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Kinetics of the reaction of acetonitrile on group VI metal carbonyls
.I. inorg, nucl, ('hem.. 1972, Vol. 34, pp. 3759-3764.
KINETICS OF THE ON GROUP
Pergamon Press
Printed in Great Britain
REACTION VI METAL
OF ACETONITRILE CARBONYLS
K H A L I E L M. A L - K A T H U M I and L. A. P. K A N E - M A G U I R E * Chemistry Department, University College, P.O. Box 78, Cardiff
(Received 15 February 1972) A b s t r a c t - A kinetic investigation of the reaction of acetonitrile on Group VI hexacarbonyls has shown the process to be stepwise, and that substitution by acetonitrile leads to increased replacement rates, A two-term rate law of the form Rate = k~[M(CO)~] + k2[M(CO)a][CH:~CN] is obeyed. Acetonitrile is seen to be as effective a nucleophile as triphenylphosphine towards Mo(CO)~;. INTRODUCTION
THE MIXED complexes [M(CO)3(CH3CN)3] ( M - Cr, Mo, W) have been prepared in high yields via the reaction of acetonitrile on the parent hexacarbonyls, using either reflux conditions [1] or u.v. irradiation [2]. In view of their extensive utility as precursors in the synthesis of mixed organometallic complexes of the type [ML3CO3] (L -- arene, olefin, etc.), we have carried out a kinetic investigation of the mechanism of their formation reaction (1) M(CO)6 + 3CH3CN ~ cis-[M(CO)3(CH3CN).~] + 3CO.
(l)
These studies were further stimulated by recent observations [3,4] that the substitution reactions of group VI hexacarbonyls with strong nucleophiles L (such as phosphites, phosphines, arsines) proceed according to a 2 term rate law, Rate = ka[M(CO)6] + k,2[M(CO)6][L].
(2)
it was suggested that the second term arose from a displacement S¥2 contribution, which is in marked contrast to the general behaviour of octahedral metal complexes, which almost invariably undergo only dissociative substitution processes [5]. Our results confirm that displacement substitution also occurs for M(CO)6 (M =-- Cr, Mo) with nitrogen donor nucleophiles such as acetonitrile. EXPERIMENTAL Materials. The metal hexacarbonyls were obtained from Strem Chemicals Inc., and used without further purification. The solvents acetonitrile and dichloroethane were distilled under nitrogen and stored over molecular sieve. The other nitrile nucleophiles were distilled immediately prior to use. The product complexes cis-[M(CO)3(CH3CN).~] were prepared by published methods [1 ]. 1. 2. 3. 4. 5.
D. P. Tare, W. R. Knipple and J. M. Augl, lnore. Chem. 1, 433 (1962). H. Werner, K. Deckelmann and U. Sch~nenberger, HelL'. Chim. Acta 53, 2022 (1970). R.J. Angelica and J. R. Graham, J, A m. chem. Soc. 88, 3658 (1966). J. R. Graham and R. J. Angelica, lnorg. Chem. 6, 2082 (1967). F. Basolo and R. G. Pearson, Mechanisms t!f Inorganic Reactions, 2nd Edn, Chap. 3. Wiley, New York (1967). 3,'59
3760
K. M. A L - K A T H U M I and L. A. P. K A N E - M A G U I R E
Kinetic studies. Acetonitrile (or freshly prepared solutions of acetonitrile in dichloroethane) was added to a weighed sample of the metal hexacarbonyl in a 10-ml Volumetric flask ([M] was generally 5 × 10-aM). The flask was shaken and sealed under nitrogen with a serum cap. All flasks were wrapped in alumina foil since the reactions are known to be light sensitive [6]. The flask was placed in a thermostatted bath (_+0.1°C) and allowed to equilibrate for at least 15 min. Samples were withdrawn by syringe at periodic intervals and their i.r. spectra were recorded over the region 21001750cm -1 on a Perkin-Elmer 257 spectrophotometer. Matched 0.5mm liquid cells with sodium chloride windows were employed. Reactions were studied by following the decrease of the strong metal hexacarbonyl band at 1990 cm -I. This was sufficiently separated from any of the product carbonyl bands as to avoid complications. During the reactions the only product carbonyl bands which appeared could be assigned to the species [M(CO)5(CHaCN)], cis-[M(CO)4(CHaCN)2] and cis-[M(CO)3(CHaCN)3] from their published spectra[6, 7]. Blank determinations using only dichloroethane as solvent established that no decomposition or loss by sublimation occurred for the metal hexacarbonyls at 70°C. Pseudo-first-order rate constants were obtained from the slopes of plots of logAt vs. time, where ,'It = absorbance at time t of the reaction solution at - 1990 cm -1. Attempts to obtain kinetic data in refluxing CH3CN (81 °C) gave inconsistent data. RESULTS
Linear first-order plots were generally obtained for at least two half-lives. Data were obtained for the reactions of Cr(CO)6 and Mo(CO), with CH3CN at different temperatures and nucleophile concentrations (Tables 1, and 2; Fig. 1). A single kinetic run was made for W(CO)6 in CHzCN at 75-2°C. All rate constants in pure acetonitrile as solvent are the average of duplicate determinations (reproducibility __+5%). The dependence of the reaction rate on the nature of the solvent and the nitrile nucleophile is summarised in Tables 2 and 3. The results indicate that for Cr and Mo a two-term rate law of the form Rate = k l [ M ( C O ) 6 ] + kz[M(CO)6][CH3CN] is obeyed in acetonitrile. The pseudo first-order kinetics arise from the large excess of nucleophile employed (~> 0.19 M). At the temperatures employed the k2 term predominated (e.g. > 97% of kob~ at 59.9°C for both Cr and Mo). The kz values in Tables 3 and 4 were calculated using the equation k2 = (kob~--kl)/[N]. Table 1. Rates of reaction of M(CO)6 with CH3CN (19-1 M) Complex
Temp. (°C)
kobs)< 106 sec -1
Cr(CO)6
64.5 68-5 75 '2 59.9 65.0 70-5 75-2
0.237 0.350 0.88 (0.42) * 20.5 35.9 70.6 0.605
Mo(CO)~
W(CO)6
*Value in brackets is for 7.64 M CH3CN in dichloroethane. 6. G. B. Dobson, M. F. Amr E1Sayed, I. W. Stolz and R. K. Sheline, lnorg. Chem. 1,526 (1962). 7. I.W. Stolz, G. R. Dobson and R. K. Sheline, lnorg. Chem. 2, 323 (1963).
Kinetics of the reaction of acetonitrile
3761
Table 2. Rates of reaction of Mo(CO)6 with C H 3 C N in CzH2C1 koh, × 106 s e c - ' 59"9°C * 68.5~+
[CH3CN] x M 0.191 0.764 1-15 1.53 1-91 3.82 19.1¢
0.44 1.21 1.56 2.42 1.93 4.20 20.5
2-42 3.34 4.21 6-23 7.14 13.3 52.5§
*k, = 0"32 × 10-6 sec ~. tk, = 1-00x 10-6sec -'. ~ C H 3 C N as solvent. §Extrapolated from data in Table 1.
12
×
©
3
0
o
L
~.o
I
1
z.o
3.o
[CH3CN ] ,
1
4o
M
Fig. 1. Plot of kobs vs. [CH3CN] for the reaction of Mo(CO)6 with C H 3 C N in dichloroe t h a n e at 68.5°C.
Table 3. Rates of reaction of Mo(CO)~ with various nitriles in C2H2CI2 (T = 64.5°C) Nucleophile
[M]
kob~ × 106 sec -~
k2 × 106M - ' sec '
CH:~CN C,~H,~CN CH2=CH--CN C6HsCN C6H4CH3CN
1 "53 1' 13 1"21 0-780 0.664
4.06* 4-37 3"22 2-79 2"80
2-29 3.42 2.21 2.87 3"39
*Extrapolated from data in Table 2.
3762
K.M.
AL-KATHUMI
and L. A. P. K A N E - M A G U I R E
AH2~ values (Table 4) were obtained from the slopes of Arrhenius plots, calculated by the least squares method. The errors quoted are the standard deviations derived from the least-squares analyses.
DISCUSSION
The production of the tris-substituted complexes [M(CO)3(CH3CN)3] from the metal hexacarbonyls would be expected to follow the step-wise process [M(CO)6]
q-
CH3CN k~.> [M(CO)5(CH3CN)] + CO
(4)
[M(CO)5(CHaCN)] + CH3CN
ks > [M(CO)4(CH3CN)z] + CO
(5)
[M(CO)4(CH3CN)2] + CH3CN
kc ) [M(CO)a(CH3CN)3] + CO.
(6)
The i.r. spectral changes during the reactions confirm this behaviour. At early reaction times the product bands indicate a mixture of only [M(CO)5(CH3CN)] ( - 2085 w, - 1940 m cm -1) and cis-[M(CO)4(CH3CN)2] (~ 1900 m, - 1880 w, 1840 w cm-1). These bands gradually disappear with the concurrent growth of strong bands at - 1915 and ~ 1790 cm -1 assigned to cis-[M(CO)3(CH3CN)3] from published spectra[6, 7]. From these spectral changes it is possible to make the qualitative observation that kB >>kA, while kc is at least as fast as kA. This result is unexpected since acetonitrile is a stronger o- donor and weaker 7r acceptor than CO. Therefore, increased substitution by acetonitrile would be anticipated to lead to decreased replacement rates [7]. Such a situation, in fact, eventually arises since substitution beyond [M(CO)3(CH3CN)3] has not been achieved. Our observations contrast with previous reports that for the corresponding reactions with phosphines [4], phosphites [4], and phenylisocyanide [8], kA is either similar or much greater than kB. However, similar labilisation of complexes [M(CO)r~L] towards further substitution have been observed for L = pyridine [9, 10], and halide ions [11]. The most reasonable explanation for the two-term rate law observed for the Table 4. Activation p a r a m e t e r s for reactions o f M(CO)6 with various nucleophiles Complex
Nucleophile
k2 × 106M -~ sec -1
AH2¢ k.cal.mole -~
AS2~ e.u.
Cr(CO)6
n-(C4Ha)3P CH3CN n-(C4H9)3P CHaCN
0.23* 0.02t 49.7* 27.0
25-5 28-0_+0.2 21-7 25.6 ___0.2
- 14"6 -7 -- 14"9 - 9
Mo(CO)6
*Calculated from data in Ref. [4]. t U s i n g k~ = 0-013 × 10-6 sec -~ (extrapolated from data in Ref. [4]). 8. 9. 10. 11.
H. W e r n e r and R. Prinz, C h e m . Ber. 99, 3582 (1966). J. R. G r a h a m and R. J. Angelici, J. A m . chem. Soc. 87, 5590 (1965). H. B e h r e n s and W. Klek, Z. anorg, allg. Chem. 292, 151 (1957). Ref. [5], p. 569.
Kinetics of the reaction of acetonitrile
3763
disappearance of Mo(CO)6 (Table 2) and Cr(CO)6(Table 1) in CH3CN, is the involvement of both an SN1 dissociative pathway (k0, and an SN2 displacement component (k2), i.e. M(CO)6
k~) M(CO)5+CO ~
fast
[M(CO).~(CH3CN)] • '
M(CO)6+ CH3CN ~ , [M(CO).~(CH3CN)] + CO. From Fig. 1, a k, value of 1-00 x 10-6 sec -~ is derived for the M o ( C O ) 6 reaction in dichloroethane at 68-5°C. This value agrees well with k~ values calculated for this temperature from published data in decalin (1" 15 × 10_6 s e c -1) [4], and a ndecane/cyclohexane mixture (1.68 × 10-rsec-a)[8]. This confirms the minimal effect of changing the solvent. In addition, an Arrhenius plot of the ks data in dichloroethane (albeit over only 9°C; Table 2) gives a 5,H15 of 29.6 k.cal.mole -1, which is in excellent agreement with reported AH1$ values in other solvents [4, 8], and the gas phase [ 12]. Even solvents of relatively high dielectric constant such as acetonitrile do not significantly effect the rates of substitution on Mo(CO)6. Thus the k2 value for M o ( C O ) 6 at 68.5°C in acetonitrile (2.70 × 10-SM -~ sec-'; Table 4) is very similar to k2 calculated in dichloroethane (3.16 × 10-SM -1 sec-'; Fig. 1, Table 2). The small role of the solvent is to be expected since both the reactant and nucleophile are uncharged. The k2 data in Table 4 show that acetonitrile is a somewhat weaker nucleophile than tri-n-butylphosphine towards M(CO)6(M----Cr, Mo). Its slower displacement rate is reflected in significantly higher activation enthalpies. The negative AS~: values obtained for CH3CN are consistent with an S~,2 process. Further comparison with Angelici's data[4] suggests that acetonitrile is similar to triphenylphosphine and triphenylarsine in its effectiveness as a nucleophile towards Mo(CO)6. As far as is known, this is the first time that acetonitrile has been placed in a nucleophilic reactivity order. The data in Table 3 show that kobs depends not only on the concentration but also on the nature of the nitrile employed. The somewhat higher k2 for C2HsCN compared to CH3CN may be considered to arise from its greater basicity (since an ethyl group has a higher inductive effect than a methyl group). The relative k2 values of CrH~CN and p-toluonitrile may be similarly explained in terms of inductive effects. Acrylonitrile is included for comparison since earlier studies [ 13] have shown that for the first two substitution steps (Eqns 4 and 5) this ligand behaves similarly to the other nitriles, i.e. coordinates to the metal via the N lone pair. Although no detailed study was made of the dependence of kobson [CH3CN] for W(CO)6, there is no doubt that a 2-pathway mechanism is also involved for this complex. The kob~(75"2°C) value obtained for W(CO)6 in acetonitrile is much larger than the k~ value calculated from other studies[4] (0-67x 10-~sec-l), requiring the existence of a dominant k2 term. In pure acetonitrile as solvent the 12. G. Cetini and O. Gambino, Atti. Acad. Sci. Torino, Classe Sci. Fis. Mat. Nat. 97, 1197 (1963); and references therein. 13. B. L. Ross, J. G. Grasselli, W. M. Ritchey and H. D. Kaesz, lnorz. Chem. 2, 1023 (1963).
3764
K . M . A L - K A T H U M I and L. A. P. K A N E - M A G U I R E
vast majority (85-97%) of/cobs for both Cr(CO)6 and Mo(CO)6 also involves the kz pathway (Table 2). The data in Table 1 therefore suggest the order Mo >> Cr W for the ease of replacement of CO by CH3CN. This agrees with the order previously observed for phosphines and phosphites [4].
KINETICS OF THE ON GROUP
Pergamon Press
Printed in Great Britain
REACTION VI METAL
OF ACETONITRILE CARBONYLS
K H A L I E L M. A L - K A T H U M I and L. A. P. K A N E - M A G U I R E * Chemistry Department, University College, P.O. Box 78, Cardiff
(Received 15 February 1972) A b s t r a c t - A kinetic investigation of the reaction of acetonitrile on Group VI hexacarbonyls has shown the process to be stepwise, and that substitution by acetonitrile leads to increased replacement rates, A two-term rate law of the form Rate = k~[M(CO)~] + k2[M(CO)a][CH:~CN] is obeyed. Acetonitrile is seen to be as effective a nucleophile as triphenylphosphine towards Mo(CO)~;. INTRODUCTION
THE MIXED complexes [M(CO)3(CH3CN)3] ( M - Cr, Mo, W) have been prepared in high yields via the reaction of acetonitrile on the parent hexacarbonyls, using either reflux conditions [1] or u.v. irradiation [2]. In view of their extensive utility as precursors in the synthesis of mixed organometallic complexes of the type [ML3CO3] (L -- arene, olefin, etc.), we have carried out a kinetic investigation of the mechanism of their formation reaction (1) M(CO)6 + 3CH3CN ~ cis-[M(CO)3(CH3CN).~] + 3CO.
(l)
These studies were further stimulated by recent observations [3,4] that the substitution reactions of group VI hexacarbonyls with strong nucleophiles L (such as phosphites, phosphines, arsines) proceed according to a 2 term rate law, Rate = ka[M(CO)6] + k,2[M(CO)6][L].
(2)
it was suggested that the second term arose from a displacement S¥2 contribution, which is in marked contrast to the general behaviour of octahedral metal complexes, which almost invariably undergo only dissociative substitution processes [5]. Our results confirm that displacement substitution also occurs for M(CO)6 (M =-- Cr, Mo) with nitrogen donor nucleophiles such as acetonitrile. EXPERIMENTAL Materials. The metal hexacarbonyls were obtained from Strem Chemicals Inc., and used without further purification. The solvents acetonitrile and dichloroethane were distilled under nitrogen and stored over molecular sieve. The other nitrile nucleophiles were distilled immediately prior to use. The product complexes cis-[M(CO)3(CH3CN).~] were prepared by published methods [1 ]. 1. 2. 3. 4. 5.
D. P. Tare, W. R. Knipple and J. M. Augl, lnore. Chem. 1, 433 (1962). H. Werner, K. Deckelmann and U. Sch~nenberger, HelL'. Chim. Acta 53, 2022 (1970). R.J. Angelica and J. R. Graham, J, A m. chem. Soc. 88, 3658 (1966). J. R. Graham and R. J. Angelica, lnorg. Chem. 6, 2082 (1967). F. Basolo and R. G. Pearson, Mechanisms t!f Inorganic Reactions, 2nd Edn, Chap. 3. Wiley, New York (1967). 3,'59
3760
K. M. A L - K A T H U M I and L. A. P. K A N E - M A G U I R E
Kinetic studies. Acetonitrile (or freshly prepared solutions of acetonitrile in dichloroethane) was added to a weighed sample of the metal hexacarbonyl in a 10-ml Volumetric flask ([M] was generally 5 × 10-aM). The flask was shaken and sealed under nitrogen with a serum cap. All flasks were wrapped in alumina foil since the reactions are known to be light sensitive [6]. The flask was placed in a thermostatted bath (_+0.1°C) and allowed to equilibrate for at least 15 min. Samples were withdrawn by syringe at periodic intervals and their i.r. spectra were recorded over the region 21001750cm -1 on a Perkin-Elmer 257 spectrophotometer. Matched 0.5mm liquid cells with sodium chloride windows were employed. Reactions were studied by following the decrease of the strong metal hexacarbonyl band at 1990 cm -I. This was sufficiently separated from any of the product carbonyl bands as to avoid complications. During the reactions the only product carbonyl bands which appeared could be assigned to the species [M(CO)5(CHaCN)], cis-[M(CO)4(CHaCN)2] and cis-[M(CO)3(CHaCN)3] from their published spectra[6, 7]. Blank determinations using only dichloroethane as solvent established that no decomposition or loss by sublimation occurred for the metal hexacarbonyls at 70°C. Pseudo-first-order rate constants were obtained from the slopes of plots of logAt vs. time, where ,'It = absorbance at time t of the reaction solution at - 1990 cm -1. Attempts to obtain kinetic data in refluxing CH3CN (81 °C) gave inconsistent data. RESULTS
Linear first-order plots were generally obtained for at least two half-lives. Data were obtained for the reactions of Cr(CO)6 and Mo(CO), with CH3CN at different temperatures and nucleophile concentrations (Tables 1, and 2; Fig. 1). A single kinetic run was made for W(CO)6 in CHzCN at 75-2°C. All rate constants in pure acetonitrile as solvent are the average of duplicate determinations (reproducibility __+5%). The dependence of the reaction rate on the nature of the solvent and the nitrile nucleophile is summarised in Tables 2 and 3. The results indicate that for Cr and Mo a two-term rate law of the form Rate = k l [ M ( C O ) 6 ] + kz[M(CO)6][CH3CN] is obeyed in acetonitrile. The pseudo first-order kinetics arise from the large excess of nucleophile employed (~> 0.19 M). At the temperatures employed the k2 term predominated (e.g. > 97% of kob~ at 59.9°C for both Cr and Mo). The kz values in Tables 3 and 4 were calculated using the equation k2 = (kob~--kl)/[N]. Table 1. Rates of reaction of M(CO)6 with CH3CN (19-1 M) Complex
Temp. (°C)
kobs)< 106 sec -1
Cr(CO)6
64.5 68-5 75 '2 59.9 65.0 70-5 75-2
0.237 0.350 0.88 (0.42) * 20.5 35.9 70.6 0.605
Mo(CO)~
W(CO)6
*Value in brackets is for 7.64 M CH3CN in dichloroethane. 6. G. B. Dobson, M. F. Amr E1Sayed, I. W. Stolz and R. K. Sheline, lnorg. Chem. 1,526 (1962). 7. I.W. Stolz, G. R. Dobson and R. K. Sheline, lnorg. Chem. 2, 323 (1963).
Kinetics of the reaction of acetonitrile
3761
Table 2. Rates of reaction of Mo(CO)6 with C H 3 C N in CzH2C1 koh, × 106 s e c - ' 59"9°C * 68.5~+
[CH3CN] x M 0.191 0.764 1-15 1.53 1-91 3.82 19.1¢
0.44 1.21 1.56 2.42 1.93 4.20 20.5
2-42 3.34 4.21 6-23 7.14 13.3 52.5§
*k, = 0"32 × 10-6 sec ~. tk, = 1-00x 10-6sec -'. ~ C H 3 C N as solvent. §Extrapolated from data in Table 1.
12
×
©
3
0
o
L
~.o
I
1
z.o
3.o
[CH3CN ] ,
1
4o
M
Fig. 1. Plot of kobs vs. [CH3CN] for the reaction of Mo(CO)6 with C H 3 C N in dichloroe t h a n e at 68.5°C.
Table 3. Rates of reaction of Mo(CO)~ with various nitriles in C2H2CI2 (T = 64.5°C) Nucleophile
[M]
kob~ × 106 sec -~
k2 × 106M - ' sec '
CH:~CN C,~H,~CN CH2=CH--CN C6HsCN C6H4CH3CN
1 "53 1' 13 1"21 0-780 0.664
4.06* 4-37 3"22 2-79 2"80
2-29 3.42 2.21 2.87 3"39
*Extrapolated from data in Table 2.
3762
K.M.
AL-KATHUMI
and L. A. P. K A N E - M A G U I R E
AH2~ values (Table 4) were obtained from the slopes of Arrhenius plots, calculated by the least squares method. The errors quoted are the standard deviations derived from the least-squares analyses.
DISCUSSION
The production of the tris-substituted complexes [M(CO)3(CH3CN)3] from the metal hexacarbonyls would be expected to follow the step-wise process [M(CO)6]
q-
CH3CN k~.> [M(CO)5(CH3CN)] + CO
(4)
[M(CO)5(CHaCN)] + CH3CN
ks > [M(CO)4(CH3CN)z] + CO
(5)
[M(CO)4(CH3CN)2] + CH3CN
kc ) [M(CO)a(CH3CN)3] + CO.
(6)
The i.r. spectral changes during the reactions confirm this behaviour. At early reaction times the product bands indicate a mixture of only [M(CO)5(CH3CN)] ( - 2085 w, - 1940 m cm -1) and cis-[M(CO)4(CH3CN)2] (~ 1900 m, - 1880 w, 1840 w cm-1). These bands gradually disappear with the concurrent growth of strong bands at - 1915 and ~ 1790 cm -1 assigned to cis-[M(CO)3(CH3CN)3] from published spectra[6, 7]. From these spectral changes it is possible to make the qualitative observation that kB >>kA, while kc is at least as fast as kA. This result is unexpected since acetonitrile is a stronger o- donor and weaker 7r acceptor than CO. Therefore, increased substitution by acetonitrile would be anticipated to lead to decreased replacement rates [7]. Such a situation, in fact, eventually arises since substitution beyond [M(CO)3(CH3CN)3] has not been achieved. Our observations contrast with previous reports that for the corresponding reactions with phosphines [4], phosphites [4], and phenylisocyanide [8], kA is either similar or much greater than kB. However, similar labilisation of complexes [M(CO)r~L] towards further substitution have been observed for L = pyridine [9, 10], and halide ions [11]. The most reasonable explanation for the two-term rate law observed for the Table 4. Activation p a r a m e t e r s for reactions o f M(CO)6 with various nucleophiles Complex
Nucleophile
k2 × 106M -~ sec -1
AH2¢ k.cal.mole -~
AS2~ e.u.
Cr(CO)6
n-(C4Ha)3P CH3CN n-(C4H9)3P CHaCN
0.23* 0.02t 49.7* 27.0
25-5 28-0_+0.2 21-7 25.6 ___0.2
- 14"6 -7 -- 14"9 - 9
Mo(CO)6
*Calculated from data in Ref. [4]. t U s i n g k~ = 0-013 × 10-6 sec -~ (extrapolated from data in Ref. [4]). 8. 9. 10. 11.
H. W e r n e r and R. Prinz, C h e m . Ber. 99, 3582 (1966). J. R. G r a h a m and R. J. Angelici, J. A m . chem. Soc. 87, 5590 (1965). H. B e h r e n s and W. Klek, Z. anorg, allg. Chem. 292, 151 (1957). Ref. [5], p. 569.
Kinetics of the reaction of acetonitrile
3763
disappearance of Mo(CO)6 (Table 2) and Cr(CO)6(Table 1) in CH3CN, is the involvement of both an SN1 dissociative pathway (k0, and an SN2 displacement component (k2), i.e. M(CO)6
k~) M(CO)5+CO ~
fast
[M(CO).~(CH3CN)] • '
M(CO)6+ CH3CN ~ , [M(CO).~(CH3CN)] + CO. From Fig. 1, a k, value of 1-00 x 10-6 sec -~ is derived for the M o ( C O ) 6 reaction in dichloroethane at 68-5°C. This value agrees well with k~ values calculated for this temperature from published data in decalin (1" 15 × 10_6 s e c -1) [4], and a ndecane/cyclohexane mixture (1.68 × 10-rsec-a)[8]. This confirms the minimal effect of changing the solvent. In addition, an Arrhenius plot of the ks data in dichloroethane (albeit over only 9°C; Table 2) gives a 5,H15 of 29.6 k.cal.mole -1, which is in excellent agreement with reported AH1$ values in other solvents [4, 8], and the gas phase [ 12]. Even solvents of relatively high dielectric constant such as acetonitrile do not significantly effect the rates of substitution on Mo(CO)6. Thus the k2 value for M o ( C O ) 6 at 68.5°C in acetonitrile (2.70 × 10-SM -~ sec-'; Table 4) is very similar to k2 calculated in dichloroethane (3.16 × 10-SM -1 sec-'; Fig. 1, Table 2). The small role of the solvent is to be expected since both the reactant and nucleophile are uncharged. The k2 data in Table 4 show that acetonitrile is a somewhat weaker nucleophile than tri-n-butylphosphine towards M(CO)6(M----Cr, Mo). Its slower displacement rate is reflected in significantly higher activation enthalpies. The negative AS~: values obtained for CH3CN are consistent with an S~,2 process. Further comparison with Angelici's data[4] suggests that acetonitrile is similar to triphenylphosphine and triphenylarsine in its effectiveness as a nucleophile towards Mo(CO)6. As far as is known, this is the first time that acetonitrile has been placed in a nucleophilic reactivity order. The data in Table 3 show that kobs depends not only on the concentration but also on the nature of the nitrile employed. The somewhat higher k2 for C2HsCN compared to CH3CN may be considered to arise from its greater basicity (since an ethyl group has a higher inductive effect than a methyl group). The relative k2 values of CrH~CN and p-toluonitrile may be similarly explained in terms of inductive effects. Acrylonitrile is included for comparison since earlier studies [ 13] have shown that for the first two substitution steps (Eqns 4 and 5) this ligand behaves similarly to the other nitriles, i.e. coordinates to the metal via the N lone pair. Although no detailed study was made of the dependence of kobson [CH3CN] for W(CO)6, there is no doubt that a 2-pathway mechanism is also involved for this complex. The kob~(75"2°C) value obtained for W(CO)6 in acetonitrile is much larger than the k~ value calculated from other studies[4] (0-67x 10-~sec-l), requiring the existence of a dominant k2 term. In pure acetonitrile as solvent the 12. G. Cetini and O. Gambino, Atti. Acad. Sci. Torino, Classe Sci. Fis. Mat. Nat. 97, 1197 (1963); and references therein. 13. B. L. Ross, J. G. Grasselli, W. M. Ritchey and H. D. Kaesz, lnorz. Chem. 2, 1023 (1963).
3764
K . M . A L - K A T H U M I and L. A. P. K A N E - M A G U I R E
vast majority (85-97%) of/cobs for both Cr(CO)6 and Mo(CO)6 also involves the kz pathway (Table 2). The data in Table 1 therefore suggest the order Mo >> Cr W for the ease of replacement of CO by CH3CN. This agrees with the order previously observed for phosphines and phosphites [4].