JOURNAL OF THE AMERICAN CHEMICAL SOCIETY (Registered in U. S. Patent Office)
(0Copyright, 1961, b y the American Chemical Society)
FEBRUARY 20, 1961
VOLUME83
NUMBER 3
PHYSICAL AND INORGANIC CHEMISTRY \ C O N T R I B U T I O X F R O M THE D E P A R T M E N T O F
CHEMISTRY, UNIVERSITY
O F S O U T H E R N CALIFORNIA, U N I V E R S I T Y PARK, L O S
ANGELBS,CALIFORNIA]
Binuclear Complex Ions. 111. Formation of Peroxo and Cyano Bridged Complexes by Oxidation of the Pentacyano Complex of Cobalt ( I I y 3 BY ALBERTHAIMAND W. K. WILMARTH RECEIVED JUNE 7, 1960 The binuclear ions [ (NC)sCorIIOOCoIII(CN)6] -6and [(r\;C)6FeIICNCorI'(c~)~] --B have been prepared by oxidation of the and Fe(CN)6-3, respectively. The one eiectron oxidation of these binuclear ions pentacyano complex of Co(I1) with 0% -5. Oxidation of the pentacyano complex of Co(I1) yields [(NC)jCoIIIOOCoIv(CN)s]-6 and [(NC)5FeIIICi\JCoIII(C?J)~] with H202, &OB-- and [(NC)sCoIIIOOCoIII(CX)s] yields (NC);CoOH2-- as the major product in each instance. Only Cr(CS)e-3 appears to be formed in the oxidation of the Cr(I1) cyanide complex with either oxygen or hydrogen peroxide. (NC)sCoO13p-- undergoes spontaneous polymerization in solution and in the solid state. The mechanism of formation of the various complex ions is discussed in terms of outer sphere and bridged activated complexes.
Introduction I n recent years, Taube and his co-workers have emphasized the importance of the information which may be obtained when a complex ion which is labile with respect to ligand substitution is converted by oxidation to a complex ion which is inert with respect to such substitution. For an oxidationreduction system which possesses such properties, the composition and geometry of the activated complex may be obtained, under favorable circumstances, by mere examination of the coordination sphere of the oxidized p r o d ~ c t . ~ This approach has now been applied to the reactions of the pentacyano complex of Co(I1) with a variety of oxidizing agents, and the results obtained provide evidence for paths in which the oxidant enters the coordination sphere of Co(I1) and is found in the Co(II1) product and paths in which ( 1 ) This work was supported by the Office of Naval Research and the Atomic Energy Commission. It is based in p a r t on a dissertation submitted b y A. Haim t o t h e Graduate School of the University of Southern California in partial fulfillment of the requirements for the Degree of Doctor of Philosophy. ( 2 ) (a) Paper I, L. R . Thompson and W. K. Wilmarth, J . Phys. Chew., 6 6 , 5 (1952). (b) Paper 11, W. K.Wilmarth, H. Graff and S. T. Gustin, THISJOURNAL, 1 8 , 2683 (1956). (3) Reported in part a t the 131st Meeting of the American Chemical Society held in Miami, Florida, in April, 1957. (4) A recent review of this subject b y H. Taube has appeared in "Advances in Inorganic Chemistry and Radiochemistry," Vol. I, Academic 195Y, Chap. 1. Press, Inc., New York, N. Y.,
the oxidant does not become a part of the coordination sphere of the Co(II1) product. During the course of the present work, Adamsons has established that oxidation of the pentacyano complex of Co (11) with bromine, iodine and chlorine produces, under some conditions, the ions (NC)~ C O B T - ~(NC) , ~ C O I -and ~ possibly (NC)6c0c1-3. I n the present work it has been established that in the oxidation of the pentacyano complex of Co (11) by oxygen or Fe(CN)eF3, the oxidant is captured in the coordination sphere of Co with formation of the binuclear ions [ (NC)5C011100CoIII(CN)5] --6 and [(NC)5FeT1CNCo111(CN)5] -6, respectively. Some of the properties of [(NC)5C O ~ ~ ' O O C O ~ ~ ~ ( Care N )reported, ~] in particular its one electron oxidation to produce the binuclear complex [ ( N C ) ~ ~ O ' ~ ~ O O C O -5~ ~ containing (CN)~] Co in the formal oxidation states I11 and IV, thus extending our limited knowledge of binuclear peroxo complexes of Co(II1, 111) and Co(II1, IV). The ion [(NC)5Fe11CNCo111(CN)~] -6 provides an interesting example of an inert bridged chemical complex having the same composition and geometry as the bridged activated complex for the reaction. It represents also an instance of an inert binuclear complex containing two different octahedrally substituted metal ions, a type of com-
509
( 5 ) A . W. Adamson, THISJOURNAL, 78, 4260 (1956).
pound which hitherto has not been definitely decomposition of the cornplcxcs with perchloric acid and separation of iron by the basic acetate nicthod . I 0 characterized in aqueous solution.6 fracticinnl precipitation referred to a l i c ~ v eWIS acliievetl In the oxidation of the pentacyano complex of byThe dissolving 3 . 2 g. of the mixture of F;,[Fc(CNoj]m t l IceCo(I1) by peroxydisulfate or [ ( X C ) ~ C O ~ ; ~ O O C O I ~C ~ [(N j 5-F e 1 I C ~ C o l I I ( C S )in ~ ] 5 nil. o f water a n d :itIding ( C N ) S ] - ~the , oxidant is not captured in the co- 5 ml. of 3 211 barium chloride. After cooling to 0" t h e prcordination sphere of the Co(II1) product, and the cipitate of Ba2[Fe(CN)s]6 H 2 0 , which amounted t o O . i g., filtered out and air dried. 30 ml. of ethanol was added product of both reactions is the mononuclear ion was to the solution and 3 . 3 g. of R a ~ [ ! S C ) j F c ~ ' C N C r ) ~ ~ ~ ( C N ) . , 1 . (NC)5CoOHz--. The reaction with hydrogen 1GH2O was obtained. peroxide also yields (NC)bCoOH*--, but in the d n d . Cslcd. for Ba?[Fe(CS),] C,H20: Fe, 0.38; (211, absence of oxygen labelling experiments it cannot 0.0. Found: Fe, 9.1; Co, 0.9. Calcd. for Bai[(KC),~] Co, 5.30; Fe, 5.01. Found: be ascertained whether the oxidant enters the co- F ~ I I C S C O I I I ( C N ).16H?O: Co, 5 . 2 5 ; Fe, 5.03. ordination sphere of Co(I1). A sample of Bas[(SC)5Fe11CXCo111( CS),] ,16H20 v a s In a preliminary r e p ~ r t some ,~ of the results treated with an excess of iodine; after 1.5 minutes the excess presented in this paper were given an alternate and iodine was titrated with thiosulfate, using starch as indicaincorrect interpretation. The revised interpreta- tor. Equivalent weight of Bas[(NC)jFer1CSCo1"( CK)s]. tion given here will be justified in the experiments 1FH2O calculated for reaction 1: 1101.3. Found: 1130. 2[ (NC)jFerICSCoIII(CNj5] -6 12 = and discussion presented below.
+
2[ ( N C ) ~ F e ~ I J C ~ C o ~ ~ -~5 (+ CN 21-) j ] ( 1 ) The one electron oxidation product of [(SC)jFeIICXCoIIJExperimental (CN)i] -6 was not isolated but was spectrophotometrically Instruments.-Absorption spectra mere obtained with a identified in solution as [( NC)iFeIIICr\'CoIII( Ch-),] - 5 since Carp Model 14PM Recording Spectrophotometer. -1 it could be reversibly reduced to [( SC)sFeIIC?JCoIII( C S ) , ] --G Beckmann Model G p H meter was used for pH measurewith sulfite, as shown in Fig. 1. ments. Current-voltage curves were obtained with a Sargent-Heyrovsky Model XI1 Polarograph. The technique described by Laitinen and Kivalo7 was followed, except for the use of a normal calomel electrode. T h e dropping mercury electrode had a column height of 69 cm. a drop time of 3.47 sec. and a value of 2.12 for m 2 / 3 t'/s. 811 solutions for polarographic investigation contained O.Ol%l, gelatin and were deaerated by bubbling oxygen-free nitrogen for 15 min. prior t o recording the polarogram. The polarographic cell and the reference electrode were kept in a thermostatat25&0.1°. Analysis.-Methods of analysis for C, H and X in the various cyanide complexes considered in this paper were developed by M r . U'illiam J . Schenck, microanalyst in this department. T o obtain complete combustion in the C and H analyses, it was necessary t o mix the sample with potassium dichromate and raise the combustion temperature to 1050-1100°. For the K analysis it was necessary t o mix the sample with copper oxide. I n our later work, when 28? 2'11; Ih, 36C T F M r . Schenck's services were no longer available, the analyses of C, H and N were discontinued, since the results obW A V E .EYGTY my tained from commercial analysts, who presumably did not Fig. 1.-, [(?;C)5Fe11C1";Co111(C?;)~] - - , [(SC)Itake the above precautions, were irreproducible and internally inconsistent with respect t o C and S . FelxrCSColl*(CN)j]- 5 ; -.-.-., [( SC)aFe1I1CNCo1II(CN)jl-6 Oxidation of the Pentacyano Complex of Co(I1) with after reduction with sulfite. Fe(CN)s-3.-The oxidation of 0.2 M Co(I1) chloride in 1 AI potassium cyanide as carried out in a nitrogen atFurther evidence to support the binuclear structure was mosphere a t 0" by adding, with stirring, an equal volume of obtained by comparing the spectrum of a solution of [ ( S C ) s 0.2 -11 potassium ferricyanide. l-isual observation sug- F ~ I I C N C O I I I ( C K ) ~ ]after - ~ , being heated a t 80" for 8 hr., gested that the reaction was complete in the two minutes with the spectrum calculated for quantitative hydrolysis to required t o mix the solutions. Addition of ethanol to Fe( C K ) G -and ~ ( r\'C)5CoOH2--. The agreement was better 80% in volume precipitated a mixture of 18% potassium than 9% for all wave lengths between 440 and 289 mp. ferrocyanide and 82YC K6[(?u'C)jFeIJCKCo1II(CN)SI, the I t was found that the yield of Fe(CS)s-4 mentioned composition being determined by various analytical pro- above, which is accompanied probably by an equivalent cedures and eventual separation of the two complex ions amount of (IYC)jCoOHt--, was somewhat variable. I n an by fractional precipitation of the barium salts. The yield, experiment carried out apparently under the same condiexpressed in terms of ISG[( SC)~F~IICSCOI~I(C was ~ ~ ) ~tions ], described above, 807, of &,[(SC)eFellCSCoIIInecL. (CS)b] and a n estimated 105% of K,[Fe(CS)ti] were obtained. Iron was analyzed spectrophotometrically according to When the reaction was carried out a t 25' and 0.1 -121 conBastian, Weberling and Palilla3 after decomposition of the centrations of Co(I1) and Fe( C ~ K ) C ~the , resulting solution romplexes with hot concentrated perchloric acid. Cobalt displayed an absorption spectrum which agreed within 6yG, was determined according t o Laitinen and Burdettg after for a11 wave lengths between 480 and 280 mp, with the spectrum of pure [(NC)jFeI'CNCoII'(CS)jl -6, a result which indicates that the yield of Fe( C N ) C - probably ~ was smaller (6) H. Taube a n d H. Myers, J . A m . C h e m . SOC.,7 6 , 2103 (1964), than loyo. The variation in the experimental conditions suggested t h a t the reaction between Cr(I1) and Fe(CN)6-3 occurs uia that would maximize the yield of [(NC)5FeIICKCoIIJa bridged activated complex with t h e cyanide bridge persisting in t h e ( C F ) j J- 8 , and correspondingly minimize the yields of Feinsoluble product. T h e y also suggested t h a t t h e intermediate green ( C > ) O - and ~ (NC)6CoOH2--, or zlice versu, was not systemcolor observed in the reaction of Ir GIs-- with Cr(I1) is due t o t h e atically investigated. binuclear complex (Hz0)CrCIIrCla. W. H. Sphiman, S. C. Foti and Hume and Kolthoff" presented polarographic evidence inW. Simon, Anal. Chem., 27, 1240 (1965), postulated t h e formation of a dicating that when 0.004 AT Co(1I) in 1 ;If potassium cyanide 1 26: -
binuclear (Fe(II), Co(II1) complex in the presence of nitroso-R- salt. (7) H. A. Laitinen and P. Kivalo, THISJOURNAL, 7 5 , 2198 (1953). ( 8 ) R . Bastian, R. Weberling and F. Palilla, Anal. Chent., 28, 459 (1956). (9) H . A. Laitinen and I.. W. Burdett, ibid.,23, 1269 (1951).
(10) W. W. Scott, "Standard Methods of Chemical Analysis," Vol. I, 5 t h Ed., D. Van Xostrand Company, Inc., New York, N. Y . , 1939, p. 558. (11) D. N. Hnme and I. hI. Knlthoff, THISJOURNAL, 7 1 , 807 (1949).
Feb. 5, 1961
PEROXO AND CYANO BRIDGED C~IMPLEXES
511
was formed for each mole of &[( NC)sCollIOOColll(CN\ij]. is oxidized with 0.1 M K3[Fe(C?j)g], the reaction products are (NC),CoOH2-- and Fe( C N ) O - ~ .The half-wave potenHzO. Smith, Kleinberg and G r i ~ w o l d 'have ~ reported that K3tial for the product of the oxidation was -1.4 v. zrs. s.c.e. [(KC)sCoOH].H20wps formed upon air oxidation of moist in agreement with the value - 1.45 which they determined K&o( CN)j. I'arious observations presented in their paper for (XC),CoOH2--. The possibility t h a t (NC)jCoOHz-- and [(NC)sFeII- led us t o suspect that the product was really Kg[(Kc),CoIIIOOCoIII( CN)6].H20, which differs insignificantly in CNCoIII( Cp\i)s]-6 had somewhat similar polarographic behavior prompted a polarographic examination of [( 1TC)j- analysis from the postulated Ks[(XC)sCoOH],H20. A FeIICNCoIII( CN):] -6. I n 1 M potassium cyanide as sup- repetition of the Smith, Kleinberg and Griswold experiment porting electrolyte, it yielded a polarographic wave with yielded a product which was identified as K6[(xC),Co111OOCoIII( CS),] .H20 by iodometric peroxide analysis, E l / , = -1.62 v. zis. s.c.e. (no correction for r I drop across spectrophotometry, acid-base titration and a com'parison the cell) and id/(Cm2/3t'/fl) = 2.14. For (KC),CoOH2--, the corresponding values obtained in the present work were of its X-ray powder pattern with that of a sample of Kg-1.42 and 2.33, respectively. When 0.004 M Co(I1) in [( K C ) 5 C o ~ I I O O C o CS),] I ~ ~ ( . H 2 0 prepared by oxygenation 1i V potassium cyanide was oxidized with 0.1 114' Fe(CS)6-3 of an aqueous solution of the pentacyano complex of Co(the conditions employed by Hume and Kolthoff) and the (11). Fe( CN)6-3 was rapidly added (20 seconds, the time required Stoichiometry of the Reaction Between the Pentacyano t o deliver the content of the pipet) t o the Co(I1) solution, Complex of Co(I1) and Oxygen.-The 60Yc yield of Kgpolarographic examination of the resulting solution yielded [(I\'C)SCOIIIOOCOIII( CK),] . H 2 0 is not an accurate indithe values E'/z = -1.63 and id/(Cm2/8t1/6) = 2.20. cation of the reaction stoichiometry, as indicated above. I n an experiment designed t o duplicate the conditions A4more accurate estimation of the yield of [(NC),CoIIIemployed by Hume and Kolthoff more closely, the Fe- OOCoIII(Cl;),] -6 was obtained by oxygenation of the pentacyano complex of Co(II), followed by acidification, (CN)6-3 was added t o the Co(I1) solution in four portions, with a 2 minute wait before each addition. Ell2 was again and iodometric analysis of the hydrogen peroxide formed. All measurements were carried out a t a cyanide t o Co(I1) -1.63, but i d / ( ~ m ' / l t ' /=~ )2.51. An estimate of the equilibrium constant of reaction 5 , ratio of 5 and a Co(11) concentration of 0.1 M . At 0" the one electron oxidation of [(SC)jFeIICr\'CoII1(CN),) -6 and with rapid bubbling of oxygen or air through the soluby Fe( C X ) C -was ~ obtained spectrophotometrically by com- tion, the yields of [(XC)jCoIIIOOCoIII(CK)jl-6 were 90 paring the spectrum of a solution containing stoichiometric and 86%, respectively. When the reaction was carried amounts of the reactants with the spectra expected for no out a t 25", the yields were 68 and 627,, respectively, with reaction and for complete reaction. The value 0.1 ob- rapid bubbling of oxygen and air. n'hen the reaction was tained from small differences between optical densities, carried out with air a t 25", using a gas buret designed t o gives only the order of magnitude of the equilibrium con- regulate the rate of admission of air to the Co(I1) solution, stant. the yield of [(KC)sCoII100CoI11(CN),] -6 was decreased to Preparation of &[( NC)jCoIIIOOCo111(CN),] .H20 .-When 0% by slow admission of air. Under these conditions, the oxygen was rapidly bubbled through a solution 0.4 M in Co- total gas absorption corresponded t o that required for (11) chloride or nitrate and 2.1 M in potassium cyanide a t quantitative formation of (XC)sCoOH2-- according t o O " , the reddish brown color'2 immediately changed t o a equation 3 but only to one-half of that required for prodarker brown indicating a rapid oxygenation reaction. duction of [( S C ) , C o ~ ~ ~ O OC?;)\ij] C o ~ -6 ~ ~according ( to equaAfter bubbling oxygen through the solution for a period of tion 6 given below. Spectrophotometric analysis of the five t o ten minutes, a time probably in excess for complete 4 CO(CN),-~ O2 6H20 = 4(NC)$oH?-40Hoxygenation, the brown crystalline compound K6[(KC),CoIIIOOCoIII( CX)j].H20 was obtained by adding t o the (3) solution an equal volume of ethanol cooled to 0". T o resulting solution indicated that (SC)5CoOH?-- was the induce crystallization and avoid formation of a n oil, it mas sole product under these conditions of intentionally slow necessary t o scratch the walls of the beaker with a glass oxygenation. stirring rod. The solid then was filtered out, washed with Acid-base Properties of [( NC)5CoIIIOOCoIII(CN),] -6.ethanol and diethyl ether and dried in a vacuum desiccator Aqueous solutions of [( XC)jCoI1IOOCoIII( CS),] - 6 were over sulfuric acid. The yield of Ks[(PiC)iCoIIIOOCoIIIfound t o be alkaline, suggesting that the complex is the (CN)j].HzO was 6 0 7 ~but , it was evident from the color of conjugate base of a rather weak acid. \Then pH titrations the mother liquor that precipitation of the highly soluble of [ ( N C ) ~ C O ~ ~ ~ O OCS),] rapidly carried out a t C O I-6' ~ were ( salt was far from complete. room temperature or lower and no more than about 0.90 Anal. Calcd. for Kg[(NC)jCor1100C0111(c~)~]~H20: C, equivalents of acid per mole of complex was added, one 18.12; X, 21.14; H , 0.30; Co, 17.79. Found: C, 18.06, proton was added reversibly to the complex, as evidenced 18.10; K,21.00,21.16; H , 0.44,0.30; Co, 17.60, 17.75. by the reversibility of back titrations with alkali. Cnder The barium salt of [( XC)jCoIIIOOCoIII( CX),] -6 was these conditions, reaction 2 described above occurred to obtained a s a dark orange powder when barium cyanide a negligible extent. The pK of the conjugate acid of [ ( S C ) S C ~ ~ ~ ~ O O C ~-6, ~ ~ measured I ( C N ) as ~ ~ the p H after was used instead of potassium cyanide in the preparation described above. The yie!d calculated on the basis of the addition of 0.5 equivalent of acid per mole of complex, was 10.5. formula Bas [( NC)jCoIIIOOCoIII( CS),] .3Hz0 was 85%. When more than about 0.90 equivalent of acid was The formula of the barium salt was assigned by iodometric -6, the analysis of the peroxide content as described below, the added t o a solution of [(n'C)~ColllOOCoill(CIT)s] rate of decomposition according t o reaction 2 became so state of hydration being uncertain by one water molecule. The presence of a peroxo ligand in the complex Ke[(NC)5- rapid that it was impossible t o carry out a reversible titraC o ~ I ~ O O C oCN),] ~ I ~ (.H20 was best demonstrated by the tion, even a t 0 " . The p H measured immediately after formation of hydrogen peroxide upon acidification of an adding a portion of acid drifted t o higher values a t an initial aqueous solution of the complex, according t o equation 2 . rate of one PH unit per minute and then much slower, indicating irreversible consumption of acid.I4 ..\fter addition of [ ( X C ) ~ C O ~ I I O O C ~ ' ~ ' ( -C6 K ) ~2]H + f 2H20 = H202 2(SC)sCoOH2-(2) (13) L. C. Smith, J. Kleinberg and E. Griswold, J. A m . Chrm. SOC., Evidence for the formation of (NC),CoOH2-- will be con- 75, 449 (1953). (14) Smith, Kleinberg and Griswold'3 reported t h a t titration wlth sidered below. Hydrogen peroxide was detected by qualitative tests with dichromate ion. I n quantitative studies, acid of t h e compound which they formulated a s (NC)aCoOH-3 and which we have correctly identified as [ (~C)nCotIIOOCoI"(CN)sl -6, excess iodide ion was added t o an acidified solution of yielded a titration curve with two inflection p?ints. A s shown above, &[( Sc)scollloocO1ll(CN),] .€LO and the iodine formed only about 0.9 equivalent of acid can be added reversibly t o one mole was titrated with thiosulfate, using starch as indicator. C N ) ~ ]addition of acid results in As expected from equation 2, 0.97 =k 0.03 mole of iodine of [ ( N C ) S C O ~ ~ ~ O O C O *-~8 ; ~ (further
+ +
+
+
+
(12) T h e color of Co(I1) cyanide solutions was found t o be dependent upon concentration and temperature. Solutions 0.1 M in Co(I1) and 0.5 M in cvanide were reddish brown a t 0' and olive green a t 2 5 ' ; solutions 0.1 M in Co(I1) and 5.0 M in cyanide were also reddish brown a t Oo b u t yellowish green a t 2.5'.
hydrolysis according t o equation 2 . A possible explanation for the results of Smith, Kleinberg and Griswold can be found in the extreme rapidity of the hydrolysis of [ ( ~ C ) ~ C O " ' O O C O ~ ~ ~-6.( C T Nh)e~acid ] added t o the solution of [(~C)sCo"'OOCoT1'(CN)sl -6 is rapidly consumed, with t h e plausible consequence t h a t the pH iersus added acid curve closely resembles a p H titration curve.
V O ~83 .
ALBERTHAIMAND 11'. I
