434
like the simple iodide in the 600-4000-cm-1 region, ion while the other known cases involve large monoexcept that the peaks were broad and the N H vibrations valent ions, except in solution studies. Also interesting unresolved, with "V broad and centered about 3140 is the observation that I? and BrS react no further with while 6xH was broad and centered about 1575 cm-I [O~(en)~]1,.2H*O than the reaction with the anion to (KC1 pellet). form trihalides. Chlorine first forms the trihalide, The stability of 13- is e ~ t a b l i s h e d ; ' ~IBr2- and IC12and, when concentrated chlorine is used or a chloriare relatively stable,22but C13- has been reported to be nated compound is exposed to the air, attack on the thermodynamcally unstablez3although it was p r e ~ a r e d ? ~ ligand and/or oxidation of the central metal ion beas early as 1923 and studied since.14,1is18z22It is gins. interesting to note that the present work provided a Acknowledgments. This work was supported by the trichloride, though not very stable, with a tripositive Robert A. Welch Foundation and the U. S. Atomic (22) J. H. Faull, Jr., J . Am. Chem. SOC.,56, 522 (1934). Energy Commission. The initial observations on the (23) R. L Scott, ibid., 75, 1550 (1953). nature of the reactions described in this paper were (24) F. D. ChattaLvay and G. Hoyle, J . Chem. Soc., 123, 654 (1923). recorded in this laboratory by Dr. E. R. Birnbaum. (25) G. C. Pimentel, J . Chem. Phj s., 19, 446 (1951).
The Reactions of Isothiocyanatobis ( ethylenediamine) cobalt ( 111) Complexes with Chromium (11) and the Linkage Isomerization of the Monothiocyanate Complex of Chromium (111) Albert Haim? and Norman Sutin
Contribution f r o m the Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973. Received September 22, 1965 Abstract: The rates of reaction of chromium(I1) with various cis- and trans-Co(en),(NCS)X"+ complexes (where X = H 2 0 , NH3, C1-, and SCN-) have been measured at 25.0" and ionic strength 1.0 M . The chromium(I1) attacks both the remote and the adjacent positions of the bound thiocyanate. Remote attack produces CrSCN2+ while adjacent attack yields CrNCS2+. The chromium(I1)-catalyzedisomerization, the spontaneous isomerization, and the aquation of CrSCN2+were also studied. The mechanisms of these reactions are discussed. t has long been believed that the transfer of an unsymwith a rate constant of 42 M-1 sec-1 in 1 M perchloric metrical bridging group in an inner-sphere, electronacid at 25.0'. I n the absence of chromium(II), CrSCN 2+ undergoes aquation and isomerization. We transfer reaction would, under certain circumstances, yield an unstable product. 3 , 4 Intermediates produced wish to report here that CrSCNz+is also produced in in this manner have recently been detected. The the reaction of chromium(l1) with cis- and trans-Coreactions of C O ( N H ~ ) ~ N O ~and ? + C O ( N H ~ ) ~ C N ~ (en)*0H2(NCS)*+ + and to describe some additional with CO(CN):~-produce the unstable species Co(CN);properties of CrSCNz+. O N O 3- and CO(CN)~NC 3-, respectively,j while the Experimental Section reaction of C O ( N H ~ ) ~ C with N ~ +chromium(I1) produces Materials. Iron(l1I) perchlorate was purified by recrystallizaCrNC2*.6 We have found that the sulfur-bonded tion from perchloric acid. A stock solution approximately 0.5 M chromium(II1) complex CrSCN*' is produced in the in iron(II1) and 1.5 M in perchloric acid was prepared from the reaction of chromium(I1) with FeNCS2+.' This comrecrystallized iron(II1) perchlorate, The concentration of the plex reacts with chromium(I1) to form the stable niiron(II1) i n the stock solution was determined by reduction of the trogen-bonded isomer CrNCS2+ iron(I1I) with a Jones reductor and titration of the iron(l1) produced with standard cerium(1V) sulfate using ferroin as indicator. The
I
CrSCN2+
k, + Cr2' + Cr2' + CrNCS2+
(1)
(1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) Visiting Chemist from the Chemistry Department, Pennsylvania State University. (3) R. L. Carlin and J. 0. Edwards, J . Ittorg. Nucl. Chem., 6, 217 ( 1958).
(4) (5) (6) (7)
D. L. Ball and E. L. Icing, J . Am. Chem. Soc., 80, 1091 (1958). J. Halpern and S . Nakamura, ibid., 87, 3002 (1965). J. H. Espenson and J. P. Birk, ibid., 87, 3280 (1965). A. Haim and N. Sutin, ibid., 87, 4210 (1965).
Journal of the American Chemical Society
perchloric acid concentration of the stock solution was determined by precipitating iron(II1) hydroxide with excess standard alkali and back-titrating with standard acid to a phenolphthalein end point. Stock solutions approximately 10-1 and 10-2 M in chromium(I1) and 1 M in perchloric acid were prepared by the reduction of chromium(II1) perchlorate solutions with amalgamated zinc. The chromium(I1) concentration was determined by titration with a standard iron(II1) solution containing a two- to threefold excess of thiocyanate. The disappearance of the red iron(II1) thiocyanate complexes was taken as the end point. The perchloric acid concentration of the chromium(I1) solution was determined by oxidiz-
1 88:3 1 February 5, 1966
435 action products were separated by ion exchange on a Dowex ing an aliquot with air and then passing the resulting chromium(II1) 50W-X8column, and the chromium content of the CrCIZ+ fraction solution through a Dowex 50W-X8 ion-exchange column in the was determined. In the second method, CrNCS*+ and CrSCN2+ hydrogen form. The acid concentration of the eluate was dewere converted into CrNCSHg4+ and Cr3+, respectively, by the termined by titration with standard base and the perchloric acid addition of Hg(C10& in 1 M HCIO,. Following this treatment, concentration of the original chromium(I1) solution obtained after hydrochloric acid was added to convert CrNCSHg4+ to CrNCS2+ correction for the zinc and chromium(I1) in the solution. The and HgCh, and the concentration of CrNCS2’ was measured speczinc was determined polarographically.* trophotometrically (e 2.90 X lo3 at 292 m p 9 . After the ratio of Sodium perchlorate solutions were prepared by neutralizing isomerization to aquation of CrSCN2+ had been established, the sodium carbonate with perchloric acid. mixtures were analyzed by measuring the amount of thioThe cobalt(II1) complexes were prepared from [ C ~ ( e n ) ~ C l ~ ] C l reaction ~ cyanate released in the aquation. using published procedures: tv~ns-[Co(en)~(NCS)~]ClO~, lo, cisKinetic Measurements. The oxidation-reduction reactions were [Co(en),(NCS)dCIO1,lo, trai~s-[Co(en)~(NCS)CI]Br, l2 cis-[Costudied by the use of the flow apparatus which has been described (en)2(NCS)CI]C1,131ru/is-[Co(en)2(NCS)0H]SCN,~~ tra/is-[Co(en)>previously. 2o The disappearance of the cobalt(II1) complexes 2 1 NH3(NCS)](SCN),,15 and ~~~-[CO(~~)~NH~(NCS)](SCN)*.~~ The was followed by observing the change in absorbance in the ultracomplexes were purified by recrystallization and by ion exchange on violet and at their absorption maxima in the visible (generally a Dowex 50W-X8 column in the hydrogen form. The complexes near 500 mp). The chromium(I1)-catalyzed isomerization of of charge +1 were eluted with 0.5 M ( t , n m ) or 1 M (cis) perchloric CrSCN2+ was followed by observing the change in absorbance in acid and those of charge +2 with 2 M (rraiis) or 3 M (cis) perchloric the range 260 to 350 mp. A large excess of chromium(I1) was acid. A solution containing an equilibrium mixturele of cispresent in all the above reactions. The aquation and the spon(82 %) and rruris-Co(en)yOHp(NCS)2 (18 %) was prepared by taneous isomerization of CrSCN2+were followed on a Cary Model adding alkali to a solution of cis-Co(en)r(NCS)Cl+ and rapidly re14 recording spectrophotometer by observing the decrease in abacidifying with perchloric acid. sorbance at 262 mp and at 630 mp, as well as by determining the Preparation of CrSCW+ Solutions. Two methods were used to amount of thiocyanate released. At the completion of the reaction prepare solutions containing CrSCN2+for the aquation and isomthe absorbance at 292 mp was measured in order to determine the erization studies. In the first method, a solution containing 1.0 X amount of isomerization which had taken place. In all cases the 10-2 M Cr2+ was gradually added to an equal volume of a wellreactions were either first order or pseudo first order. The rate stirred solution containing 1.1 X M Fe3+ and 9.0 X M constants were obtained from the slopes of plots of log ( D l - Dm) SCN-. The iron(I1) produced in this reaction was oxidized to vs. time, where D t and D , are the optical densities at time t and at iron(II1) with excess sodium peroxydisulfate. In the second method the end of the reaction, respectively. All the kinetic measurements a slightly less than stoichiometric amount of Cr2+ was gradually were made at 25.0” and an ionic strength of 1.0 M . added to a well-stirred solution 1 X M in tru/is-Co(en)iOHyThe amount of thiocyanate transferred from the cobalt to the (NCS)2+. The CrSCN2- and CrNCS2+were adsorbed on a Dowex chromium in the oxidation-reduction reactions was obtained in 50W-X8 column and eluted with 1 M perchloric acid. The ionthe following manner. After the reaction between the cobalt(II1) exchange procedure was carried out with chilled solutions. complex (usually in slight excess) and chromium(l1) had prmeeded The solutions obtained by elution with 1 M perchloric acid to completion, the reaction mixture was analyzed for free thio(which contained chromium(II1) complexes of charge + 2 ) were cyanate and for thiocyanate bound to chromium(II1). The ratio analyzed in the following manner. Total chromium was deof thiocyanate bound to chromium(II1) to the sum of the free and termined spectrophotometrically as Cr0,2- (e 4.83 X 103 at 372 bound thiocyanate was taken as the fraction of thiocyanate transmp) after oxidation with alkaline peroxide. l7 Free thiocyanate ferred. was determined spectrophotometrically as FeNCS2+ after the addition of excess iron(II1) ( E 3.60 X I O 3 at 460 mp when [Fe(III)] = 3.12 X IO-* M , [SCN-] N M , and [HCIO1] = 1 M ) . For the Results determination of total thiocyanate (free and bound) the solution The Oxidation-Reduction Reactions. The secondcontaining CrSCN*+ and CrNCS*- was made slightly alkaline prior to addition of the iron(II1) and perchloric acid solutions. order rate constants for the oxidation of chromium(I1) This treatment releases SCN- very rapidly from both CrNCS2+ by the Co(en)2(NCS)X”+ complexes are presented in and CrSCN 2 - . The [SCN-]/[Cr(III)] ratios of three different Table I. With the exception of cis- and trans-Co(en),preparations were 0.99, 1.02, and 1.03. (NCS)Cl+, quantitative transfer of thiocyanate from Analysis of Mixtures of CrSCN2+ and CrNCS2+. Two methods cobalt to chromium is observed, indicating that these were devised for determining the composition of solutions containing both CrNCS” and CrSCN2’. In the first method, chlorine reactions proceed via a thiocyanate-bridged transition was bubbled through the solution; this treatment converts CrNCS2+ state. The chloro complexes, on the other hand, react and CrSCN2+ into Cr3’ and CrCI2+, respectively.ls The re&
(8) J. J. Lingane, 117d.Eng. Chem., Anal. Ed., 18, 429 (1 946). (9) W. C. Fernelius, Ed., “Inorganic Syntheses,” Vol. 11, McGrawHill Book Co., Inc., New York, N. Y . , 1946, p. 222. (10) A. Werner, Anrt., 386, 198 (1912). (1 I ) These cobalt(II1) complexes were prepared by L. S. Hegedus. (12) A. Werner, A m . , 386, 133 (1912). (13) A. Werner, ibid., 386, 139 (1912). (14) A. Werner, ibid,, 386, 158 (1912). (15) A. Werner, ibid., 386, 206 (1912). (16) C. I lo?, [CrNCS2+]/[CrSCN2+] = 0. (b) The CrNCS2+is formed by the incorporation of "free" thiocyanate. This reaction appears to proceed very efficiently. For example, CrNCS*+ is quantitatively produced in the thiocyanate-catalyzed iron(II1)chromium(I1) reaction which does not involve FeNCS*+ as a reactant.' For example, on mixing a solution 2 X lo-? M i n NaSCN and 4.2 X lo-' M in chromium(l1) with an equal volume of a solution 5.2 X lo-' M in iron(III), the yield of CrNCS2+based on the thiocyanate was 22 %. The value predicted from the rate constants for the Fe3--Cr2& and Fe3+-Cr2+-SCN- reactions is 26 %. Air oxidation of chromium(I1) solutions containing thiocyanate also produces CrNCS?+ in good yield. Thus, bubbling 0, through a solution 1 X M in NaSCN and 2.1 X 10-1 M in chromium(l1) produces about 40 % of CrNCS2+,based on the amount of thiocyanate initially present. Similarly, when 1.2 X M C O ( N H ~ ) ~ C reacts I ~ + with 1 X M chromium(I1) in the presence of 1.0 X M NaSCN, the yield of CrNCS2+ is 8%. Although these effects are quite striking, they can account for only a small proportion of the CrNCS2+ formed in the trans-Co(en)20H2(NCS)2+-Cr(II) reaction. (c) The chromium(I1) attacks the nitrogen atom of the thiocyanate ligand. The steric restrictions of this type of attack are severe and Cr(H20)62+probably has to lose two of its coordinated water molecules in order to form a bond to the n i t r ~ g e n . ~Nevertheless, adjacent attack of the thiocyanate offers the most reasonable explanation of the CrNCS2+ produced. Since the ratio of [CrSCN2+]/ [CrNCS *+I is not significantly increased on going from 25.0 to 0 O, this interpretation requires that the entropies and heats of activation for adjacent and remote attack of thiocyanate not be very different. This implies that the unfavorable steric factor for the adjacent attack
Soc., 19,
Journal of the American Chemical Society
(37) R. T, M. Fraser, ibid., 2. 954(1963).
88:3 1 February 5, 1966
439 is largely compensated by the increase in entropy accompanying the loss of the two water molecules coordinated to the chromium(I1). Moreover, although adjacent attack yields stable CrNCS2+, this favorable enthalpy change appears to be largely cancelled by the energy required to remove the coordinated water molecules. In our previous communication,’ we suggested that the stable isomer of iron(II1) was nitrogen bonded. However, since both CrNCS2+ and CrSCN2+ were formed in the reaction between chromium(I1) and the iron(II1) thiocyanate complex, we mentioned the possibility of forming CrSCN2+ by adjacent attack on the sulfur-bonded isomer FeSCN2+. In view of the formation of both CrSCN2+ and CrNCS2+ in the reaction of Cr2+ with C O ( ~ ~ ) ~ O H ~ ( N CaS complex )~+, in which thiocyanate is definitely nitrogen bonded, we can now conclude that the stable monothiocyanate complex of iron(II1) is nitrogen bonded. Chromium(I1)-Catalyzed Isomerization. The chromium(I1)-catalyzed isomerization of CrSCN2+ (eq 1) proceeds by attack of chromium(I1) at the nitrogen atom. The position of attack of the chromium(I1) on CrNCS2+ in the Cr 2+-CrNCS2+ exchange reaction4 is less certain. As has been seen, chromium(I1) attack occurs at the nitrogen and sulfur atoms of CoNCSz+ and FeNCS2+ with roughly equal probability. It is therefore likely that the CrNCS 2+-Cr 2+ exchange also proceeds by both adjacent and remote attack. Under these conditions the rate constant for the exchange of labeled chromium between CrNCS2+ and Cr2+ is equal to the sum of the rate constants for adjacent and remote attack, i.e.
kex = ( k ,
+ 2ks)
(12)
where k , and k, are defined by the reactions CrNCS2+ CrNCS2+
+ Cr2+
.k kn
CrNCS2+
+ Cr2+
ks + Cr2+ e CrSCN2+ + Cr2+ k,
(13) (14)
The coefficient 2 in eq. 12 is necessary because thereactants and products of the remote attack, but not of the adjacent attack, are different.38 If it is assumed that the ratio k,/k, is the same for the CrNCS2+-Cr2+and FeNCS2+-CrZf systems then k,, = 4k,. Since k,, = 1.4 X M-’ sec-l at 25.0°,4 we estimate that and k, = 7.0 X 10-5 M-l sec-l at k, = 3.5 X 25.0’. For purposes of comparison we may correct the rate constants for any differences in the standard free energy changes of the reactions. This can be accomplished by means of the Marcus relation31~39 kiz
=
(kiikzzKiz)”’
tack of Cr2+ on CrSCN2+ to give CrNCS2+ and Cr2+ (or vice versa) with zero free energy change. We thus obtain kOex = k12K12-1/2= 4 X 10-2 M-1 sec-1. Comparison of koexwith k, shows that for zero free energy change, remote attack on thiocyanate is approximately l o 3 times more favorable than adjacent attack. It is noteworthy that koexis smaller than the rate constant for the CrN32+-Cr2+reaction (6.1 M-1 sec-l a t 25.0’).40 This suggests that the azide-bridged reaction is faster than the thiocyanate-bridged reaction even after allowance is made for the difference in the standard free energy changes of the reactions. In other words, this comparison suggests that, in the chromium(I1)-chromium(II1) system, azide is a better bridging ligand than a “symmetrized” thiocyanate group. The above argument also implies that remote attack of chromium(I1) on is about IO3 times more favorable than adjacent attack. An acid-dependent path similar to the one found in the CrSCNz+-Cr2+reaction Cr(SCN)OH+
+ Cr2++Cr2+ + Cr(NCS)OH+
(16)
has not been observed in other CrX2+-Cr2+ exchanges.4840 The rapid rate of reaction 16 (k > 2 X IO4 M-l sec-l if Kh’is greater than the hydrolysis constant of Cr(H20)63+19) indicates a large effect of hydroxide as a nonbridging ligand or, more likely, perhaps, a double-bridged (hydroxide and thiocyanate) transition state. A double-bridged transition state, with hydroxide as one of the bridging groups, has been proposed for the cis-Co(NH&OH(OAc)+-Cr(I1) reaction. Aquation and Isomerization of CrSCN2+. The rates of aquation of CrX2+complexes decrease in the order I- 2 2 > SCN- > Br- 4 2 > CI- 2 2 > NCS-.19 Azide and fluoride are not included in this series since there is some evidence that C T N ~and ~ + CrF2+dissociate into CrOH2+ and HN3, and CrOH2+ and HF, r e ~ p e c t i v e l y . ~ ~ ~ ~ ~ , ~ ~ As expected from Pearson’s theory44 the complexes formed between the hard acid chromium(II1) and the soft bases I- and SCN- are not very stable and dissociate relatively rapidly. The above order of the ligands is similar to the spectrochemical series I- > BrSCN- > C1- > NCS- and suggests that, in these systems at least, the same factors are important in determining the order of the ligands. The rate of isomerization of CrSCN2+is comparable to its rate of aquation. For the isomerization to occur it is necessary that the chromium-nitrogen bond be formed before the thiocyanate leaves the sphere of influence of the chromium(II1). Some mechanisms satisfying this requirement will next be considered. (a) Intramolecular Isomerization. In this case the isomerization proceeds via a seven-coordinated transition state N
(15)
where klz = k,, KIZ = k,/k, (or k, and k,/k,, respectively), and kll = k22 = ko,,, the rate constant for the hypothetical exchange reaction involving remote at(38) R. J. Campion, T. J. Conocchioli, and N. Sutin, J . Am. Chem. 4591 (1964). (39) The rates of a large number of outer-sphere electron-transfer reactions have been successfully correlated by means of this expression. See, for example, R. J. Campion, N. Purdie, and N. Sutin, Inorg. Chem., 3, 1091 (1964). The validity of this expression as applied to innersphere reactions has not yet been established. Soc., 86,
Haim , Sutin
/
(40) (41) (42) (43) (44)
R. Snellgrove and E. L. King, Inorg. Chem., 3 , 288 (1964). K. D. Kopple and R. R. Miller, Proc. Chem. Soc., 306 (1962). F. A. Guthrie and E. L. King, Inorg. Chem., 3, 916 (1964). D. Seewald and N. Sutin, ibid., 2, 643 (1963). R. G. Pearson, J. Am. Chem. Soc., 85, 3533 (1963).
Is0thiocy ana tobis( e thy lenediarnine)cobalt( I l l )
Complexes with Cr(11)
440
A similar transition state has been proposed for the isomerization of nitritopentaamminecobalt(III).45~46 This mechanism, although reasonable in the nitrito case, is not likely in the thiocyanate system because of the prohibitive amount of energy required to bend the linear thiocyanate group. On the other hand, a sevencoordinated transition state, in which an entering water molecule assists the removal of thiocyanate, cannot be ruled out for the aquation of CrSCN2+. A solventassisted dissociation has been proposed for the aquation of other complexes. 47 (b) Ion-Pair Mechanism. In this case a five-coordinated intermediate is formed. A reasonable mechanism of this type is RSCN2'
RNCSz+
ROH13+.SCN-
ROH23+
+ SCN-
where R stands for Cr(OH&. If the dissociation of the ion pair into R3+and SCN- is relatively slow compared to the internal return of the ion pair, then, in the absence of added thiocyanate, ki = kdk,/(k-d k, k,) and k , = kdkw/(k+ k , kw). The assumption that the dissociation of the ion pair is relatively slow is supported by our observation that when the aquation-isomerization is carried out in the presence of 0.8 M C1- the yield of Cr(OH2)5C12+is appreciably less than the amount calculated from data on the
+
+ +
+
(45) R. I
