ru(EDTA)(thiourea)]complex formed.” The pseudo-first-order rate constants were calculated from plots of log (A, -A,) vs time.
463
M. M. TAQUI
464
RESULTS
KHAN
AND DISCUSSION
1. Stoichiometry of the reaction of thiourea with complex 1 The mole ratio method was used to determine the stoichiometry of the complex formed by the displacement of water by thiourea in complex 1. The absorbance at 468 (I,,, of [Ru(EDTA-H) (thiourea)]) was measured as a function of mole ratio of [Ru(H-EDTA)H,O] to [thiourea]. The plot of absorbance vs mole ration in Fig. 1 clearly indicates that a 1 : 1 complex is formed immediately after mixing of the reactants. The equilibrium constant for reaction (1) was determined from the ratio of intercept to slope of the plot of 1 mvs
and R. M. NAIK
Table 1. Thiourea dependence of the observed pseudofirst-order rate constant (kabs)for its reaction with [Ru(H= 5 x 1O-4 M, EDTA)H @I,, [Ru(H-EDTA)H,O], pH = 3.OkO.02, p = 0.1 M (KCl) and T = 25°C 10’ x [Thiourea] (M)
lo-‘xk,, (s- ‘)
0.6 1.1 2.1 4.0 5.0
1.21 2.58 5.43 7.30 9.70
lo-3xk, (M- ’ s- ‘) 2.11 2.34 2.01 1.82 1.94
k’;’ = (2.04kO.17) x lo3 M-’ s- ‘.
2. Kinetics of the substitution reaction
1 [Thiourea]
(Fig. 2). The value of equilibrium constant Kdetermined is 9.04 x 103.
1.21
I
The reaction between 1 and thiourea follows firstorder dependence in 1 as well as in thiourea concentration over a wide range of concentrations. The pseudo-first-order plots of log (A, -A,) vs time were linear for at least three half lives of the reaction. The results in Table 1 show that the reaction is firstorder in both the reactants over a wide range of concentration and PH. The values of pseudo-firstorder rate constants (kobs) obtained from the plot of log (A, -A,) vs time are plotted against thiourea concentration (Fig. 3). 3. Dependence of rate on pH
I
0
I
1
04
2.0 24 2.8 1.6 (CL/ [RdH-EDTA)H20])
0.8 1.2 Mole ratio
3.2
Fig. 1. Determination of stoichiometry by the mole ratio method. [Ru(H-EDTA)H,O] = 5 x lop4 M, p = 0.1 KCl, pH = 3.0 and T = 25°C.
The kinetic measurements over a wide pH range of 1.5-8.5 (Fig. 4) (Table 2) show the pH dependence of the reaction. The rate constant of the reaction was found to increase in the pH range 1.5 4.5, remain constant in the pH range 4.5-6.5 and decrease in the higher pH range (6.5-8.5) (Fig. 4).
I
2
L
1
6
[Thiourea]
1
,
a
10
,ti’
Fig. 2. Determination of the stability constant for IRu(H-EDTA)(thiourea)l at u = 0.1 M and T = 25°C. I_
.
[Thiourea]
x 102, M
Fig. 3. Thiourea dependence of observed pseudo-firstorder rate constant for the reaction of thiourea with [Ru(EDTA)H,O]. Reaction conditions as in Table 1.
465
Reaction of thiourea and [Ru”‘(H-EDTA)(H,O)]
The total concentration of Ru’u complex [Ru (EDTA)(H,O)], can be expressed by a sum of the concentration of the three species la, lb and lc. [Ru(H-EDTA)H,O],
= [Ru(H-EDTA)H,O] la
+[Ru(EDTA)H,O]0
2
L
6 - log[li’]
6
+[Ru(EDTA)OH]‘-.
10
Fig. 4. pH Dependence of the reaction of thiourea with [Ru(H-EDTA)H,O]. [Ru(H-EDTA)H,O], = 5 x 1O-4 M, mourea] = 5 x lo-* M, b = 0.1 M (KCl) and 7’ = 25°C.
Substituting the pK,, and pK+ values6 of [Ru(EDTA)H,O]and [Ru(EDTA)OH12-, eq. (3) can be finally expressed in the form of eq. (4) :
NW-EDTAW,Ol, = 1+ k [Ru(H-EDTA)H,O]
This interesting behaviour of the reaction over the entire pH range can be explained in terms of different reactivities of species 1 towards the nucleophile SC(NH&. Complex 1 can exist in three forms viz.
protonated [Ru(H-EDTA)H,O] la, neutral [Ru (EDTA)H,O]lb and hydroxo form [Ru (EDTA)OH12- lc. The species lb was found to be the most reactive species over the whole pH region. The rate constants due to these species can be calculated by an algebraic assignment of the specific rate constants outlined below. The rate of reaction (1) may be expressed by eq. (2). [Ru(H-EDTA)H,O]
+ [thiourea] G% k-1
[Ru(H-EDTA)(thiourea)]
+ H20.
Rate = kr [Ru(H-EDTA)H201T[thiourea].
(1) (2)
(3)
lc
lb
[H+l +
K, x K, [H+]2 )
where Ka, and Ka2 are expressed as follows,
K = [WEDTWWIW+l ” [Ru(H-EDTA)H,O] = 4.27 x lop3 M, K =
a2
(ref. 12)
[WEDWW-012-[H+l [Ru(EDTA)H,O] = 2.34 x lo-* M.
(ref. 12)
The pH dependence of the rate constant for reaction (1) can be analysed in terms of the following reaction pathways : [Ru(H-EDTA)H,O]
+ L& [Ru(H-EDTA)L] + H,O,
Table 2. pH Dependence of the rate constant (k,) for the [Ru(HEDTA)H20h = 5 x 10m4 M, [Thiourea] = 5 x lo-* M P= 0.1 M (KCl) and T = 25°C
[Ru(EDTA)H,O]-
[Ru(EDTA)L][Ru(EDTA)(OH)]‘-
lO-3x k, (M-’ s-‘)
1.50 2.02 2.50 3.00 4.02 4.70 5.30 6.02 6.52 7.02 7.30 7.60 8.02 8.62 9.30
0.55 0.94 1.66 1.95 2.30 2.65 2.64 2.70 2.65 2.30 1.70 1.10 0.76 0.35 0.30
(5)
+LA
reaction of thiourea with [Ru(H-EDTA)H,O],
PH
c4)
+ H20,
(6)
+ OH-,
(7)
+L& [Ru(EDTA)L]-
where L = thiourea. The rate constant (k,) given by eq. (2) can be expressed in terms of the specific rate constants k,, kb and k, of the three species in the form : [Ru(H-EDTA)H20h [Ru(H-EDTA)H,O]
) = k+k(&)
+kc
.
(8)
In order to simplify the evaluation of the rate parameters of eq. (8), one can consider the sections of the pH rate profile of Fig. 4. In the pH range 1.5-4.5, it is assumed that kb >>k,
466
M. M. TAQUI KHAN and R. M. NAIK
or k,. Thus, it is reasonable to neglect the contribution from path k, (eq. 8) in the above pH range. Similarly, in the pH range (4.5-8.5), the contributions from path k, (eq. 5) can be ignored. Applying the above approximations to eq. (S), the rate expression finally reduces to eqs (9) and (lo), respectively :
(atpH < 4.5)
(9)
k,(l+~) =k~+k~(~). (atpH >, 4.5)
(10)
The plot of the left hand side of eq. (9) vs 1,&l+], yields a straight line (Fig. 5) which gives the values of the rate constants (k, = 5 x 10’ M- ’ s- ‘) and (kb = 2.34 x lo3 M- * smi) from the intercept and slope of the above plot, respectively. Similarly, a plot of the left hand side of eq. (IO) vs fH+] gives a straight fine where the values of the constants (k, = 0 M- ’ s- ‘) and (kb = 2.6 x 10’ M- ’ s- ‘) were calculated from the intercept and slope of the plot, respectively (Fig. 6). The intercept of Fig. 6 is almost zero within experimental error. This dearly shows that the species [Ru(EDTA)(OH)J2- is the least reactive of the three species. The reactivities of different species of 1 over the entire pH range, thus follow the order [Ru(EDTA)~OH)12- < ~Ru(H-E~TA)H*O]
Pig. 6. Resolution of rate constant due to the reaction of [Ru(EDTA)H,OJ- and [Ru(EDTA)(OH)j2- with thiourea. Plot of ki
atp==O.l MIWIand
T-25%,
< ~Ru(EDTA)H~O~- . The substitution of the aqua group by thiourea, thus proceeds through the major path kb in the entire pH range studied. Similar rate vs pH profiles were observed in several other systems.i’$‘*
The reaction was studied over a wide temperature range. The activation energy E, was calculated from the plot of log (k,) vs l/T (Fig. 7 ; Table 3). The enthalpy of activation, AHt and entropy of activation, AIIsiwere also calculated. The values of the
3.8 3.7 36 g-3.5 k 3.4 3.3
Pig, 5. Resolution of rate constants due to the reaction of [Ru(H-EDTA)H*O] and [Ru(EDTA)H,O]- with thiourea. Plot of
3:l
3.2
3.3
3.4
3
(#x103
Fig. 7. Plot ‘of log (k,) vs l/T for the detemxination of activation parameters E,, A# and A&‘*. [Ru(HEDTA)H,O] = 5 x 10e4 M, vhiourea] = 5 x 10ee3M, p = 0.1 M KG, pH = 3.OkO.02.
Reaction of thiourea and [Ru”‘(H-EDTA)(H,O)] Table 3. Temperature dependence of the rate constant (k,) for the reaction of thiourea with [Ru(HM, [Ru(H-EDTA)H,O], = 5 x lO-4 EDTA)H,O], [Thiourea] = 5 x 10e3 M, pH = 3.OkO.02 and p = 0.1 M (KCl) lO-3x k, (M-Is-‘) 2.06 2.56 3.45 4.30 4.90
Temperature (K) 293 293 308 313 318
467
of complex 1 indicates that hydrogen bonding between the free carboxylate group, the coordinated water molecule and thiourea appears favourable. The highly negative value of A$ again supports steric crowding in the transition state and distortion in the EDTA ligand in complex 1.Other structurally and similar complexes’ 6*I 7 viz. [Fe(EDTA)H,O][Cr(EDTA)H,O]also follow an associative mechanism like complex 1 in their reaction with nucleophiles.
REFERENCES
parameters, E,, A@ and AS% were found to be 26.7 kJ mol- ‘, 24.4 kJ mol- ’ and -97.2 JK- ’ mall’, respectively. The high negative value of ASt favours an associative Si mechanism for the displacement of water by thiourea in complex 1. The values of enthalpies of activation for substitution reactions of Ru”‘-OH2 species with other ligands were found to be in the range 8&100 kJ mol- ‘. I3914These data clearly indicate a high energy barrier for cleavage of the M---OH2 bond and a dissociative nature of these reactions. In the displacement reaction of water by thiourea in complex 1, however, the enthalpy barrier is lowered to an appreciable extent possibly due to an associative mechanism. It is to be emphasized here that the associative mechanism is only indicative from the second-order mechanism which is always obtained by the Id replacement of water. The reactivity of complex 1 towards substitution reaction through an associative activation is possible because of the presence of free carboxylate group in complex 1 and availability of labile coordination sites on the complex. Recently, the single crystal X-ray analysis has confirmed that EDTA is five coordinate in complex 1 and in [Ru(EDTA-H)Cl,], EDTA is four coordinate. I5 This flexibility of coordinated EDTA in complex 1 thus allows the simultaneous coordination of thiourea and the H,O molecule in the transition state and supports a low energy associative pathway. The space filling model
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