Indian Journal of Chemistry

Vol. 43A, November 2004, pp. 2307-2314

Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of [FeL(OH)]2-n complexes with cyanide ions [Ln

- =trans-l ,2-diaminocyclohexanetetraacetic acid (CYDTA) and nitrilotriacetic acid (NT A)]

Radhey M Naik*, Desh D Chaturvedi, Neena Srivastava, Amit K Verma, Anjani K Tewari & Abhinav Agarwal

Department of Chemistry, Lucknow University, Lucknow 226 007, India

Email: naik_rm@rediffmail.com

Received 8 May 2003; revised 25 August 2004

The kinetics and mechanism of exchange of CYDTA4- in [FeCYDTA (OH)f and NTA3- in [FeNTA(OH)]l- with

cyanide ions (CYDTA 4- = trans-l,2-diaminocyclohexanetetraacetic acid and NTA = nitrilotriacetic acid) have been investigated spectrophotometrically at 395 nm (}"lI1a. of [Fe(CN)sOH]3-) under the conditions, temp. = 45 ± 0.1 °C, pH = Il±0.02 and J= 0.25 M(NaCI04) for CYDTA and temp. 25±0. loC, pH =9.0 ± 0.02 and 1=0.4 M (NaCI04) for NTA. Both the reactions exhibit three observable stages leading to the formation of [Fe(CNhOHf-, [Fe(CN)6t and [Fe(CN)6t respectively. The [FeCYDT(OH)f-CN- as well as [FeNTA(OH)t-CN- systems show variable order dependences in [CN-] in their first stage, ranging from one to two and zero to one at low and high cyanide concentrations. The second stage of reaction is common for both the systems and follow a first order dependence each in [Fe(CN)sOH3-] and [CNT The third stage of reaction follows a overall second order kinetics, first order each in [Fe(CN)sOH3-] and [Ln-] (Ln, = CYDT A 4- and NTA'·). The thermodynamically unfavourable reverse reaction of [Fe(CN)sOH]3- with [CYDTAt and [NTAt have also been studied under forcing conditions by taking large excess of [CYDTA]4- and [NTA]3-. These reactions exhibit first order dependence each in [Fe(CN)sOH3-] and [CYDTA 4-] or [NTA 3-] and an inverse first order dependence in [CN-] which makes it possible to identify the fourth step as rate determining one. The dependence of forward rate on ionic strength also confirms that the fourth step is rate determining in the porposed reaction mechanism for the first stage of reaction. A five step mechanistic scheme is proposed for both the systems in their first stage of reaction involving the presence of four cyanide ions around the central iron atom in the rate determining step. The activation parameters for both forward and backward reactions of the first stage of reaction are evaluated and they support the proposed mechanism.

In the last several years, Nigam et al. I have been interested in investigating the kinetics and mechanism of formation of tetracyanonickelate (II) from the reactions of arninocarboxylatonickelate (II) with cyanide ions, where a four-step mechanistic scheme has been established. The substitution reactions of [FeL(OH)]2-n with cyanide ions (L = arninocarboxy­lates viz., EDTA4-, HEEDTA3-, DTPAs- and TTHA6-) have also been studied by Nigam et al. 2

-4

. In case of Fe(III) systems, a five-step mechanistic scheme has been proposed for the formation of [Fe(CNhOH]3-. A feature different from the Ni(ll) system is the existence of two more stages in this reaction involving the formation of [Fe(CN)6]3- followed by its eventual reduction to [Fe(CN)6]4- by the ligand released in the first stage of the reaction.

The overall reactions for first stage in each case are represented by Eq- (1)

[FeL(OH)]2-n+5CN--7[Fe(CN)sOH]3-+Ln- _. -(1)

where L = CYDT A 4- or NT A3-

The overall aim of the present investigation is to enhance our knowledge on the exchange of a polydentate ligand from the coordination sphere of Fe(III) by cyanide ion, an un identate ligand. The ligand CYDT A contains a cyclic ring and provides extra stability making it least labile towards substitution ' in comparison to the corresponding complex containing a tridentate ligand NT A.

The experimental observations have revealed that the forward rates of above reactions are inversely proportional to the stability constants of [FeL(OH)fn

complexes_ Thus, the [FeCYDTA(OH)f having high stability constant due to a cyclic ring present in it makes it least labile_ The structures of these complexes play an important role for their reactivities with cyanide ions but we are unable to comment precisely in this regard due to lack of information on the structures of the present complexes. The

2308 INDIAN J CHEM. SEC A. NOVEMBER 2004

experimental observations on forward and reverse rates of the reaction (1) reveal that step four is rate determining in the proposed mechanistic scheme. The fifth cyanide finally adds to produce [Fe(CN)50H]3-in a fast step. The reverse rate studies further strengthen the proposed mechanism. Additional evidences are also gi ven in support for the proposed mechanism.

Materials and Methods Na2H2CYDT A (AR, BDH), NTA (Reanal,

Hungary), ferric nitrate (Thomas and Baker, UK), NaCI04 (E Merck, FRG) and potassium ferricyanide (GR, SDC), NaCN (May and Baker, England), were used in this study. Sodium aquapentacyanoferrate (II) was prepared from sodium nitroprusside (AR, BDH) by a literature method5 and was oxidized to aquapentacyanoferrate (m) by H20 2

6. Ferric

perchlorate was prepared by dissolution of a precipitate of Fe(OH)3 in a calculated amount of HCl04 and standardized complexometrically using sulphosalicylic acid as an indicator7

. Ferric perchlorate was used for preparation of [FeL(H20)]3-n complexes by taking 10% excess of ligands over stoichiometric requirements in order to ensure complete complexation. These complexes were converted to [FeL(OH)fn by adjusting their pH to any value higher than 9.0. NaCN was standardized argentometricall/. The pH of the reactants was adjusted by the addition of NaOH or HCI04. Buffers were not used in order to minimize complexity of the reaction. Doubly distilled water was used throughout the study.

A Shimadzu double beam spectrophotometer model UV -240 fitted with a circulatory arrangement of water for thermostating the cell compartment was used for monitoring the reaction as well as for repetitive spectral scans of the reaction mixture. The pH measurements were made on a Toshniwal digital pH meter model CL-46. The standard BDH buffers were used for standiudization of pH meter.

The rate of reaction between [FeL(OH)fn and CN- was followed spectrophotometrically for both the systems at 395 nm (Amax of [Fe(CN)50Ht, C = 1740 MI cm-I)9 at pH = 11.0 ± 0.02, temp. = 45±0.1°C and I = 0.5 M (NaCI04) under pseudo-first order conditions by taking large excess of cyanide. The pseudo first order rate constants (kobs) are obtained from the slopes of log CA versus time plots, which are linear for at least 70% of the reaction in all the cases_

A correction is applied for the absorbance of [FeL(OH)fn at 395 nm and it can be shown that

C =cBC~-AI A .. • (2)

cB-cA

where CAo and CA are concentrations of [FeL(OH)fn

at t = 0 and t = t respectively, B represents [Fe(CN)50H]3- and CA and CB the molar extinction coefficients of A and B in order.

The kinetics of reverse reaction of Eq. (1) i.e. between [Fe(CN)50H]3- and C - was followed spectro­photometrically by monitoring the decay of a peak at 395 nm under pseudo-first order conditions by taking large excess of [LnT The rate constants for this reaction were evaluated by a general expression (vide supra).

Results and Discussion Kinetics of forward reaction

Reaction (1) is thermodynamically favourabl e because there is a large difference in the stability constants of reactants [log KFeCYDTA(O H) = 27.48] 10 and [log KFeNTA(OH) = 25.8] 10 and product [Fe(CN)50Ht [log 135 = 43.9] II . The rate of reaction between [FeL(OH)]2-n and [CN-] are monitored under pseudo­first order conditions in presence of large excess of cyanide ions.

There is an appreciable change in absorbance immediately after following the mixing of reactants. This instantaneous change in absorbance is an indication for the formation of mixed ligand complexes during the course of reaction. Similar observations on [NiL]2-n - CN- reaction were made by Nigaml, Stara Kopanica l2 and Margerum et a1. 13

-16

The [FeCYDT A(OH)]2--CN- reaction system exhibits a variable order dependence in [CN-], changing from two to one as cyanide concentration was varied from low to high. On the other hand [FeNTA(OH)]I--CN- reaction has shown zero and first order dependence in [CN-] over a wide range of cyanide concentration. A rate expression consistent with variable order dependence in [CN-] is given by Eq. (3).

Rate = dldt[Fe(CN)sOH3-]

= kf[FeL(OH)(CN)/-n-xH CN-]4-X

= kobs[FeL(OH)(CN)x 2-n-x] . .. (3)

where kobs = kf [(CN-)4-X] and x is the number of cyanide attached to initial [FeL(OH)fn complex and can have values of 2 ,md 3 for [FeCYDTA(OH)f-

NAIK et at.: KINETICS AND MECHANISM OF PENTACYANOHYDROXOFERRATE(III) FORMATION 2309

CN' reaction system. The [FeNTA(OH)t-CN' reaction system is found to have the value of x equal to 4 and 3 only. The observed pseudo-first order rate constants (lOs kobs) are 1.30, 1.54, 2.46, 3.84, 5.25,

I 2 ' 5.76, 7.67, 10.55, 15.60 and 17.70 s' for 10 [CNh = 2.5, 3.0, 3.5 , 4.0, 6.0, 8.0, 10.0, 15.0, 20.0 and 25.0 mol dm·3 respectively at [FeCYDTA (OHl] = 5.0 x 10.4 mol dm' , pH = 11.0 ± 0.02, temp = 45°C and 1 = 0.25 mol dm') (NaCI04). The values are 2.23, 2.37, 2.34, 3.52, 4.31, 5.29 and 5.90 S·I when 103 [CNh =

, 3 3.0, 5.0, 7.0, 10.0, 12.5, 14.5 and 17.5 'mol dm-respectively at [FeNTA(OHrl] = 5 x 10.4 mol dm·3

,

pH = 9.0 ± 0.02, 1 = O.IM (NaCl04), and temp. = 25°C. A plot of log kobs versus log [CN'] is shown in Fig. 1. The variable orders in [CN'] are inferred from the slopes of the above plot. This indicates that monocyano, bicyano, tricyano and tetracyano complexes are formed during the course of above reactions.

Kinetics of reverse reaction

The species [Fe(CN)sOHt is highly stable in the pH range 8.0-11.5 and no noticeable dissociation is observed even after several hours. The reverse reaction of Eq. (1) i.e. the reaction of [Fe(CN)sOH]3' with C ' is not favoured thermodynamically and it can be forced to occur by adding a relatively large excess of [Ln,] compared to [Fe(CN)sOH3'], The reaction was monitored by disappearance of a peak at 395 nm (/."max of [Fe(CN)sOH] 3.) and the reaction followed a first

-3 .6 -2.7 -2 .5 - 2.3 · 2.1 - 1.9 -1 .7 '4.0

-3.8 '4.1

·4.0 '4.2

. -4.3

- 4. 4 '4.4

-4.6 . -4.5

-4.8 '4.6

·1 .6 -1 .4 -1.2 -1.0 -0.8 -0.6

log [CN' j,

Fig. J - The cyanide dependence of observed pseudo-first order rate constants for [FeL(OH).(OH)](3.n.x) -CN' (The conditions as

given in Tablel )

order dependence each in [Fe(CN)s(OHl] and [Ln.} and an inverse first order dependence in [CN'], The rate law consistent with the above observation can be given by Eq. 4.

- ~[Fe(CN)sOH3.]= dt

kr [Fe(CN)sOH3'][C' ]/[CN-]

But, [CN']=[CN'h and kr[Ln-]=klobS

... (4)

... (4a)

The [CN'h in Eq. (4a) can be expressed by Eq. (4b)

[CN'h == 5 {[Fe(CN)sOH3'] i - [Fe(CNhOH3.]} . . . (4b)

where [Fe(CNhOH3-]j and [Fe(CN)sOH3.] are the

concentrations of [Fe(CN)sOHt at t = 0 and t = t. Thus, from Eqs (4), (4a) and (4b), one can easily obtain Eq. (4c).

5{[Fe(CN).1 OH3' 1-[Fe(CN\ OH3. J}

d[Fe(CN)s OH3. ] .

--:::"O----------::i=_ == k d t [Fe(CN)s OH3

. ] obs ' .. .(4c)

Integration of Eq. (4c) on both sides between the limits t == 0 and t = t gives Eq. (4d).

[Fe (CN)s OH3' 1-[Fe(CN)s OH3'l +

[ Fe(CN) OH3- ] k' [ Fe(CN) OH3- ] in .5 1 =_ ob.l ·

t ... (4d) s i [Fe(CN)s OH3'1 5

The absorbances of [Fe(CN)sOH] 3· at initial and any time t can be expressed using Lambert-Beer's Law

Ai == C [Fe(CN)sOH3-]i . I

At == c [Fe(CNhOH3']t . I

... (4e)

where, I = path length == 10 mm = 1 cm; c = molar extinction coefficient of [Fe(CN)sOHt .

Equation (4d) can be transformed to Eq. (5) using Eq. (4e).

Ai - At + A j In {A/Ad = - {c/5} I k1 obs.t ... (5)

where k10bs = kr [Ln.], kr being the second order reverse

rate constant. The values of klobS for [FeL(OH)fn-CN- reaction system are obtained from slope of the plot of left hand side of Eq. (5) versus time. The evaluated rate constants are listed in Table 1.

The reaction between [Fe(CN)6]3- and L D·

We have studied independently the kinetics of reaction between [Fe(CN)6]J· and C' (Ln. = EDT A 4·

2310 INDIAN J CHEM, SEC A, NOVEMBER 2004

and HEDT A3-)_ It is observed that the rate of these

reactions follow first order ldnetics each in [Fe(CN)/J and [Ln

-] and a overall second order ldnetics. Similar studies are made by Nigam et al. 17 for the reaction between [Fe(CN)6]3-, and TTHA6

-. It is, therefore, inferred that CYDT A 4- and NT A3

- will also follow the same mechanism.

EfTect of pH on forward rate

The rate of forward reaction for [FeCYDT A (OH)f-[CN-] system has been studied as a function of pH in the range lOA to 11.2 and for [FeNT A(OH)(-CN- reaction in the pH range 8.0-11.0 at a fixed [cyanide ion] in the first order region by keeping all other variants constant. The rate is found to increase in pH range lOA to 11.0 and then levels off for [FeCYDTA(OH)f-CN- reaction. Similar behaviour is observed in case of [FeNTA(OH)t-CN- reaction where the rate increases from 8.5 to. 10.0 then levels off. These initial increases in the rate can be attributed to the conversion of reactant [FeCYDT A(H20)] 1- to [FeCYDTA(OH)f.

The increase in the rates of reaction in case of [FeCYDTA(OH)f-[CNr system is not due to the conversion of HCN to CN- because the cyanide exists as [CN-] only in the above pH region (pKHCN = 9.18)18. While on the other hand the intial increase in the rate of reaction for [Fe NTA (H20)]-CN- system can be

Table I - Kinetics of decomposition of [Fe(CN)sOHt by L'" [Fe(CN)sOH3

' ] = (1-5) x 10.4 M. pH = 11.0 ± 0.Q2,

(A) CYDTA4. : [Fe(CN)50H3-] = (1-5) x 10.4 M, pH = 11.0 ±

0.Q2, 1= 1.5 M (NaCI04) , Temp. = 45.0 ± 0.10 °C

102[CYDTAh. (M) 108 kl obs (Ms-I) 107 k./ (S-I)

8.0 1.55 1.93

10.0 1.80 1.80

12.0 1.93 1.61

15.0 2.50 1.67

k, (av) = (1.75 ± 0.12) x 10-7. S·I

(B) NTA3. : [Fe(CN)sOH3-1 = (2-5) x to-4 M, pH = 11.0 ± 0.02, 1= 0.3 M (NaCI04) , Temp. = 25.0 ± O.loC

102[NTAh. (M) 108 kobs (Ms- I

) 107 k,* (S-I )

1.0 3.42 3.42

1.1 3.58 3.25

1.2

1.3

4.22

4.72

3.52

3.37

attributed to the conversion of HCN to CN- and also of [FeNTA(H20)] to [FeNTA(OH)(. The rate constants cannot be resolved due to the complexity in the above reaction systems. The values of rate constants as a function of pH are given in Table 2.

Dependence of forward rate on ionic strength

The rate of forward reaction has been studied as a function of ionic strength in the first order region of [cyanide] in both the cases keeping all other variants fixed. It is observed that the forward rate obeyed Bronsted-Bjrerrum-Christiansen 19 relationship as shown in Eg. (6).

Log kf = log ko + 1.018 ZA ZB "11(1 + "I) ... (6)

The plots of log kf versus "1/(1 + "I) are linear and the values of the products ZA ZB on the reactants A and B obtained from the slopes of these plots are approximately 5.0 in case of [FeCYDTA(OH)f-CN­reaction and 4.0 in [FeNTA(OH)t-CN' reaction . The experimental values of ZA ZB so obtained are in good agreement with their theoretical expected values as evaluated from the rate determining step envisaged in the proposed mechanism. . These fairly good agreements between experimental and theoretical values of the product ZA ZB lends additional support that the step four is rate determining one in the proposed mechanism (vide supra). The kinetic data on the reverse reaction further confirm the above conclusion.

Table 2- Dependence of pH on reaction between [CNr with [FeL(OH)f"

(A) [FeCYDTA(OHi'1 == 5 x 10-4 M, [CN-] = 0.2 M. 1= 0.25 M (NaCl04) , Temp. = 45.0±0.1 uC

pH 105 kobs (Ms· l

) 104 kf *(S·I )

10.4 0.60 0.30 10.6 1.10 0.55 to.8 5.54 2.77 11.0 15.60 7.80 11.2 20.47 10.24 11.5 20.60 10.30

(B) [FeNTA(OHrJ = 5 x 1O-4M, [CN-] = 1.25 x 1O.2M, 1== 0.1 M (NaCI04) , Temp. = 25.0 ± 0.1 °C

pH kobs (Ms· l) kf* (S· I)

8.0 8.5

8.75 9.0

9.5 10.0

11.0

2.5 X 10-7

4.3 x 10-7

7.03 x 10-6

4.51 X lO-s

1.57 x 10-4

1.98 X to-4

5.58 x 10.4

2.0 X lO's

3.45 X to·s

5.62 x 10.4

3.61 X 10-3

1.26 X 10.2

1.59 X 10.2

4.47 X 10.2

NAIK et al. : KINETICS AND MECHANISM OF PENTACYANOHYDROXOFJ::RRATE(III) FORMATION 2311

Temperature dependence of forward and reverse reactions The activation parameters for the forward reaction

of [FeL(OH)f-[CN-] systems are evaluated at the condition of flrst order dependence in [CN-] in the temperature range 25-45°C. The activation parameters for reverse reaction .are also calculated in the same temperature range. The activation parameters are evaluated using Arrhenium and Eyring equations.

For [FeCYDTA(OH)f-[C~]system for forward reaction Ea = 31.25 kJ mor l, Ml = 28.64 kJ mor l

and ~st = 30.10 JKI mor l; and for [Fe NTA(OH)]1- -[CN-] system Ea = 35.57 kJ mor l, flHi- = 32.97 kJ mor l and ~Si- = 179.4 JKI mOrl. For [Fe(CN)50H]3-­[CN-] systeml9, Ea = 38.91 to J mOrl Wi- = 36.5 kJ mor l and ~Si- =168.01 JKI mOrl. For reverse reactions Ea = 44.86 kJ mor l Wi- = 42.29 kJ morl

and ~Si- = 367.72 JKI mOrl for CYDTA system and En = 47.11 kJ mOrl, Wi- = 44.86 kJ mor l, ~Si- = 218 .6 JKI mOrl for NTA system.

Our results on the forward and reverse reactions of stage one for [FeL(OHln] -[CN-) system suggest a mechanism given by Eqs (7) to (11). The order dependence in [CN} - shows that four cyanides are added in stepwise manner around the central iron ion to bring the 4th rate determining step. The flfth cyanide adds very rapidly and displaces the remaining glycinate segment of the ligand flnally producing [Fe(CN)50H] 3-.

A similar observation was made by Nigam et al. 2-4

and by Serratrice20 on other [FeL(OH)]2-n complexes for their reactions with cyanide ions. Thus the earlier proposed mechanism is reproduced below in conflrmation with the mechanism obeyed by the system at hand.

[FeL(OH)]2-n + CN­

[FeL(OH)(CN)]I-n

[FeL(OH)(CN)]1-n + CN­

[FeL(OH)(CN)2rn

[FeL(OH)(CNhrn + CN­

[FeL(OH)(CNbr(n+ 1)

[FeL(OH)(CNhr(n+ 1) + CN-

... (7)

... (8)

... (9)

[FeL(OH)(CN)4r(n+2) (r.d.s.)

[FeL(OH)(CN)4r(n+2) + CN-

... (10)

ks - --

(fast) ... (11)

where L = CYDTA4-, NTA3

- and n = charge on the ligand anion. A general expression can be derived which accounts for all the observed orders in [CN-l [Eq. 12].

kobs = K I K2K3K4[CN-]4-/{ 1+K1[CN-]+

... (12)

This expression can be reduced to fourth, third, second and first order dependences in [CN-] at appropriate cyanide concentration. In earlier systeml-2

the first, second and third order dependences in [CN-] . were observed. However, we failed to observe the third dependence in [CN-] for the [FeCYDTA(OH)]2-­CN- reaction system. We also failed to observe the expected fourth order dependence in [CN-] for the above reaction and also in case of any reaction of [FeL(OH)]2-n with cyanide ion investigated so far. Two reasons may be attributed for these behaviours: (i) The reaction is too slow to be observed experimentally in the low concentration range of cyanide where fourth order dependence is expected. The third order dependence in [CN-] could not be observed for reaction of [FeCYDTA(OH)f with cyanide ion in present case. This is due to the presence of cyc10hexane ring in CYDT A which provides additional stability in [FeCYDTA(OH)]2-complex on chelation. (ii) Stable 1: 1: 1 and 1: 1:2 complexes viz. [FeCYDTA(OH)(CN)]3- and [FeCYDT A(OH)(CN)2t are produced immediately following mixing. Therefore, only two cyanides may be added in a stepwise manner up to rate determining step (Eq. 10) giving rise to second and flrst order dependence in [CN-] for [FeCYDTA (OH)f-CN­system. The reaction between [FeNTA(OH)2f and cyanide ion exhibits a slightly different behaviour, where only zero and flrst order dependence in [CN-] are observed at low and high cyanide concentrations respectively. The zero order dependence in [CN-] in this case can be attributed due to cyanide independent dissociation of [FeNT A(OH)2]2- to [FeNT A(OH)t according to Eq. (13).

2312 INDIAN J CHEM, SEC A, NOVEMBER 2004

[FeNTA(OH)r +OH- ",(13)

The [FeNT A(OH)( complex so formed follows similar mechanistic pathway as suggested for [FeCYDTA(OH)]2'-CN' reaction system (Eq::; 7-11), Similar behaviour was observed in the case of [FeTTHA(OHh]5'-CN' reaction system21. This may be attributed to the relatively low stabilities of the mixed ligand intermediates formed during the course of these reactions. Steady state approximation can be used in deriving the general rate law for reverse reactions. Applying the steady state approximation to the intermediate [FeL(OH)(CN)3rcn+2) formed in the rate determining step (Eq. 9) in the proposed mechanism, one can get Eq. (14).

-d [FeL(OH)(CNk(n+2)]ldt =

K4 [FeL(OH)(CN)/n+I)] [CN']

-k.4 [FeL(OH) (CN)4'Cn,2)]-ks [FeL(OH) (CN4r(n,2)]

[CN'] + k,s[Fe(CN)sOHJ'][Ln,] = 0

or [FeL(OH)(CNk(n,2)] = [k4[FeL(OH)(CNh,(n, I)]

[CN'] + k,s [Fe(CN)sOH3'][L n']1

. . . (14)

In the presence of large excess of ligand, the first term in the numerator can be omitted. The experimental observation on the forward and reverse rates has already confirmed that the step four is rate determining one and step five is a fast one. Thus k4 and k-4 both are relatively small. Therefore, even if [CN'] is small, the k5[CN'] will be still quite larger than the term k,4' Hence, the term k,4 can also be omitted in the denominator of equation. Thus Eq. (14) now reduces to Eq. (15)

[FeL(OH)(CN)4,(n,2)] = (k,5 I ks).

{[Fe(CN)s OH3'][e']} I [CN'] ... (15)

The rate of reverse reaction is given by Eq. (16)

Rate = k.4 [FeL(OH)(CN)4,(n,2)] ... (16)

Now substituting the value of [FeL(OH) (CNk(n,2)] from Eq, (15) in Eq. (16) one gets Eq. (17).

Rate = (k'4 : k,5 I k5)' {[Fe(CN)50H3,]

[L n,]} I [CN'] --, (17)

" " iii -e o

O.soo

0.375

~ 0.250

0.12S

Scan No. I 2 3

Ti m!.: (mi n/h) 2 (,

IR 30 1.0 1.5 ~ ,O

2.' 3.5 16

0.000 .t-, ----.---:-----,-----~

200 JOO 400

Wavel ength (nm)

Fig. 2 - Repetitive scan of the reaction mixture during a typical kinetic run. [FeNTA(OHrJ = 5 x 10'4 M. [CN'] = 125 x 10.2 M.

pH = 9.5 ± 0.02, 1= 0.1 M (NaCI04) and Temp. = 35U C

The rate of decomposition of [Fe(CN)sOHf is given by Eq. (18)

Rate = -d I dt [Fe(CN)50H3'] . .. (18)

Now from Eqs (17) and (18) one can be get Eq. (19)

-d I dt [Fe(CN)50H 3,] = [kr [Fe(CNhOH3']

[Ln']/[CN' ] ... (19)

where kr = k,4 ks I k,5 = k,4K,'1

Equation (19) is in conformity with the observed rate law as given by Eq. (4). The observed forward rate in cyanide concentration is found to be 4-x where x = 2,3 in case of [FeCYDT A(OH)f -CN' system and x = 4,3 in case of [FeNT A(OH)( -CN' system. These observations on the forward rates In

combination with the experimental observation on reverse rate enabled us to propose a 1: 1 : 1 intermediate of the type [FeL(OH)(CN)] J'n formed immediately after mixing in both the cases at any cyanide concentration.

Thus, it follows that (a) the order two in [CN'] as observed in the case of [Fe CYDTA(OH)f-CN' system, the two cyanides are added simultaneously to the reaction intermediate [FeCYDTA(OH)(CNht (b) whereas only first order dependence is observed when one cyanide is added to the intermediate [FeCYDT A (OH)(CNh]5' and [FeNTA(OH)(CN)3t. Thus, these two systems also follow the general proposed

h . d ]. 2,420 mec amsm as propose ear ler . .

::-' .

NAIK et al.: KINETICS AND MECHANISM OF PENTACY ANOHYDROXOFERRATE(III) FORMATION 2313

The complete unwrapping of coordinated ligands to [FeL(OH)fn complexes by cyanide ions to produce [Fe(CN)sOH] 3- is relatively slow. The second, third and fourth cyanides add to initial reactant [FeL(OH)(CN)(n (Eq. 7) to form [FeL(OH)(CN)4r(n-2) in rate determining step (Eq. 10). This explains why a fourth order dependence in [CN-] is not observed in any case. Now as the concentration of cyanide increases, the initial reactant is converted to [FeL(OH)(CN)2r n and [FeL(OH)(CN)3r(n-J ) and accounts for the observed second and first order dependences in [CNT

The formation of similar intermediates ' were observed in the [NiL]2-n -CN -and [MnCYDT At­CN-, [Fe(IDA)(OH2)3t-S032- and [Fe(SALM)(OH2ht sot reaction systemsJ,22-24. The relatively low values of energy of activation and highly negative values of entropy of activation (.tS") for forward reaction in comparison to reverse reaction favour for an associative mechanism rather than dissociative one. This observation is consistent with the rate determining step given in Eq. 10 where bond formation is taking place in the forward reaction and the bond breaking in the reverse reaction. The effect of ionic strength provides additional support and pinpoints that step (10) in the mechanism is rate determining one.

A repetitive spectral scan for [FeNTA(OHn-CN­system is given in Fig. 2 as an illustrative example. The [FeCYDTA(OH)f-CN- reaction system also exhibits similar pattern. The continuous increase in absorbance at 395 nm in the beginning of reaction to a maximum value is attributed to the formation of [Fe(CN)sOHt as the first major product in the first stage of reaction. Now a gradual decrease in peak height at 395 nm and its eventual shift towards 410-420 nm in the later part of reaction is due to the formation of [Fe(CN)6]3- in the second stage of reaction. This observation is further supported by the appearance and continuous · increase of three new peaks in UV region at 320 nm, 300 nm and 257 nm respectively.

The peak heights at 300 nm and 320 nm finally disappear indicating reduction of [Fe(CN)6]3- to [Fe(CN)6]4- by the ligand released in the first stage of reaction. Lastly, the two isosbestic points which are observed at 345 nm and 313 nm during the course of scan indicate the coexistence of [FeL(OH)fn-[Fe(CN)sOHt and [Fe(CN)6]3--[Fe(CN)6]4- at a time.

-0 · 3

-0·5

-0'7

-0 ·9

-1 ·1

-1 · 3

-1·5 m

~1 ' 7 .o,z "" Of) -1 · 4 C7I ~ ~

-2 ·1

-2 ,3

-2 '5

-2·7

-2·9

-3 · '

-3 ' 5 38'0 3~ ' 0

10tJ~ 11 41 '0

36·0 37-0 3'a'0 39'0 40'0 41 ·0

log ~I II

Fig. 3 - Plot of log (rate constant) versus log (stability constants) of mixed intermediates: log k4 versus log ~ 113;

log k4k3 versus log ~ 113

This observation strongly supports that the original reacting complex is being replaced by one or more than one products in the reaction maintaining their constant ratio in their respective stages.

In an earlier communication, Nigam et al. 25 have established a linear free energy relationship (LFER) between the stepwise rate constants and the overall stability constants, f3 lmn of the intermediate reacting in that particular step for [FeL(OH)f"-CN- reaction where L = EDTA4-, HEDTA3- and DTPAs-. The mathematical expressions for these LFER' s can be easily obtained by equating forward and reverse rate for [FeL(OH)fn-CN" reactions for the rate determining step and the final equation is given by relation (20).

.. . (20)

where kr = k4- kS-1 = reverse rate constant and f35 and

f3FeLOH are the overall stability constants of [Fe(CN)sOHt and [FeL(OH)]2-n respectively.

The reaction of [FeCYDT A(OH)f with cyanide ion has exhibited only first and second orders in [CNT Thus, Eq. (20) can be transformed to Eqs (21 ) and (22) respectively.

2314 INDIAN J CHEM, SEC A, NOVEMBER 2004

k4 [kr~5] / ~FeLOHKIK2K3 = kr ~5 / ~113

k4K3 = [kr~5] / ~FeLOH KIK2 = kr~5 / ~112

... (21)

... (22)

where k4 and k4K3 are the first and second order rate constants.

The denominator ofEqs (21) and (22) are defined as:

~1l3 = ~Fe L OH KIK2K3

~112 = K2Kl~11O = K2Kl ~FeLOH

.. . (23)

. .. (24)

~112 and ~113 are overall stability constants of [FeLOH(CNhr- and [FeLOH (CNhr(n-l) (see Eqs 9 and 10).

Thus it can be shown that

log k4 = log k4~5 -log ~11 3 (First order) ... (25)

log k4k3 = log kr~5 - log ~112 (Second order) ." (26)

These relationships were tested earlier but their authenticity was little doubtful due to the availability of rate data on only three systems viz EDT A 4- ,

HEDTA3. and DTPA5

-. We have found that the present system [FeCYDTA(OH)f--CN- also fits on the log k4 versus ~11 3 and log k4k3 versus ~112 plots (Fig. 3). This lends additional support to the LFER proposed earlier25 . This confirms that [FeCYDTA(OH)f-CN- system also follows similar mechanistic scheme. However, in the case of [FeNTA(OH)t-CN- system only zero and first order dependences in [CN-] are observed. The values of klk2 and k3 cannot be evaluated kinetically. Therefore, it has not been possible for us to include this system in the proposed LFER. A linear relationship has been established between I:::..H" and 1:::..1' values for these systems which also points that all the systems viz [FeL(OH)fn (L = EDTA4

-,

HEDTA3-, DTPA5- and CYDTA4

-, NTA3-) follow similar mechanistic path way for their reactions with cyanide ion. There is a slight deviation from linearity in case of (FeNTA(OH)2f-CN- system. This is probably due to slightly different reaction conditions used in this case.

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