6516 (5)J. S. Taylor, P. Mushak, and J. E. Coleman, Proc. Nafl. Acad. Sci. U.S.A., 87, 1410 (1970). (6)E. Grell and R. C. Bray, Biochim. Biophys. Acta, 236,503 (1971). (7)F. S.Kennedy, H. A. 0. Hill, T. A. Kaden, and 9. L. Vallee, Biochem. Biophys. Res. Commun., 48, 1533 (1972). (8) P. H. Haffner and J. E. Coleman, J. Biol. Chem., 248, 6630 (1973). (9)J. A. Fee, J. Biol. Chem., 246,4229 (1973). (10)9. L. Vallee, J. F. Riordan, D. S. Auld, and S. A. Latt, Philos. Trans. R. SOC. London, Ser. 6.. 257,215 (1970). (11)T. A. Kaden, 9. Holmquist, and 9. L. Vallee, Biochem. Eiophys. Res. Commun., 46, 1654 (1972). (12)J. S. Taylor, C. Y. Lau, M. L. Applebury, and J. E. Coleman, J. Biol. Cbem., 248,6216 (1973). (13)G. Rotilio. L. Calabrese, and J. E. Coleman, J. Biol. Chem., 248, 3855 (1973). (14)J. E. Coleman and R. V. Coleman, J. Biol. Chem., 247,4718 (1972). (15)9. Holmquist, T. A. Kaden, and 9. L. Vallee, Biochemistry 14, 1454 (1975). (16)S.A. Latt, D. S. Auld, and 9. L. Vallee, Biochemistry, 11, 3015 (1972). (17)W. Dew. Horrocks, Jr., 9. Holmquist, and 9. L. Vallee, Proc. Natl. Acad. Sci. U.S.A., 72,4764 (1975). (18)F. A. Ouiocho and W. N. Lipscomb, Adv. Protein Chem., 25, 1 (1971). (19)For a detailed discussion of phenomenologicalligand-field parametes see: M. Gerloch and R. C. Slade, "Ligand Field Parameters", Cambridge University Press, London, 1973. (20)C. K. Jorgensen, R. Pappalardo, and H. H. Schmidke, J. Chem. Phys., 39,
1422 (1963). (21)C. E. Schaffer, Struct. Bonding(Berlin), 5, 68 (1968). (22)D. W. Smith, Struct. Bonding(Berlin), 12,49 (1972). (23)9. N. Figgis, M. Gerloch, and R. Mason, Acta Crysfallogr., 17, 506 (1964). (24)M. A. Porai-Koshits, Tr. Inst. Krisfallogr., Akad. Nauk SSSR, IO, 117
(1954). (25)J. Ferguson, J. Chem. Phys., 38, 116 (1963). (26)J. P. Jesson, J. Chem. Phys., 48, 161 (1968). (27)R. P. van Stapele, H. G. Beljers, P. F. Bongers, and H. Zijlstra. J. Chem. Phys., 44,3719 (1966). (28)9. N. Figgis, M. Gerloch, and R. Mason, Proc. R. SOC.London, 279,210 (1964). (29)A. Bose. S.Mitra, and R. Rai, Indian J. Phys., 39,357 (1965). (30)A common bisector was assumed for both angles. (31)M. Tinkham, "Group Theory and Quantum Mechanics", Mc&aw-Hill, New York, N.Y., 1964,p 162 ff. (32)D. A. Burlone, Ph.D. Thesis, The PennsylvaniaState University, 1975. (33)W. Dew. Horrocks, Jr. and D. Dew. Hall, Coord. Chem. Rev., 6, 147 (1971). (34)M. Gerloch and J. R. Miller, Prog. lnorg. Chem., 10, 1 (1966). (35)For consistency the coordinate system demanded by the Cpvpoint symmetry of structure 2 was retained throughout: C, = z axis, uv = xz plane, u i = yz plane. Using this coordinate system the ground 4F and excited 4P states of the cobalt(l1) ion in a regular tetrahedral field split into the following levels in the order of increasing energy: 4A,(F), 4T,(F), 4TdF),4TdP), rather than having the more common symmetry labels 4AT(F),4TdF), 4T,(F), 4T7(P),respectively, which apply when the xand y coordinates are rotated by ~ 1 4The . symmetry labels appropriate to D p d symmetry in our Coordinate System are indicated in Table II. The choice of coordinate system for Td and 0 2 6 symmetries is arbitrary: however, this is not true for C ,, symmetry. Failure to appreciate this point has resulted in the incorrect labeling of the energy levels of the C ,, complexes M[(C6H5)3P]2C12,M = Co [C. Simo and S. Holt, lnorg. Chem., 7,2655(1968)land M = Ni [R. J. Fereday, 9. J. Hathaway, and R. J. Dudley, J. Chem. SOC.A, 571 (1970)]. (36)See F. A. Cotton, 0. D. Faut, D. M. L. Goodgame, and R. H. Holm, J. Am. Chem. SOC., 83, 1780 (1961),footnote 15. (37)W. Dew. Horrocks, Jr.. lnorg. Chem., 13, 2775 (1974).
The Kinetics of Ligand Detachment from Labile Cobalt( 11)-Amine Complexes in Aqueous Solution J. Lilie, N. Shinohara, and M. G . Simic*' Contribution from the Hahn-Meitner-lnstitut f u r Kernforschung Berlin, GmbH, I Berlin 39, West Germany, and Food Engineering Laboratory, U.S. Army Natick Research and Development Center, Natick, Massachusetts 01 760. Received September 15, 1975
Abstract: The eaq- reduction of some Co(II1)-amine complexes and the kinetics of ligand detachment of the resulting Co(l1) complexes have been examined by pulse radiolysis with kinetic conductivity detection (time resolution -1 ps). The halide ligands in Co(NH3)5CI+, Co(en)zC12, and Co(trien)Clz are lost very rapidly ( < I ps). Similarly all but three NH3 ligands from C O ( N H ~ ) and ~ ~ other + amine complexes are lost in < I ps. The last three NH3 ligands from all monodentate complexes are replaced by water with half lives of 1 I , 81, and 630 ps, independent of pH (3-4.5). Ethylenediamine ligands in C0(en)3~+and Co(en)zClz are detached in well-defined consecutive steps. The detachment rates of ethylenediamines are much slower than for ammonia ligands (t1/2 = 0.5, 3, and 10 ms at pH 3.5) and are pH dependent. In Co(trien)Clz the elimination of trien proceeds by a single observable pH-dependent step in the ms range. N o detachment from CoEDTA- was observed as expected for stable complexes.
The alternating current (ac) kinetic conductivity technique has been coupled successfully with pulse radiolysis2 and exploited in the study of charged free radicals over a wide p H range (2-12). This combination, referred to as "conductometric pulse radiolysis" (CPR), was subsequently demonstrated to be useful for studying the kinetics of ammonia detachment from some simple Co(I1) ammine complexes3 and was suggested as a powerful method for the study of the fast kinetics (time resolution of -1 ps) of basic-ligand detechment from labile Co(I1) complexes. More recently, CPR was successfully exploited in the study of ammonia detachment from Rh( 11) c ~ m p l e x e sThis . ~ technique can also be used to monitor intramolecular electron transfer processes3 as in the case of (p-nitroben~oate)Co'~~(NH3)~*+, where initial addition of an electron to the nitro group is followed by the intramolecular electron transfer to the metal enter.^ We present here kinetic data concerning the detachment of ammonia and some polyamines from labile Co(I1) complexes that are generated by rapid (T < 1 ps) reduction of stable Journal of the American Chemical Society
/
98:21
/
Co(II1) complexes in aqueous solutions. These kinetic data provide also an insight into the mechanism of detachment of polydentates from Co(I1) and all partially aquated intermediates. In addition, the significance of the participation of those intermediates in the chemical reactions can be assessed from their lifetimes and reactivities as in the case of the Rh"(NH3)4*+ 0 2 02Rh11(NH3)42+r e a ~ t i o n . ~
+
-
Experimental Section A IO-MHz ac conductivity cell with two pairs of platinum electrodes, one serving as a compensatory pair, was irradiated using 0.5 ps pulses of 1.6 MeV electrons from a Van de Graaff generator. The cell can be used for simultaneous conductivity and optical measurements with 1-ps resolution. Since the intermediates in the systems examined have very small absorbances, only induced conductivity changes were recorded. The associated optical and electronic equipment have been described elsewhere.6 The ac kinetic conductivity cannot be used above lo-* M ionic concentration, Le., below pH 2. The solutions were made up from ultrapure water generated by a millipore system. All chemicals were of highest commercial grade
October 13, 1976
6517 available. The amine complexes were prepared by standard methods. Oxygen was eliminated from solutions by purging with 02-free argon. Perchloric acid was used for pH adjustment. The complexes were added to purged solutions and investigated within 15 min. The dose per pulse and the conductivity cell constant were always determined before each series of experiments using M tetranitromethane solutions in the presence of M 2-pr0panol.~The dose was calculated7 from G(C(N02)3-) = 6.0 and 4C(N02)3-) = 14 000 M-I cm-l. The cell constant was calculated from equivalent conductivities:* A(H3O+) = 350 R-' mol-' cm2 and A(C(N02)3-) = 40 R-l mol-] cm2 all at 25 OC. The equivalent conductivities of intermediates were calculated from the dose/pulse and G(eaq-) = 2.7. In most of the experiments between 0.5 and 1 krad/pulse was used which corresponded to 1.3-2.6 X M hydrated electrons. The time dependent conductivity signals were digitized and stored in a Biomation 8100 transient recorder, the digital data then transferred to paper tape and later evaluated on a PDP 11 computer for the best kinetic fit. Because of the overlapping kinetics of ligand detachment, the k values have an estimated error of about 20-30%.
Results Pulses of high-energy electrons produce in aqueous solutions two main reactive species-hydrated electrons, eaq-, and hydroxyl radicals, OH. The overall reaction is given below H20'-eaq-
(2.7), O H (2.7), H3O+ (2.7), H (0.6) (1)
with G values (number/100 eV absorbed) indicated in parentheses. The O H radicals, which are strong oxidizing agents, are usually removed in the reaction with tert-butyl alcohol )O.l M solutions were sufficient for complete scavenging). OH
+ C4H90H
H20
+ - C4HsOH
(2) The resulting tert-butyl alcohol radicals do not show any acid-base properties in the pH region investigated9 and were unreactive towards Co complexes as expected for P-hydroxy radicals.I0 H atoms disappeared by reaction with tert-butyl alcohol or radicals from tert-butyl alcohol, since the rates with the complexes used were relatively low." The rapid reduction of Co(II1) complexes was accomplished using hydrated electrons which are unreactive towards tertbutyl alcohol," e.g. +
+
eaq- Co(NH3)63+-CO(NH3)62+ (3) The reaction rate of the above reaction and of eaq- with other complexes used in this work is extremely high," k 8 X 1 O l o M-l s-l. Hence, with the concentrations of complexes used, > M, the reduction reactions were over in a time shorter than the time resolution of kinetic conductivity (-2 ps). Formation of charged species on irradiation of aqueous solutions, reaction l , results in a transient change in conductivity after the pulse that is determined by A(eaq-) = 180 Q-' mol-' cm2 and A(H30+) = 350 f 2 - I mol-' cm2, Le., AA = 530 Q - l mol-' cm2. If an electron acceptor is present, the conductivity due to eaq- reaction will be lost after the completion of the ea!reaction. The exact level of the remaining conductivity will depend on the nature of the electron acceptor. For example, if one starts with a neutral solute, S , the overall conductivity change at the end of the eaq- reaction will be AA = A(H30+) A(.S-); in case of a positively charged metal or a metal complex, it will be AA = A(H30+) A(M(fl-l)+) - A(Mfl+). In some cases the exact values of A(.%) and A(metal complex) a r e not known, e.g., for unstable complexes, and have to be approximated with those for similar species. Those A values associated with change in charge of metal ions are much smaller than A(H,O+); hence only a small error may result from these approximations. For instance, for reactions 1 and 3 AA = 350 - 60 = 290 f2-I mol-' cm2 is expected and a small error in AA = A(CO(NH3)63f) - A(CO(NH3)62+) = 60 Q-' mol-' cm2 can be neglected.
-
+
+
If H30+ does not enter into any subsequent reaction, AA = 290 R-I mol-' cm2 is the conductance, Ai,, that one would observe at 2 ps. On the other hand, if protonation reactions take place, a corresponding decrease in conductivity occurs. In most cases the protonation reactions are too fast to be followed, 7 < 1 ps, by the equipment used. In the Co(II1) hexaamine example, the reduction reaction (eq 3) leads to a labile Co(I1) complex that under our conditions quantitatively loses all ammonia ligands: 6H2O
-
+ C O ( N H ~ ) ~ ~C+O ( H ~ O )+~ 6NH3 ~+
(4)
In order to simplify the presentation, as is customary, the coordinated water molecules replacing the detached ligands are hereafter omitted. Under our experimental conditions, the pH of the solutions was kept below 4.5 and the released ammonia ligands would be protonated in less than 2ps or even faster or lower pH since: NH3
+ H30+
k = 4.3 X
-+
+ H20
NH4+
1 O l o M-'
(5)
s-I
For every ammonia released a decrease in conductance AA = A(H3O+) - A(NH4+) = 350 - 75 = 275 3-' mol-' cm2 takes place. For hexaammino complex a total conductivity change of A, = - 1350 9-I molp1cm2 should be reached when all six ammonias are detached; while for pentaammino complexes, A, = - 1075 Q-I mol-' cm2. In most cases about 90% of those values are observed which is within the experimental error of the technique (10-20%). Protonation of ammonia by water can also be achieved; however, the results would not be changed inasmuch as NH3
+ H20
-+
is followed by a very fast (t 1 1 2 tonI2 at pH
