J. P. Barbier, C. Koppenrtein, and R. Hugel ~niversitdd e Reims Reims, France
The Hydration Isomers of Chromium(lll) Chloride
The following experiment for undergraduate students majoring in chemistry was started two years ago a t our Faculty and found great interest from our students. The experiment takes about eight hours of laboratory work and is a good illustration of lectures in transition metal chemistry on subjects like isomerism and crystal field theory. There iq :i good prirt of prrpur:~rivrand nnalyricd \vork, an wrll 3s use of physirnl mrthods likr nbwrption spectrophotometry, conductibility measurements, etc. The text of the experiment given to the students is reproduced below. Introduction
As well as geometrical and optical isomerisms we find other types like ionization isomerism, coordination isomerism, linkage isomerism, and the kind we want to illustrate in this experiment: hydration isomerism. This type is not very common and only a few examples are given in the literature, they are all complexes of "inert" d S or d6 cations like chromium(II1) or cobalt(111). We find [Co(NH& (HIO)](NO& and [Co(NH3)s NOal (NOa)x.H%O(1) or [Co(NHa)4 (HzO) Cl] Br2 (8) and [Co(NH& Brz] C1.H20 (8) the latter illustrating both hydration and ionization isomerisms. The best known example is given by the hydrated chromium(II1) chlorides (4) such as the dark green dihydrate of dicblorotetraaquochrornium(II1) chloride [Cr(H,O)a CL] (31.2 H 2 0 (A), the green monohydrate
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of chloropentaaquochromium(111) chloride [Cr(HzO)s Cl] C12.H 2 0 (B) and the greyish-blue hexaaquochromium(II1) chloride [Cr(H20)6]C13 (C). Isomer (A) is the commercial product, the two ot,hers are prepared in the experiment. Experimental Procedure
Preporofion of [Cr(MO)6Cl]C12~Hz0 The method given by Pass and Sutcliffe is used (5). Coal 300 ml of diethylether in ice and satmate i t with a stream of dry hydrogen chloride (obtained by reaction of sulfuric acid on sodium chloride). Dissolve 14 g of diehlorotetra~quochromium(II1) chloride in 20 ml water and reflux for 10 min. Cool the solution in ice and saturate with a stream of hydrogen chloride gas. Pour this salution into 200 ml of the hydrogen chloride solution in ether. Stir the mixture mechanically for ahout 30 min on the ice bath. The crystals are removed by filtration and washed with the remaining 100 ml of the ether saturated with hydrogen chloride. Dry on filter paper. NOTE: I t is necessary to work under s. HOOD. The excess of hydrogen chloride produced by the generating system can be absorbed in placing the outlet tube of the systemin alarge volume (11) of aooncentrrtted (5 M ) sodium hydroxide solution.
Preparation of [Cr(f&O)e]CI, Prepare a. solution of 10 g of chrome alum, KCr(SOI)2.12H%0, in 40 ml of eoncentraied hydrogen chloride and 10 ml of water. Cool in ice. Filter the so1ut;on and pour into a washing bottle cooled with ice. Saturate the solution with hydrogen chloride gas (care should be taken to avoid obstruction of t,he gas inlet tubing with crystals), until the solution is practically colorless. Filter off the greyish-blue cryst&. Wash with acetone and ether and dry on filter paper.
Conductances (in Microsiemens or Micro MHO) Concentration
KC1
BaCL
K,Fe(CN)I
A
B
C
10-=M 10-'M
1230 133
2150 250
3400 420
870 120
2020 270
2860 395
Spectra (8) Weigh 0.5 g of isomer for 50 ml of solution. Use 1 cm quartr. cells. Record the spectra for isomers (A) and (B) from 350 to 750 nm and for isomer (C) from 200 to 750 nm. For isomer (A) the spectrum is recorded every 20 min, and the evolution of the spectrum is followed for 1 hr. Questions
Recorded spedro for the three isomers; [ A ) -lid line, (81 dol-dash line, and D third bond at 262 nm vitn r = 5. The svolvtion of the spectrum of isomer (A) h given for time intenoh of 20
(C)broken line. For isomer (C)there h
Determinotion of the Amount of Ionic Chloride and the Total Amount of Chloride in the Isomers The chloride ions present in the solution of each isomer are titrated with mercuric nitrate using sodium nitmprusside Nar [Fe(CN)sNO] as an indicator (6). Mercuric halides (except the fluoride) are undissoeiated in solution whereas salts with oxoanions are wmpletely dissociated. I n addition of mercuric nitrate to a solution of a metallic chloride we have the reaction Hgl+
+ 2 C1-
-
HgC5 (undissoeiated)
After complexation of all the chloride ions by Hg'+ the first excess of mercuric nitrate gives a white precipitate due to the fonns, tion of the insoluble mercuric nitroprusside Hg[Fe(CN)sNOl. For each isomer weigh about 100 mg. Add 20 rnl of cold water, two drops of concentrated nitric acid and two drops of a 10% aqueous solution of sodium nitroprusside. A 0.02 M solution of mercuric nitrate is used for titration. To prevent transformation of (A) and (B) into isomer (C), PIP pare the solution just before titration, cool in ice, and titrate as fast as possible. The total amount of chloride in each isomer is obtained in the same way after boiling 10 ml of each solution for 10 min. REMARK: The amount of chromium in the isomers is titrated in another experiment by potentiometry as bichromate and by colorimetry as chromate ions (7). Conductance Measurements The conductance measurements are a way to reach the number of ions present in solution for the three isomers in comparison of typical 1-1, 1-2, and 1-3 electrolytes. Aqueous solutions of pctassium chloride, barium chloride, and potassium ferricyanide with wncentrations 0.01 M and 0.001 M are prepared and their wnductanees measured. A 0.01 M solution of each isomer is used, the conductance is measured immediately after preparation. Dilute the solution ten times and measure the conductance. Follow the wnductance a few minutes.
1) Calculate the yields. 2 ) Explain the results obtained from the chloride analysis, the codductance measurements and from the spectra. 3 ) Spectra (a)How many absorption bands would you expect from the energy level diagram for a dl ion? How many did you pet? Explain. (b) Between HzO and C1; which ligand gives the greater d orbital splitting? (e) Discuss the intensity and the form of the actual hands in tenns of symetry of the complexes. (d) What kind of indication can you have from the study of the evolution of the spectra of isomer (A) about the reaction going on and the rate of this reaction.
Results
The yields are around 20% for [Cr(H20)&1]Clv Hz0 and around 85% for [Cr(H20)a]Clr. The conductance measurements for one team of two students is given in the table, and the recorded spectra in the figure. Conclusion
The above experiment has introduced the student to some interesting aspects of transition metal coordination chemistry. The majority of students were able to get good results in the time given for the experiment. Preparation of the pentaaquoisomer ( B ) is one of the difficult points, an improvement can be made in using a HCl gas cylinder instead of the generating system. The questions concern the experimental work done as well as the lectures on coordination chemistry the students heard before the experiment. With the last question a possibility exists for further extension of the work. Literature Cited F.,A N D PDARBON. R.. "Mech&niam8of Inorganic Reactions" (2nd sd.). J.Wiley & Sons, Ino., New York, 1967, p. 13. (2) WERNER.A,. J . P ~ a k tCham.. . 42,215 (1890). (3) Wofie.;na,A., Bsr., 38,994 (1905). , Anorp. Chem.. Chrom No 52. B,228. (4) G r e s r ~Hand. (5) P A ~ RG.. . ANY S U T ~ I F PH.. E , "Pri%~tic&L Inorganic Chemistry." C h a p man and Hall,London, 1968, p. 102. (8) Ca~nbor.G.. "Methodes de la ohimia analytique," Maaaon. Paris. 1966, p. 689. (7) C m n ~ oa~p .,dl.. p. 700. (8) D V ~ N T. E , G., J. Cnau. Eono.,44, 101 (1957). (1)
BASOLO.
Volume 49, Number 3, March 1972
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