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The Journal of Physical Chemistry, Vol. 82, No. 20, 1978
(11) R. H. Rein and J. Chipman, Trans. Metail. SOC.AIME., 233, 415 (1965). (12) A. Nahotsky, R. C. Newton, and 0. J. Kleppa, @whim. Cosmochim. Acta, 37, 2497 (1973). (13) M. J. Holdaway, Am. J . Sci., 271, 91 (1971). (14) P. A. M. Anderson and 0.J. KleDDa, Am. J . Sci.. 267. 285 (1969). (15) P. A. M. Anderson, R. C. Newton, and 0. J. Kleppa, Am. i.Sci:, 277, 583 (1977). (16) 1. Eliezer and R. A. Howald, J. Chem. Phys., 65, 3053 (1976). (17) N. Eliezer, R. A. Howald, M.Marinkovic, and I. Eliezer, J. phys. Chem., 82, 1021 (1978). (18) D. F. Weill, Geochim. Cosmochim. Acta, 30, 223 (1966). (19) S.J. Schneider. Pure ADD/. Chem.. 21. 115 (1970). (20) E. N. Fomichev, P. B. Kaitor, and V. V. Kandyba, T;. Metrol. Inst. SSSR, 110, 135 (1971). (21) V. Ya. Checkhovskoi, M. M. Kenlsarln, and V. V. Kuzichkin, Izmer. Tekh. (Metrol.),8, 16 (1972). (22) M. W. Chase, J. L. Curnutt, A. T. Hu, H. Prophet, A. N. Syverud, and L. C. Walker, J. fhys. Chem. Ref. Data, 3, 311 (1974). (23) E. N. Fomlchev, V. P. Bondarenko, and V. V. Kandyba, H/gh Temp. H/gh Pressures, 5 , 1 (1973). (24) E. E. Shpil’rain, D. N. Kagan, and L. S. Barkhatov, H/gh Temp. H/gh Pressures, 4, 605 (1972). (25) J. F. Shairer and N. L. Bowen, Am. J. Sci., 253, 714 (1955). (26) D. M. Roy and R. Roy, Am. Mineral., 49, 952 (1964). (27) N. A. Toropov and F. Ya. Galakhov, Dokl. Akad. Nauk. SSSR, 78, 299 (1951). (28) N. A. Toropov and F. Ya. Galakhov, Izv. Akad. Nauk SSSR, Ofd.
(29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44)
Khim. Nauk, 8 (1958); Eksp. Tekh. Mineral. Petrogr. Mater. Soveshch. 7th, 1964, 3 (1966). P. P. Budnikov, S. G. Tresvyatski, and V. I. Kushakovski, Dokl. Akad. Nauk SSSR, 93, 291 (1953). W. H. Bauer, I. Gordon, and C. H. Moore, J. Am. Ceram. Soc., 33, 140 (1950). R. F. Davis and J. A. Pask in “High Temperature Oxides”, Refractory Materials, A. M. Alper, Ed., Vol. 5, Part IV, Academlc Press, New York, N.Y., 1971, p 37. R. F. Davis and J. A. Pask, J . Am. Ceram. SOC.,5 5 , 525 (1972). J. H. Welch, Nature (London), 166, 545 (1960). T. Horibe and S. Kuwabara, Bull. Chem. SOC.Jpn., 40, 972 (1967). E. M. Levin, C. R. Robbins, and H. F. McMurdie, “Phase Diagrams for Ceramists”, American Ceramic Society, Columbus, Ohio, 1964. S. H. Risbud, Ph.D. Thesis, University of California at Berkeley, 1976, LBL-5453. J. F. McDowell and G. H. Deall, J. Am. Ceram. SOC.,52, 17 (1969). J. A. Pask, private communlcatlon. C. L. Thomas, “Catalytic Pracesses and Proven Catalysts”, Academic Press, New York, N.Y., 1970. SNAM Progette S.p.A., Spanish Patent 440497 (1977); Chem. Abstr., 87, 85492f(1977). F. C. Kracek et al., Annual Report of the Director of the Geophysics Laboratory, Geophysics Laboratory Paper 1215 (1953). T. Yokokawa and 0.J. Kleppa, J. Phys. Chem., 68, 3246 (1964). R. Barany, U . S . Bur. Mines, Rep. Invest., No. 6251, 8 (1963). J. H. Hlldebrand, J. M. Prausnitz, and R. L. Scott, “Regular and Related Solutions”, Van Nostrand, New York, N.Y., 1970.
Aquation and Spontaneous Reduction of the Diaquotetrakis(pyridine)cobalt(III) Ion in Aqueous Perchloric Acid K. E. Hyde’ and G. M. Harris’ Department of Chemistry, State University of New York, College at Oswego, Oswego, New York 13126, and Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 (ReceivedJanuary 12, 1978) Publlcation costs assisted by the John D. and Frances H. Larkln Foundation of the State University of New York at Buffalo
The redox properties of the trans-diaquotetrakis(pyridine)cobalt(III)ion, which we first observed during an investigation of the acid hydrolysis of the carbonatotetrakis(pyridine)cobalt(III) ion, have been separately examined in aqueous perchloric acid. The reaction series that ultimately leads to the spontaneous reduction of the cobalt(II1)complex to aqueous Co(I1) and oxygen involves stepwise replacement of coordinated pyridine by water molecules, starting with the previously identified species, t r u n s - C ~ ( p y ) ~ ( H ~ OThe ) ~ ~more + . highly and the monomeric aquated complexes, f~c-Co(py)~(H~O)~+, polynuclear forms of cis- and truns-C~(py)~(H~O)~+, ~ i s - C o ( p y ) ~ ( H ~ have O ) ~ +been , isolated in solution and characterized spectrally. Both trans-Co(py)4(HZO)$+ and f~c-Co(py)~(H~O),~+ undergo pyridine dissociations which lead to the same relatively slow reduction step. undergo depolymerization and, for the trans form, The condensed forms of cis- and trun~-Co(py)~(H~O),~+ rearrangement to give monomeric cis-C~(py)~(H,O)~~+. The latter is proposed as the reducible complex and it oxidizes water at a rate which involves a reciprocal hydrogen ion concentration dependence. The reaction sequence for the hydrolysis and spontaneous reduction of the tr~ns-Co(py)~(H~O)$+ ion is compared with that observed for a number of related ammine complexes.
Introduction The loss of coordinated NH3from cobalt(II1) centers has been noted often during the study at elevated temperatures of the reactions of CO(NH~)~XP+ ions, a process which necessarily causes complications in studies of the high temperature aquation kinetics of such species. Recently, reactions involving loss of NH3 from a number of aquoamminecobalt(II1) complexes of the type [Co(NH,),(H20)6-n]3+have been subjected to detailed kinetic analysis.l-* The studies show that the aquation rate increases with increasing numbers of coordinated water molecules, and that the reactions are decelerated by increasing acid concentration. These observations have been noted4 to be consistent with the increasing availability of 0022-385417812082-2204$01.OO/O
conjugate-base reaction paths as water ligand substitution for ammonia proceeds. Not unexpectedly, the oxidizing ability of the complex and the rates of spontaneous reduction by internal electron transfer also increase with increasing numbers of coordinated water molecules. For the aquoamminecobalt(II1) series of complex ions, spontaneous reduction is first observed for the fuc-[Co(NH3)3(H20)3]3fion, while the more fully aquated ion c i ~ - [ C o ( N H ~ ) ~ ( H undergoes ~ 0 ) ~ 1 ~ +the redox process at a considerably enhanced rate. The monoammine complex, [ C O ( N H ~ ) ( H ~ O )has ~ ] ~been + , reported5p6to disproportionate to form Co(I1) and C O ( N H ~ ) ~ ( H ~ Oa) ?rather +, anomalous reaction indeed. Several types of chelated aquoamminecobalt(II1)complex ions also have been found 0 1978 American Chemical Soclety
Aquation Properties of frans-Co(py),(H,O)~+
to undergo spontaneous thermal reduction. Three of these, ~is-[Co(en),(H,O)~]~+ (en = eth~lenediamine),~ cis-[Co( t r ~ ) ~ ( H ~ O )(tn ~ l ,=+ trimethylenediamine),, and fac[C~(dien)(H,O)~]~+ (dien = diethylenetriamine)8 are, as expected, very much more resistant to internal redox than are the aquoammine species. However, the species is mer-[Co(dpt)(H20),l3+(dpt = dipr~pylenetriamine)~ unusually prone to spontaneous reduction, with rate parameters for this process of magnitude similar to those for c~s-[CO(NH~)~(H~O),]~+. When ammine ligands coordinated to cobalt(II1) are replaced by pyridine, the reactivity of the resulting metal center is altered considerably. These pyridine complexes of cobalt(II1) possess relatively labile metal-pyridine bonds,1° robust metal-ligand (nonpyridine) bonds,11-14and enhanced oxidizing ~r0perties.l~ During our previous work on the acid hydrolysis of the carbonatotetrakis(pyridine)cobalt(III) ion in aqueous perchloric acid,12we noted that complications were encountered at moderate ionic strength and acid concentrations ( I = 1M, [H'] < 1MI. Under these conditions, the expected diaquocobalt(II1) complex was not observed as a product, but instead a comparatively rapid redox reaction occurred after the rate determining decarboxylation with the result that aqueous Co(I1) was the only identifiable cobalt containing species. In this communication we report the results of a kinetic investigation of the decomposition reactions of several aquopyridinecobalt(II1) ions and compare these results with those for the aquoamminecobalt(II1) centers mentioned above. In addition, the relationship between electronic and structural features of the pyridine ligand and the reactivity of the cobalt(II1)-pyridine complexes is discussed.
The Journal of Physical Chemisfty, Vol. 82, No. 20, 1978 2205
450
500
550
A,
600
650
nm
Flgure 1. Visible absorption spectra of aquopyridinecobalt(II1)ions: fran~-[Co(py)~(H,O),]~+ (5 M HC104), - - -; fac-)Co(p~)~(H,0),1~+(5 M HC104), -;monomeric ~is-[Co(py),(H,O)~]~(1 M HCIO,), -.
---
-
chlorodiaquo complex in acid resulted in an immediate color change from green to red. Attempts to isolate the solid complex by addition of cold concentrated perchloric acid to a cold concentrated solution of the triaquo ion were unsuccessful. Its spectral parameters [see Figure 1; Am(€) 521 nm (52); Amin(€) 442 nm (15) and a possible shoulder near 370 nm] are similar to those for other CO(N)~(H,O)?+ chromaphores1?l6and on this basis the triaquo ion is assigned the 1, 2, 3, or facial configuration. The complex tran~-K[Co(CO~)~(py)~]-3H~O was prepared by the methods previously described17 and gave satisfactory analyses (Calcd: C, 33.50; H, 3.75; N, 6.51. Found: C, 32.98; H, 3.31; N, 6.84). The corresponding cis complex was prepared by the method of Davies and Hung18 and gave an excellent analysis for the dihydrate (Calcd. C, 34.93; H, 3.42; N, 6.79. for ci~-K[Co(py)~(C0~)~].2H~O: Found: C, 35.14; H, 3.42; N, 6.61). The tetraaquobis(pyridine) complexes, cis-Co(py)zExperimental Section (HzO),3+and tran~-Co(py)~(H,0),3+ in their polynuclear forms, were obtained in solution by adding solid cis- or Preparation o f Compounds. The preparation of complex to a perchloric acid solution trans-[C0(py),(H~O)~](C10~)~~4H~O from [C0(py)~C0~]- trans-KC~(CO~)~(py)~ of the desired acidity and ionic strength. By analogy with C104.H20was accomplished by the previously reported the similar ammine complexes, these condensed tetraaquo pr0~edure.l~ The new compound, chlorodiaquotris(pyrspecies are believed to be dihydroxy bridged d i m e r ~ . ~ l ~ J ~ idine)cobalt(III) perchlorate dihydrate, [Co(py)3ClThe spectrum of the polynuclear cis-tetraaquo complex (Hz0)2](C104)2.2H20, is prepared by dissolving 2 g of the obtained in this work (Amm(€) 536 nm (93)) agrees with corresponding carbonato complex14[Co(py),C03C1] in 10 respect to the position of the maximum with that previmL of 70% perchloric acid. Formation of a dark green ously noted18 (Arnm 540 nm). The spectrum of polynuclear solution and precipitate accompanied the rapid evolution t r a n s - C ~ ( p y ) ~ ( H ~has O ) ~not ~ +been reported previously of carbon dioxide. Green crystals were collected when the and is difficult to obtain because of an apparent rapid stirred and cooled mixture was filtered through a fritted depolymerization and isomerization to the monomeric cis glass funnel. Several milliliters of cold 70% perchloric acid isomer. An estimate of the spectral features of the trans was used to wash these crystals. Recrystallization involved complex was obtained by adding a known amount of the dissolving the complex in a minimum amount of cold 0.12 solid bis-carbonato salt to a solution of 1M perchloric acid M HC104. This solution was cooled after filtering and at about 2.5 "C in the thermostatted spectrophotometer mixed with 10 mL of cold 70% perchloric acid. On cell. Maxima at approximately 566 ( E -82) and 376 nm standing, dark green crystals formed and these were (c -102) and minima at 468 ( E -22) and 358 (c nm -95) collected by filtration and washed with cold 4 M perchloric acid followed by ether. The recrystallization procedure were observed on the first scan but these rapidly shifted was repeated a second time and the final crystals were toward shorter wavelengths as depolymerization and dried in a desiccator for several hours. The yield of the isomerization proceeded. Acidified solutions of either polynuclear cis- or transsolid complex was -2 g (70%). Anal. Calcd for [CoC0(py)~(H,0),~+ complexes ultimately form solutions with (py)3Cl(HZO),](C104)2.2Hz0:C, 29.90; H, 3.84; N, 6.97. Found:37 C, 29.55; H, 3.62; N, 7.10. The visible spectrum identical visible spectra and we take this to be the result of the complex in 1 M HClO, indicated two maxima of the formation of monomeric c i ~ - C o ( p y ) ~ ( H ~ OThis )~~+. [Amm(c): 602 nm (32.3); 534 nm (33.5); Amin(€): 579 nm assignment is based on the observed similarity of the final (32.1); 472 nm (18.0)]. Although geometrical isomerism spectral parameters of the above solutions (see Figure 1; is possible for the [C0(py)~Cl(H,0)~]~+ ion, it could not be Amax(4 546 nm (421, Amin(€) 448 nm (7) and a possible determined from the limited spectral studies which isomer shoulder near 375 nm; 1 M HClO,) with those1J6p20of monomeric c ~ s - C O ( N H ~ ) ~ ( H ~ O ) ~ ~ + . had been isolated. The triaquotris(pyridine)cobalt(III) cation Co(py),All chemicals used to produce stock solutions of NaC10, (H20),,+ was prepared in 5 M perchloric acid by the and HC104 as well as those used in the syntheses of the mercury(I1) catalyzed aquation of C0(py),Cl(H,0),~+. above complexes were of reagent grade and were used Addition of solid mercury(I1) acetate to a solution of the without further purification. Deionized distilled water was
2208
The Journal of Phys;cal Chemjstry, Vot. 82,No. 20, 1978
K. E. Hyde and G. M. Harrls
used in the preparation of all solutions. dichromate. After thermal decomposition of the solution, the Co(I1) content was determined by a potentiometric Kinetics. A Cary Model 118C recording spectrophotitration with ferricyanide. Pyridine was determined by tometer was used to obtain spectra of the various comdirect measurement of the UV absorption of the deplexes. The kinetics of spontaneous reduction of the aquo composed solutions. (We find an t of 5488 M-l cm-l at 256 ions were followed spectrophotometrically at fixed nm from solutions containing known quantities of pyridine wavelength by use of the Cary Model 118C or a Beckman in 2 M HC104.) These analyses showed that the py/Co DU instrument with a Gilford optical density converter. ratio was 1.99 for the complex in this eluate. When the For most runs the reaction was monitored at 490 nm (a maximum in the absorption spectra of tr~ns-[Co(py)~- same chromatographic separation procedure was used for polynuclear tr~ns-K[Co(py)~(CO~)~].2H,O, only one band (HzO),3+])and several runs at other wavelengths indicated was observed. This was eluted with 2 M HC104 and that the kinetic parameters for the slow step in the reaction identified spectrally as monomeric c i s - [ C ~ ( p y ) ~ ( H ~ O ) ~ ] ~ + . sequence were independent of the monitoring wavelength. Finally, a sample of partially reduced monomeric Ion-Exchange Chromatography. Attempts to separate ~is-[Co(py),(H,O)~]~+ was passed through a 5 cm Dowex the various [ C ~ ( p y ) , ( H ~ o ) ~ -ions , ] ~ +by ion-exchange 50-X2 (200-400 mesh) cation exchange column at room chromatography were only partially successful because of temperature as a test of the existence of a more fully the high charge on the complexes, the tendency of these aquated Co(II1) intermediate, [ C 0 ( p y ) ( H ~ 0 ) ~which ]~+ complexes to aquate in acid solution with loss of pyridine, would be analogous to the reported [CO(NH,)(H,O),]~+ and the observed spontaneous reduction of the complexes. i ~ n . ~The ! ~ only identifiable cobalt-containing species Approximately 0.1 g of tr~ns-[Co(py)~(H~O)~](C10~)~.4H~O eluted by 2 M HC104were unreacted Co(py)?+ complex was added to 5 mL of 3 M HC104 (I = 5, NaC104) at 60 and aqueous Co2+,showing that the suggested intermediate "C. The reaction mixture was incubated at 60 "C for about is not formed or, perhaps more likely, is too unstable to 30 min. This solution was placed on an ion-exchange reduction to survive the separation procedure. column (Dowex 50W-X2,200-400 mesh) which had been Oxidation-Reduction Products. Additional analytical prewashed with 2 M HCIO1. The column was 13 mm X experiments showed that the reducing species in this 15 cm and was cooled to about 5 "C by circulating water. reaction is water, since the final Co2+solutions contain Eluting the column with 175 mL of 2 M perchloric acid approximately equivalent amounts of 02.The dissolved brought the lower band (red-violet) to the bottom of the oxygen was determined by the procedure described by column. Additional 2 M HC104eluted this band from the McCormick.21 In some experiments the oxidation-recolumn. A second red band was eluted with 5 M HC104. duction reaction was carried out in a sealed container so Absorption spectra of the middle portions of eluate from that gaseous products were prevented from escaping. In each band permitted the assignment of unreacted trans other experiments a flow of Nz through the reacting sodiaquo complex to the red-violet band and fuc-[Colution swept the gaseous products from the reaction vessel. (py)3(HzO)3]3+ as the slower-moving red band. The The gas stream was bubbled through a cold manganese position of the absorption band maxima and minima for sulfate solution to trap the evolved oxygen and the reboth species agreed to within a few nanometers with the sulting solution was analyzed for oxygen content. In all assignments for the independently prepared complexes. experiments the determined oxygen content after corExtinction coefficients, calculated from the observed recting for the blank was within 25% of the calculated absorption spectra of the eluate from the middle portions amount for reduction of a Co(II1) complex by water. of both bands and a total cobalt analysis, were about 10% lower than the expected values. This observation is Results consistent with partial decomposition of the complexes to In preliminary experiments carried out at I = 5.0 M and aqueous Co(I1). For aquopyridinecobalt(II1) complexes over a range of acid concentrations (1.0-5.0 M, HC104) the the more highly aquated complexes are found to be held visible spectrum of the diaquo complex was scanned at more tightly by the ion-exchange resin, which is the opvarious time intervals. The absorption was found to inposite of the behavior of aquoamminecobalt(II1) on ioncrease briefly in certain wavelength regions but no isobestic exchange columns.6 points developed.22 The above process was followed by a If [ C 0 ( p y ) ~ C l ( H ~(C104)2.2H20 0)~] is incubated for 10 slower one where the absorption decreased at all wavemin at 60 "C in the presence of 2 M HC104 and Hg(I1) ion, lengths. These events were independent of whether the only one band results when the resulting solution is passed tetra-, tri-, or condensed diaquo complex was the reactant. through a cooled cation-exchange column with 2 M HC104 In each case spectral scans at various time intervals reveal as the eluant. This band is identified by the positions of that the original spectrum of the aquo ion rapidly changes absorption band maxima and minima as monomeric during the "induction period" to one with a maximum near ~is-Co(py),(H~O)~~' and results from the loss of py from 550 nm and a minimum near 450 nm, which corresponds the Co(py)&H20)2+cation. to the spectrum of monomeric c i s - C ~ ( p y ) ~ ( H ~ O ) ~ + . Similar experiments with the condensed forms of cisAfter completion of reaction, the spectrum had one and tran~-[Co(py)~(H,O)~]~' were attempted. Two bands 5 M-l cm-l), indicative of minor peak at 510 nm (6 separated when polynuclear ci~-K[Co(py)~(C0~)~].2H~O aqueous cobalt(I1)ion as the only spectrally distinguishable was acidified in 1 M HC104 for 10 min at room temperproduct of the reaction under these conditions. These ature and charged on a column cooled to
