1867
J. Phys. Chem. 1980, 84, 1867-1868
for the description of positronium chemical reactions in weakly polar liquids developed in ref 5-7 allows us to explain the extreme temperature dependence of the positronium reaction rate constant and to understand the correlation between the temperature at which the reaction rate is a maxi.mum and the solvent polarity. One can believe that future improvements of the theory will bring about not only qualitative but also quantitative agreement with the expeiriment.
References and Notes V. I. Goldanskii et al. Dokl. Acad. Nauk SSSR, 203, 4, 381 (1971). A. P. Buchikhin, V. I. Goldanskii, V. P. Shantarovich, Dokl. Acad. Nauk SSSR, 212, 6, 1353 (1973). I. B. Kevdina, "Positronium Reactions in Liquids", Candidate Dissertation. MIDSCOW. 1975. E. Hall, W. J. Madia, and H. J. Ache, Radiochem. Radioanal. Lett., 23, 283 (1975).
(5) R. A. Marcus, d . Chem. Phys., 24, 966 (1956);26, 867 (1957). (6) A. A. Ovchinnikov, M. Ya. Ovchinnikova, Zh. Eksp. Teor. Fiz., 56, 1287 (1969). (7) R. R. Dogonadze and A. M. Kuznetsov, "Results of Science", Vol. 2, VINITI, Moscow, 1974. (8) V. M. Byakov et al., Chem. Phys., 24, 91 (1977). (9) V. M. Byakov, V. I. Goldanskii, V. and V. P. Shantarovich, €lektrokhimiya, 13, 804 (1977). (10) Yu. I. Harkats, Hektrokhimiya, 10, 612 (1974). (11) I. Si-Ylh Wang and C. Kittel, Phys. Rev. 57,713 (1973). (12)E. P. Prokopiev, Khim. Vys. Energ., 12, 172 (1978). (13) K. P. Areflev, S. A. Vorobev, and A. A. Tzoy, fiz.; Tekh. poluprovcdn., 10, 2086 (1976). (14) P. Colombino and B. Fiscella. Nuovo Cimento 5.3, 1 11971). (1 5) T. M. Usachyova, "Dielectric Radiospectroscopy of Liquid Alcanes", Dissertation, Moscow, 1977. (16) A. Vaisberger et al., "Organic Solvents", Invstr. Liter., Moscow, 1958. (17) M. Ya. Ovchimnnikova and A. A. Ovchinnikov. Opt. Spekfrosk.,28, 964 (1970). (18) V. M'Byakov, B. S. Klyachko, and A. A. Ovchinnikov, Preprints ITEP, No. 27, 1973;No. 4, 1975.
COMMllNICATIONS TO THE EDITOR Reaction of the Binuclear Triethylenetetraminehexaacetatonickel(I1)Complex with Cyanide ][on. A Reinvestlgation
1
I
-5.0-
Sir: In general, little attention has, been paid to the mechanism of substitution reactions of binuclear complexes of Ni2+. Some rate studies, however, have been reported on the substitution of Pt(I1) complexes.1-6 Of course, cyanide exchange on mono(aminocarboxylat0)nickel(I1) compllexes has been investigated in some detail and a four-step mechanism was p r ~ p o s e d ~and - ~ confirmed1"13 for this class of reactions. We had occasion to reinvestigate the reaction14 NiL;!-"
+ 4CN- * Ni(CN)42-+ L"-
-
Ni2L(CN)22-n (fast)
(2)
k
Ni2L(CN)22-"+ 6CN- & 2Ni(C!N)42-+ Lnkr
-4.0-
-rn -3.O--
(1)
where L is triethylenetetraminehexaacetic acid (TTHA). We found a mlechanism different from that proposed earlier by Stara and Kopanica15but in agreement with the earlier work of Margerum et al.,7-9Nigam et a1.,l0-l2and others.13 Later we also reinvestigated the cyanide exchange reaction of the l~inuclearcomplex Ni2TTHA4-under conditions previously used by Stara and Kopanica15for this same study (Le,, pH 11.0 f 0.2, p = 0.1 M (NaClOJ, and 25 "C). They hald found that the forward reaction was first order in Ni2(TTHA)and second order in cyanide. On this basis they postulated the following mechanism for this reaction: Ni2L4-n+ 2CN-
OI
n
(3)
The explanation for this proposed mechanism is unclear. We have extended the forward rate study to a much wider concentration range of CN- than chosen by these workers and found that the reaction becomes zero order in cyanide at concentration levels lower than 1.0 X M (Figure 1). (The actual rate constants published by Stara 0022-3654/80/2084-1867$01 .OO/O
-2.0 -1.0
-2.0 -3.0 log [ C d T
-4.0
Flgure 1. Cyanide dependence of the observed forward rate constants in the Ni,(lTHA)-CN- reaction system at [Ni,TTHA] = (1.5-7.5)X M, 25 f 0.1 O C , p = 0.1 mol dm-3, and pH 11.0 f 0.2.
lo-'
and Kopanica15 correspondingto second-order dependence in cyanide agree with those obtained in this study.) Our observation shows that NizL dissociates very slowly into NiL and Ni2+(aq),which react further with excess CNpresent in the system to form Ni(CN),2-. A study of the reverse reaction carried out by us12showed that, in contrast to the results of Stara and Kopanica,15 the reverse reaction is first order in Ni(CN)42-,first order in L"-, and inverse first order in CN-. This observation clearly brings the rate-determining step (eq 7) one step earlier than the final step of the reaction (eq 8). There is no evidence for the formation of NizL in the reverse reaction. The secondorder dependence of the forward reaction in CN- at higher concentration points to the formation of an intermediate NiL(CN) which reacts with CN- to give Ni(CN)42-. Other evidence that supports this assumption is discussed later. Based on these observations we propose the following mechanism for the title reaction: 0 1980 American
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1868
J. Phys. Chem. 1980, 84, 1868-1869
NlzL2-
+ c i
5-
$ N I L ( C N I + NI2+(aqi
NiL(CNI6- + CN-
& NiL(CN)2s-
NiL(CN)26-+ CNNiL(CN)?-
k
(5)
(fast)
NiL(CN)?-
References and Notes (fast)
(6)
(rds)
(7)
k-3
+ CN- 2Ni(CN)42-+ L6-
(fast) (8)
The important step in the scheme is the cyanide-assisted dissociation of Ni2L to give NiL(CNI5- and Ni2+(aq)both of which react with cyanide to form Ni(CN)42-. The last three steps are in line with the reactions of other mono(aminocarboxylato)nickel(II)complexes investigated earlierS7-l3The second-order dependence in cyanide at higher concentration is interpreted to mean that NiL(CN) reacts with two cyanides to produce the intermediate NiL(CN)3 in the rate-determining step rather than the formation of Ni2L(CN)2as assumed by Stara and Kopanica.15 The mechanism suggested above is supported by the following additional facts: (I) Addition of dimethylglyoxime to a solution of NizL in the presence of a low concentration of cyanide gives the characteristic precipitate of the Ni(dmg), chelate. No such change is observed in absence of CN- even on long standing. This shows the presence of Ni2+(aq)as a consequence of dissociation according to eq 5. (2) Addition of CN- to about a tenfold excess of NizL gives the same product as is obtained by similar addition of CN- to NiL. This product is NiL(CN) and its stoichiometry and stability constants were established by the mole ratio method.12 (3) A large absorbance change is observed after mixing the reactants (Le., Ni2L and cyanide) as a result of formation of Ni(CN)t- not from displacement of TTHA from Ni2TTHA but from cyanide-assisted dissociation of NizTTHA according to eq 5 and rapid formation of Ni(CN),2- from Ni2+(aq). Thereafter, the reaction follows steps 6-8. The observed absorbance jump equals that expected from Beer's law and stoichiometric conversion according to Ni2L4-n+ 5CN-
-
dissociation of Ni,TTHA to NiTTHA and Ni2+(aq)followed by subsequent steps. (7) Preliminary investigations on the reaction of Ni2DTPA17 (where DTPA = diethylenetriaminepentaacetic acid) with cyanide point to similar conclusions as arrived at for the Ni2TTHA reaction.
NiL(CN)5- + Ni(CN)42-
(9)
(4)The rather small activation energy (E, = 6.3 kcal mol-l, determined from a temperature dependence study) of the foward reaction where cyanide dependence is second order shows an associative mechanism to be operative in which bond breaking and bond making are taking place simultaneously. This value is comparable to that of other mono(aminocarboxylato)nickel(II) reactions with CN- investigated earlier.l*12 Compared to this, the activation energy for dissociation according to eq 4 is 14.82 kcal mol-l. ( 5 ) Kinetic investigations on the reactions of two bis complexes, viz., Ni(IDA)2and Ni(MIDA)2 (where IDA is iminodiacetic acid and MIDA is N-methyliminodiacetic acid), with cyanide8 showed that the bis complexes must first dissociate to give mixed complexes of the type NiL(CN) which react further with excess cyanide to produce Ni (CN)42-. (6) In their study of the substitution reaction (eq lo),
(1) Pearson, R. G.; Muir, M. M. J. Am. Chem. SOC.1968, 88, 2163. (2) Muir, M. M.; Cancio, E. M. Inorg. Chim. Acta 1970, 4, 565. (3) Muir, M. M.; Cancio, E. M. Inorg. Cbim. Acta 1970, 4, 568. (4) McMane, D. G.; Martin, Jr., D. S. Inorg. Cbem. 1966, 7, 1169. (5) Teggins, J. E.; Gano, D. R.; Tucker, M. A.; Martin, Jr., D. S. Inorg. Cbem. 1967, 6, 69. (6) Teggins, J. E.; Martin, Jr., D. S. Inorg. Cbem. 1967, 6 , 1003. (7) Margerum, D. W.; Bydaiek, T. J.; Bishop, J. J. J. Am. Cbem. SOC. 1961, 83, 1761. (8) Coombs, L. C.; Margerum, D. W. Inorg. Cbem. 1970, 9, 1711. (9) Coombs, L. C.; Margerum, D. W.; Nigam, P.C. Inorg. Cbem. 1970 9,2081. (10) Kumar, K.; Nigam, P. C.; Pandey, G. S. J. Phys. Chem. 1978, 82, 1955. (1 1) Kumar, K.;Nigam, P. C. J. Pbys. Cbem. 1979, 83, 2090, (12) Kumar, K.; Nigam, P. C. J. Pbys. Cbem. 1980, 84, 140. (13) Pagenkopf, G. K. J. Coord. Cbem. 1972, 2, 129. (14) Kumar, K.; Nlgam, P.C. J. Coord. Cbem. 1979, 9, 139. (15) Stara, V.; Kopanica, M. Collect. Czech. Cbem. Commun. 1972, 37, 2882. (16) Stara, V.; Kopanica, M. Collect. Czech. Cbem. Commun. 1972, 37, 80. (17) Kumar, K.; Bajaj, H. C.; Nigam, P. C. J. Phys. Chem. Submitted for publication. Department of Chemistry Indian Institute of Technology Kanpur-2080 16, U.P., India
Received August 15, 1979: Revlsed Manuscript Received Marcb 7, 1980
Volume Increase on Comlcellization of Fluorocarbon and Hydrocarbon Surfactants as Evidence for the Mutual Phobicity in Comicelles
Sir: Two or more surfactants spontaneously form mixed micelles above the critical micellization concentration (cmc). This phenomenon may be important in relation to the formation of biological membranes, which are composed mainly of lipids and proteins.' Micelles have different kinds of properties: some can be explained best in terms of aggregation whereas others can be explained best in terms of a micellar phase. A recent theory2 which deals with a micelle as a small system3 may be the most rigorous.1 Fluorocarbons are oleophobic as well as hydrophobic, as exhibited by properties such as vapor pressure, solubility, heat of mixing, and volume ~ h a n g e . This ~ ! ~ property of bulk fluorocarbons might be observed in small systems such as micelles of fluorocarbon surfactants; mixed micelles of fluorocarbon and hydrocarbon surfactants have a limited mutual solubility and higher cmc values than those expected from ideal m i ~ i n g . ~ ! ~ In this work, we report volume expansion on comicellization of fluorocarbon and hydrocarbon surfactants as direct evidence for mutual phobicity in micelles. The chemical structure of Neos Ftergent (NF) used is CCF3)2CF,
0022-3654/80/2084-1868$01 .OO/O
,CF3
(cF3 ) 2 c ~ =c c \ o ~ 3N~a o
NizTTHA + 2Cu2++ CuzTTHA + 2Ni2+ (10) Stara and KopanicalG have themselves postulated the
K. Kumar P. C. Nlgam"
The apparent molal volume 4 of a mixture of N F and 0 1980 American
Chemical Society
