Complete Characterization of tran~-[Co(en)2C1~]~[Fe(ox)~] 45H20 An Extended Undergraduate Project Involving an Unknown Metal Complex Graham S. Bull and Graeme H. Searle Universlty of Adelaide, Adelaide, South Australia 5001, Australia In a previous article we briefly reviewed project experiments involvine the complete characterization of "unknown" inorganic compouuds ( I ) . In these projects the compounds are of unknown formulation (although the constituents are presumed to be known, either from the synthetic procedure or from information provided), and from the results of various analyses and measurements the formulations and geometrical structures are required to he deduced. Such experiments require a more critical interpretation of experimental results than the more usual experiments involving known compounds and require the integration of results from a variety of techniques. Relatively few such exoeriments seem to be available. and our orevious article descrihed projects with unknown complexes containing the labile comolex inns ICnC1,I2- and ICuCLl' . We no; describe'an eGended project of the same type employing the combination complex compound trans[Co(en)2Cl2]3[Fe(ox)3] 4%H20 (en = ethane-1,2-diamine, - ) , has been specifically devised. This comox = C Z O ~ ~which pound: denoted X, contains six dift'e;ent components in both inert and lahile contplex ions, and it thus provides for a variety of measuremmts. The individual experiments in the proiect include ion-exchangr separarions of the component ioniplex ions, qualitative chemical tests for the components in each complex ion, analyses for all the components, and various nhvsical measurements. The individual oarent comand ~ ~ [ ~ e ( o..Hz0 x j ~ ](2, plexes trans-[~o(en)~~l~]~1(2-4) 5-10) have been popular in undergraduate synthesis and characterization experiments, and the new project incorporates some of these characterizations. Our full projrct is assigned to small numhers of our advanrrd students (final yrar HSr) working in pairs, and requires two full days. Thecompound X i s prepared by lahoratory staff, and th? studenu are provided with samples as an "unknown", though with the six components specified. The two students apportion the various separate experiments and collate and interpret the results collaboratively. They are required t o show how the formulation and structure of X are deduced from the exoerimeutal results and to account for and explain all their bhservations and quantitative results from the various determinations on the basis of the deduced structure.
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Information Provlded About 5.0 g of comoound X is provided for each determination (studint air): T h r com~bnentsare given as Fe, Co, CI, ox", en, and H20, hut the oxidation states of the metals are not specified. The proportion of hydrate water (as determined by weight loss at 100 OC) is provided (6.9% wlw) to avoid any uncertainty in the water mole ratio which is nouintegral (H20/Fe = 4.5). The compound X is stated to be moderatelv soluble in water and conductine. indicatine it to be "ionic': This and the presence of the t& metals sLould sueeest the likelihood of two different c o m ~ l e xions in X. This problem is more complicated than previous "unknowns", so that we prescrihe the various experiments to he carried out and give some guidance for the methods and quantities of materials to be used.
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Journal of Chemical Education
In our instructions i t is suggested that the following characteristics for the compound he considered and that the various possibilities he progressively limited as the experiments proceed. 1) The likely oxidation states for the two metals. (These are Limited
to +2 and +3 with the possible Ligands). 2) The possibilities of high- and low-spin configurations for each metal ion.
3) l'hr po~siblenurnhers of unpaired el~ctrons. 4 ) The posrilditier of the complex ions hemg inert or labile. 51 How,ronsequently.X might observably renrr with acid or alkali.
Knowledge of the empirical formula and formula weight are required for interpreting the tests and calculating some of the results (conductance.. maenetic moment. and ion exchange), so that it is advantageous that the'analyses be com~letedfirst. Students may be unfamiliar with multicomplex-ion compounds so that we indicate the formula weight range (1000-1400). However, the correct formula weight should emerge without difficulty from the analysis results, even allowing for errors of about 5% in each analysis. The project involves the following 10 experiments.
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1) Separatlon of the Component ions Uslng Ion Exchange Separate solutions of X (0.1 g in 10 mL of water) are passed through small columns of cation-exchange resin (Dowex 50W-X2, 200-400 mesh, Na+ form; regenerate the resin with 1 M NaOH, then wash until the effluent is neutral) and anion-exchange resin (Amberlite IRA-400, NO3form; regenerate the resin with concentrated H N 0 3 until free of C1-, then wash until the effluent is neutral). Columns 1.2 cm diam X 4 cm long of wet-bed resins are suitable. The respective effluents contain Na3[Fe(ox)s] (pale green-yellow) and trans-[Co(en)zC12]N03 (deep green), and are collected for measurements (2) and (3) below. 2) Vlslble Spectra The visible spectra of these solutions are measured as obtained from the columns (neutral solutions) and are remeasured after acidification a s a test for lability (afew drops of concentrated H N 0 3 are added to each solution cell). The spectrum of the green complex cation shows onevisible band a t 617 nm, and this is essentially unchanged with HN03, suggesting an inert complex (Co(II1)). The spectrum of the oale ereen-vellow comolex anion shows no hands in the visible, and the color is evidently due to charge-transfer absorption extending into the visible with a shoulder a t 455 nm. With H N 0 3 the color becomes more intense yellow immediately, indicating a labile complex, and the shoulder disappears. (The absorbing species is now essentially Feaq3+). These characteristics might suggest that the anion is a d5 high-spin system, and hence is Fe(II1). 3) Qualitative Tests to Determine the Components in Each Ion The following qualitative tests should provide information on the disoositions of Fe. Co. ox2-, and C1- hetween the two complex ions, on the oxidation states of the metals, and on the labile or inert character of each complex ion.
Add NaOH. Red-brown,dirty green, or blue precipitates indicate Fe(III), Fe(II), or Co(II),respectively. No precipitate suggests an inert complex. Fdl0 and/or o$ Add HISO,. A n immediate color change indicates a lahile cum-
plex. Warm and add KMnO, dropwisp, when decolorination indicates Frtll) andlor 0x2-, and C02huhhles indicate ox2-. Oxalate in the Presence of Fe(l0
Add NaOH, and filter off any hydroxide precipitate. To the filtrate add HzSOa and KMnOd. Decolorization and COz bubbles indicate ox2-. Chlorlde
Add HNOx.. (an . immediate color chanee indicates a labile eomplev and AgNO,. An immediate white precipitate shows Weither ionic or fnm a lalde complex. A precipitate appearing on warming uf C1- from an inert complen. suggrsls hvdrolv~i~ Oxalate ff Chloride is Absent
Add AgN03. An immediate white precipitate of Agzoxshows 0x2either ionic or from a lahile complex. Agzox is confirmed by its solubility in HN03. Fe(lN), Fdl0, or Call)
Add HzSOI, then KSCN. A deep red color shows Fe(III), confirmed by decolarizatian with SnClz; no color suggests Fe(II), confirmed by the temporary appearance of red Fe(II1)with HzOz;apale pink-mauve color suggests Co(l1). Add H2SOCthen KSCN. A deep red color shows Fe(III),which is confirmed by decolorization on adding NHIF solid. can)
Add H2SOs then KSCN solid. A mauve solution indicates Co(I1). (Add NHaF solid to remove Fe(II1) if present). Add amyl alcohol, when deep blue Co(II) extracts into the alcohol layer. CdNI), if Co(l0 is Absent
Add H&Oc then SnCln and warm. A gradual change to pink suggests Co(II1) is being reduced, confirmed on addition of KSCN solid and amyl alcohol. These tests should show that the pale green anion is lahile and contains Fe(II1) and ox2-. The green cation is shown to be inert and to contain Co(II1) and coordinated Cl-. This halide hydrolyzes slowly in acid and rapidly with NaOH. 4) Analysls for Coban Cobalt may he determined by atomic absorption a t 304.4 nm. Assume 10-20% wlw cobalt in the compound. 5) Analysis for Chloride
Transfer asample of X (0.2-0.3 g, accurately weighed) to a 250-mL heaker. Add 30 mL of 1M NaOH and antibumoine granules, and heat the solution a t just below boiling for 5 min. Cool, add 30 mL of 2 M HN03, and add water to adjust the volume to about 120 mL. Titrate the solution potentiometrically with standard 0.05 M AgN03, with the solution stirred magnetically.
6) Analysis for Oxalate and Iron A solution of X in water (about 0.6 g in 30 mL) is passed throuah a column (1.2 cm diam X 5 cm lone. wet-bed) of 50W-X2 cation-exchange resin, 200-400 mesh, inthe Na+ form. The cation is retained. and the total effluent olus 'washings (until the effluent becomes colorless) is collected in a titration flask, H2SOl is added (25 mL of 2 M), and the solution is titrated.with standard 0.02 M KMn04. Zinc powder (2 g) is then added t o the flask and contents, and the solution is maintained a t just below hoiling for 10
ow ex
min. After cooling, the solution is filtered through sintered glass (washing with 2 M HzS04). then is titrated again with 0.02 M KMnO*. The first titer corres~ondsto the ox2- content [plus Fe(fI) if it were present] A d the second titer to total Fe, and the theoretical ratio of the titers is 6. Titrations of the 0x2- and Fe(I1) directly on a solution of the total compound proved unreliable due t o color interferences by the complex cation and Co(I1) in the two titrations, res~ectivelv.The above orocedure confirms that the Fe and oxi- are hoih in the com&ex anion. 7) Analysls for Ethane-1,2-dlamlne This may he carried out by decomposing the complex with sodium sulfide, then removing the en in a continuous distillation with toluene, as described previously (11).About 0.3 g of X should he used. The analytical results for all the components may he calculated both as % wlw (to check their total) and as mole of component per gram of X. The mole ratios of the components are then calculated as multiples of the lowest value (Fe = I), and give the empirical formula of the compound as FeCo3Cken~ox3(H20)~ ~,z.Theformula weight can then be calculated (FW = 1151). 8) Conductance
Using the formula weight determined as above, solutions in water ol0.016,0.008,0.004, and 0.001 Mare prepared, and the ronductanres at 25 'C are measured. From a plot of A,. versus (concentration)'/2, the molar conductance it infinite dilution A,0 is estimated, and hence the number of charge units per formula weight is deduced. indicating The Am0ohtained is about 420 ~hrn-'cm~mol-~, six charge units ner formula weieht. This information together with the pievious results eiahles X t o he formulated as [CO(III)]~[F~(III)]~-. 9) Magnetlc Moment
The magnetic moment is determined by the Gouy method usina the calculated formula weieht. ~ 6 p.nohtained, e around 6.0 B~M.,indicates five unpaired electrons per mole, and this is consistent only with Fe(II1) high-spin and Co(II1) low-spin. 10) Displacement of CI- by Anion Exchange
Prepare a column (1.2 cm diam X 5 cm long, wet-bed) of Amherlite IRA-400 anion-exchange resin, C1- form, and wash the column with water until the effluent is free of C1-. Pass a solution of X in water (use 0.15-0.20 e) slowlv through the column, and collect thekffluent and wishings i; a beaker until essentially all displaced C1- has been removed from the column. Hydrolyze this effluent with NaOH (as with the chloride analysis above), then acidify with HN03. Titrate the solution potentiometrically with standard 0.05 M ApNOx. Determine the number of moles of CI- disolaced per mole of compound, and interpret this result. Nine moles of CI- per mole of compound are displaced (experimental 8.3). ~ h e s arise e as three displaced by the 3anion and six hydrolyzed from the green cation that had passed through the column unchanged. Evidently the [Fe(0x)~]3-ion has high stability' since it does not appreciahlv dissociate on the column: such dissociation would eive u six CI- displaced by three ox; dissociated. All the information obtained allows X to be formulated as . 4SH20. Finally, the isomer of the [C0(en)~C1~]~[Fe(ox)~] cobalt(II1) complex cation is identified as trans by its green color.
' Ka is about 4, B3 about 20. Volume 63
Number 10
October 1986
903
This is a challenging project, in which students have to adapt their experience with measurements on simpler or known complexes previouslv to a new situation. It also draws on students' knowledge of the various features and types of behavior of transition metal complexes, such as high- and low-spin configurations, labile and inert complexes, and thermodynamic stability, and the various possibilities are required to be matched with the experimental results ob-
tained .-- --- -.
Other educative features of the experiment include the following. The students can appreciate in a first-hand manner how a variety of measurements may have to he applied t o a prohlem if i t is "unknown", as in real research situations. The necessity for such internation of measurements mav not he evident in the more usual undergraduate experiments with inorganic compounds (2,6,12.13) in which svnthesized or "known" comp&nds are characterized only by selected analyses or measurements. The usefulness of ion-exchange operations in manipulating complexes in aqueous solution, a technique infrequently incorporated in experiments in the texts (14,15), is well demonstrated in this project. The necessary role of chemical tests (2, 6, 12) in investigating unknown compounds is also made apparent, whereas contemporary courses seem to have decreased emphasis on qualitative analvsis. The pioject has been developed to provide the maximum benefit in its "unknown" format. However. i t could be abbreviated if the formula or structure of the'compound were partly or wholly provided, and students could then be required to demonstrate how the results of selected measurements are consistent with the information eiven. Such an abbreviated format could provide instructivi and interesting work for less advanced students. Extended projects of this same type could be devised with other combination complex compounds. Some readily ohtained complex ions, all with simple ligands, that could he incorporated are the following: [Ni(en)J2+, [Al(o~)~]3-, [Cu(ox)z]- and [ZnCI4l2- (all labile), and [C0(en)~]3+, [Co(NH&I3+, [Co(en)2oxIf, [Co(en)zC03]+, c i s ~Co(en)dN02)21+,[Co(NHds(H20)I3+, [Co(edta)]-, [Co(N02)~l3-and [Cr(o~!~]3-(inert). Ease of preparation and reasonable solubility ~nwater are requirements for suitable complex compounds.
dull green powder (16, 17). This product contains a little cobalt(II),3and this should be removed to avoid the possible separation of cobalt(I1) oxalate4 during the subsequent preparation of the mixed complex. The cobalt(I1) is extracted5 by stirring a suspension of trans-[C~(en)~Cl~]CI in ethanol. then filterine off the comalex. . . washine with acetone. and air-drying. Solutions are prepared of tran~-[Co(en)~Cl~]Cl (10.0 g in 3H20 (4.9 g i n 35 40 mL of ice-cold water) and K~[Fe(ox)~] mLof warm water, 35OC). The solutions are quickly filtered, and the iron(II1) solution is then added to the magnetically stirred cobalt(II1) solution. On cooling the mixture in an ice hath the product separates as fine green crystals. These are filtered off after 10 min,4 washed with portions of ice water (until the filtrate is free of C1-), then with ethanol and acetone and air-dried. Yield 6.2 g, 54%. Analysis found: C, 18.7: H. 4.9: N. 14.5: Cl. 18.4: weieht loss a t 100 'C. 6.9%. [ C O ( C ~ H ~ N ; ) ~ C I ~ ] ~ ~ ~4112~~20 ( C & ) ~requires ]'. C, 18.8; H,. 5.0;N. 14.6:Cl. 18.5:H70.7.0%. t r a n s - [ ~ o ( e & ~ l ~ ] ~ [ ~ e ( 4II2H20 o x ) ~ undergoes no apparent change over some days in subdued light. However, like K3[Fe(ox)3] 3H20, it is photosensitive and gradually changes to light brown with strong light, and the compound should thus he stored in the dark. The compound dehydrates a t 100 "C. I t rehvdrates on standing inthe atmosphere at room temperature;apparently to the 4'2-hydrate,and thin precludes the anhydrous compound being used in this project.
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(1) C u d s , N. F.;Hay,R.W.; House,0. A.:Sedc, 0.H.J. Chem.Edur., 1986,63,899. (2) Adams, 0. M.;Raynor, J. B. '"Advanced Practical Inorganic Chemistry"; Wiles London.1966.
Preparation ol trana[Co(en)2CI2b[Fe(o~)~]4% H20 (X)
(15) Rc113.p63. (16) B a i h , J. C. Inorg Synth. 1946.2.222, (17) Springvrg. J.; Schaffer. C. E. Inorg. Synfh. 1973.14.68. (18) Dwer, F. P. In "Aduan~esin the Chemistry of the Cwrdination Compounds": Kirshner, S., Ed.; Maanillan: New York, 1961: p 21.
Ks[Fe(ox)a] 3H20 is prepared by a modification of a previous method (5).Asolution of Fe(N03)3. 9Hz0 (16.2 g i n 20 mL water, 0.04 mol) is added slowly t o a stirred hot solution of KZOX.Hz0 (2.40 g i n 40 mL water). After cooling in an ice hath the pale green product is filtered off, washed with ice water, ethanol, and acetone, and air-dried.2 Yield 18.6 g, 94%. tran.~-[Co(en)~Cl~]C1-2H~O .HC1 isobtained by oxidation of a mixture of CoC12 6 H 2 0 and en with Hz02 (3.4). The hydrate water and HCI are subsequently removed completely by heating a t 110 OC to give tran~-[Co(en)~Cl~]Cl as a
Product which is not used immediately can be recrystallized from hot water. Cobalt(l1)apparently arises both from me starting CoCI, and from reduction during heating (17, 18). 'If significant cobalt(l1) is present, cobait(l1) oxalate may commence to separate (as a pale pink insoluble powder) if the crystallization of the complex is prolonged. Cobalt(ll)can be tested for by allowinga sample of the compound to hydrolyze in NaOH. The solution is then acidified. and KSCN is added. The solution is shaken wim amyl alcohol to extract and intensifyany blue Co(li)color.
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Journal of Chemical Education
