Inorganic Chemistry, Vol. 15, No. 11, 1976 2643
Aquo Exchange C0(en)z(OH2)2~+
Contribution from the Departments of Chemistry and Physics, The University of Calgary, Calgary, Alberta, Canada T2N 1N4
Kinetics of Aquo Exchange of Diaquobis(ethylenediamine)cobalt(III) Ion in Acidic Aqueous Solution. Pressure Effect and Mechanism SHIU BOR TONG,la H. ROY KROUSE,lb and THOMAS W. SWADDLE*la Received June 27, 1975
AIC50449V
The volume of activation for aquo exchange in tran~-Co(en)2(OH2)2~+ in aqueous HC104 (ionic strength 2.0 mol kg-l, 308 K) is +5.9 & 0.2 cm3 mol-' and is independent of pressure over the range 0.1-300 MPa at least. These observations, taken together with those of Stranks and Vanderhoek (preceding paper), for the isomerization of trans-Co(en)2(OH2)z3+, are interpreted in terms of a dissociative interchange mechanism involving a tetragonal-pyramidal intermediate for aquo exchange and a dissociative mechanism with a trigonal-bipyramidal intermediate for the isomerization.
Kruse and Taube2 showed that, in the isomerization of
trans-diaquobis(ethylenediamine)cobalt(III) ion in aqueous solution, one aquo ligand is exchanged with solvent and that stereoretentive aquo exchange in the trans isomer is faster (by some 70%, at 298 K in 1 m HClO4) than isomerization. These data suggest, but do not prove, a dissociative (D) or dissociative interchange (Id) mechanism for both processes. Kruse and Taube2 pointed out that their kinetic data for the isomerization and aquo exchange of cis- and tran~-Co(en)2(OH2)2~+ cannot be explained on the basis of a single five-coordinate intermediate having trigonal-bipyramidal geometry (mechanism I) but suggested one in which aquo exchange occurs mainly N-N
I HpO-c'o-oH, Ul
:
+
Nr'" H20-L...
N"N
+ H20
I\
N
d
trans
+
Y?N I..
H,O-CO-N
~ 8 N' J l Cis
via a primary tetragonal-pyramidal intermediate which may also rearrange to trigonal-bipyramidal geometry and so permit isomerization on recombination with water (mechanism II).3
trans
Cis
We now present data on the effect of pressure on the rate of the aquo-exchange reaction of the trans isomer, which, taken in conjunction with corresponding measurements by Stranks and Vanderhoek4 on the kinetics of the isomerization reaction and anation processes, indicate that dissociative mechanisms are indeed involved, that the rearrangement of the presumed five-coordinate intermediate in isomerization is accompanied by extensive desolvation in the transition state, and that stereoretentive aquo exchange does not involve the same
rearranged intermediate as does isomerization.
Experimental Section Preparation of trans-[Co(en)2(0H~)(OH)](C104)2. trans-[Co(en)2Cl2]Cl was made by a standard method5 and recrystallized from aqueous HCl. A mixture of this product (10 g), silver oxide (12.2 g), and water (8 cm3) was stirred well and filtered. The filtrate was ice-cooled and acidified carefully with 71% HC104 (1.2-1.8 cm3). The precipitated mixture of cis- and trans- [Co(en)z(OH2)2](C104)3*xH20was filtered, washed with acetone, and air-dried (1.66 g). This material was redissolved in 28 cm3 of water, the pH was adjusted to 7.58-7.66 with 4 m NaOH, Norit charcoal (0.3 g) was added, and the mixture was set aside a t room temperature for 14 h. The filtered solution was treated with NaC104 to precipitate tram-[Co(en)2(0H)(OH2)] (c104)2, which was filtered, washed with acetone, and dried in vacuo over P205; yield 0.5 g. Anal. Calcd for C O N ~ C ~ H ~ ~N, C ~13.56. ~ O ~Found O : N, 13.40. The visible spectrum agreed well with that reported6 for the trans isomer. The H2180-labeledsalt was made in the same way, using 1.5% H2180 water as solvent in the conversion of the dichloro complex. Kinetic Measurements. A solution (0.2 m ) of H2180-labeled trans-[Co(en)2(OH)(OH2)](c104)2 in 0.8 m HC104 was prewarmed and then pressurized (10.5MPa) in the thermostated (*0.01 K) pressure apparatus previously described;' it was established that the outlet capillary of tantalum could be replaced by one of stainless steel without affecting the reaction rate. Samples (about 1.5 cm3) were withdrawn periodically (rejecting "hold-up"), chilled with ice water, and treated with just enough concentrated HC1 (1 .O-1.6 cm3) to precipitate trans-[Co(en)2(0H2)2] Cl3, which was filtered, washed with acetone, and dried in vacuo over P205 for at least 18 h. The coordinated water was recovered on a vacuum line by heating the salt at 363-413 K and was converted to C 0 2 as described previously8 for mass spectrometric analysis to determine the I 8 0 abundance. Kruse and Taube2 precipitated the tran~-Co(en)2(0H2)2~+ ion to the exclusion of the cis isomer by addition of hexachlorothallate( 111) ion in aqueous HC1 and reported that the precipitate contained two molecules of water of crystallization as well as two aquo ligands per cobalt atom. In our hands, the Kruse-Taube procedure yielded mainly trans- [Co(en)2(0H2)2]C13, and the above procedure was therefore adopted as being simpler and yielded a product free of both the cis isomer and lattice water. Nevertheless, the exchange kinetics were not in general followed beyond 50% completion, since the effectiveness of the procedure was reduced thereafter by accumulation of the isomerized complex in solution. Other potential non-oxygen-containing precipitants which were investigated included HBr (which precipitated both the cis and the trans isomers together) and NaBF4 and HBF4 (which gave no precipitate).
Results and Discussion First-order rate coefficients k,, for the aquo-exchange reaction at 307.97 K and ionic strength I = 2.0 m are collected in Table I. The datum for atmospheric pressure (ca. 0.1 MPa) was obtained using ordinary laboratory glassware and agreed well with the value of 6.33 X s-l extrapolated from measurements made in the pressure assembly and with 6.32 X s-l interpolated from the data of Kruse and Taube* (which yield AEF = 126.2 kJ mol-' and AS* = 84.2 J K-'
Tong, ICrouse, and Swaddle
2644 Inorganic Chemistry, Vol. 1 5 , No. I 1 , 1976 Table 1. First-Order Kate Cocfficieiits k,, f o r the Cxchaiige of H Z r 8 0between t~uns-Co(en),(r80H,),3' and Solvent Water as a Function of Pressure a t 307.98 Ka -._x
Pressure, MPa
O.lb
6.4 102.5 102.5 152.4
105k,,, s-'
6.29 t 6.21 f 4.90 i5.20 + 4.23 f
0.3V 0.24 0.18 0.31 0.27
^ _ _ ll_ _ _ l_ .l.l
Pressure, MPa 105kk,,,s-' _llllll___-_.
201.3 250.0 301.9 301.9
4.17 t 3.52 ? 3.09 f 3.23 +
0.17 0.14 0.16
0.08
a [Co(en),(OH,),(ClO,),] = 0.200 rn; [HClO,] 2 0.80 rn. Rate data refer to exchange of both a q u o ligands. lri ordinary glass Standard errors cited refer to uncertainties within indivessels. vidual kinetic runs.
'Thus, acjuo exchange ill ~ r a r e s - C o ( e n ) z ( O H * proceeds )~~'~ al most exclusively :iia a .tetragorpal-pyraniidal five-coordinate intermediate without significant solvational change. This rirechanism is best described as Id rather than D, since the lack of significant presslire dependence of Lave,* shows that the secoiid coordination sphere (solvation sheath) does not relax before six-coordination is The isomeriz,ation reaction must involve rearrangement of the tetragonal-pyraniidal intermediate to trigonal bipyrarnidal, with extensive concomitant desolvation which is reflected in the strong pressure dependence of AVi,,*; this mechanism should therefore be classified as D.' Desolvation during the rearrangement of a tetragonal-pyramidal interrnediate to trigonal bipyramidal also provides a rationale9 for 'robe's observation14 that AS* is usually much more positive for aquations of complexes of the type of CoLACln+ (where L = (en)2, trien, cyclani, etc., and A is an "inert" ligand) when there is marked steric change than when the reaction is stereoretentive. Finally, the closeness of the value of h V e x * to the volumes of activation obtained by Stranks and Vanderhoek'j for stereoretentive anation within ion pairs of the type C0(en)2(0&)2~+,X* indicates that both anation and solvent exchange proceed by I d mechanisms in Co (en) 2 (OH$ 23 .
mol-' for the exchange of both aquo ligarids, under the same conditions). A plot of In kex vs. pressure is linear within the experiinenlal uncertainty (which is somewhat larger than usual because the reaction could be followed only over the first 50%) arid so yields a pressure-independent volume of activation Av" = +5.9 f 0.2 cm3 mol-'. It follows that there is predominant Co-OH2 bond breaking with little or no change in solvation on going to the transition state of this simple, symmetrical reaction, since significant solvational changes manifest thcmselves in a pressure dependence of the volume of activation."." Thus, Acknowledgment. We thank Drs. I>. R. Stranks aind N. the mechanism is Id or D; the numerical value of AV,,* is too Vanderhoek for discussions arid for communicating results large to accommodate extreme arguments (as in footnote 10 prior to publication and the National Research Corincil of of ref 11) for an 1, or A process. This result again illustrates Canada for financial support. the anomalous behavior in substitution of Co(II1) complexes Registry No, liuns-[Co(en)z(OH 2) (OH 11(ClO4)2,, 14099-22 0; among cationic octahedral complexes of the trivalent transition t ~ ~ n s - 6 3 0 ~ c r i ) ~ ( i ) Y 1 2 ) 19314-32.0; 2~$, truvrs-[Co(en)zY:i21(:I, metals, which in general yield negative values of AVe," in 14040-33-6; H20, 7732-18-5. solvent-exchange reactions, implying an 1, m e ~ h a n i s r n . ~ ~ ~ ~ ~ References arid Notes This result lends support to Hunt and Taube's12 interpretation of their value of Av* = 4-12 cm3 mol-' for aquo ( 1 ) (a) Departnierlt of Chemistry. (b) Departrnerit of Physics. (2) W. Krose and I i . 'Taube, J . Am. Chem. SOC.,83, 1280 (1961). exchange in aqueous Co(NH3)j0€-123+. The larger LIP',,* (3) An alternative scheme may be conceived in which aquo exchange through value for trun~-Co(en)2(0H2)23+probably reflects in part tetragonal-pyramidal species and isomerization through a trigonalrelaxation of the greater steric compressions which are bipyramidal interrnediate are parallel, rather than consecutive, reactions. It is to be expected, however, that interconversion between the various presumably present in the initial state vis-a-vis Copyramidal and bipyramidal interniediates will occur readily. since this (NH3)50H23+,by analogy9 with Foxman's observations" on proms involves small displacements of ligands within high..eiiergy species, Co(NH2CH3)5C12+,but could also contain a contribution from and indeed it is not even tiecessary to suppose that the trigonal-bipqraniidal species exists othzr than instantaneously as the transition state for reaquation of the trigonal-bipyramidal species of mechanism interconversion between the isomers of the tetragonal intermediates (cf. I1 to the trans configuration. Stranks and Vanderhoek4 found, ref 2). Mechanism I1 is therefore considered more realistic, for the for the isomerization of trans-C0(en)2(0H2)2~+,AViso* (at purwses of this discussion, but the eshential conclusions drawn here could also be presented in terms of parallel aquo-exchange and isomerization zero pressure) = +14.3 and +13.2 cui3 mol-' at I = 0.05 and pathways. 1.O M, respectively; this isomerization presumably proceeds (4) D. R. Stranks and N. Vanderhoek, lnorg. Chem., preceding paper i n via the trigonal-bipyramidal species of mechanism 11, there this issue. being no indication of C e N bond fission, and so the very large (5) J. C. Bailar, Jr., and C. W. Rollillson, Inorg. Synth., 2, 222 (1946). (6) J . Bjerrurn and S. E. Rasmussen, Acta Chem. Scand., 6 , 1265 (1952). size of AP',,,* relative to AV&* precludes the possibility that ( 7 ) L. R. Carey. W. E, Jones, and T. W. Swaddle, Iriorg. Chem., 10, 1S66 a significant fraction of aquo exchange goes through the I1 971-, 1. trigonal-bipyramidal intermediate. Furthermore, A?&,* is (8) S. 13. Toiig aud 'T,W, Swddle, l n o ~ g L'hem., . 13, 1538 (1974). (9) T. VI. Swaddle, Cooid. Chem. Reo., 14, 217 (1974). strongly pressure dependent, with -.(aaV*/3P)~ = 0.01 and E. Jones, L. k?. Carej, and T. W . Swaddle, Can. J . Chem., 50,2739 0.1 cm3 mol-' MPa-' at I = 0.05 and 1.0 M, respecti~ely,~ (10) W. (1972). whereas no pressure dependence of dVe,* can be detected up ( I I ) D. R. Stranks and'r. W. Swaddle,.I. A m Chem.Soc., 93, 2983 (1971). (12) H. R. Hurit and fI. Tauhe, J . A m Chem. Soc., 80, 2642 (1958). to 300 MPa. In any event, the isomerization equilibrium lies (13) B. M. Foxnian, J. Chem. SOC., Chem. Commun., 515 (1072). heavily in favor of the cis isomer in acidic solution;2 that is, (14) M. L. Tobe, lnorg. C h e m , 7, 1260 (1968). reaquation of the trigonal-bipyramidal intermediate to the (1.5) D. R. Stranks and N. Vanderhoek, Iriorg. Chern., following paper in this issue. trans configuration is not favored. +
\ - -
