Cobalt(lll) Ammines-"Werner"
Complexes
An Undergraduate Experiment Anthony M. Greenaway and Robert J. Lancashire University of the West Indies, Mona, Kingston 7, Jamaica The history of modem coordination chemistry can be traced back to 1893 (.1 ).. for it was in that vear that a 26-vear-old unsalaried, university lecturer, with perhaps less than three vearsawareness of the field and a~rtainlvlittleewerimentnl background, proposed a revolutionary new theory. The man was Alfred Werner and the theory concerned the octahedral configuration of transition metal ion complexes. According to Werner's most famous student, Paul Pfeiffer, "The inspiration came to him like a flash. One morning at two o'clock he awoke with a start; the long sought solution of this problem had lodged in his brain. He arose from bed and hy five o'clock in the afternoon, the essential points of the coordination theory were achieved" (2). The salient feature of the new theory was the number of isomers, hoth geometric and optical, that was predicted for different configurations. Only the octahedral configuration led to the prediction of optical isomers and the correct number of geometric isomers. For compounds of the type [M(AA)s], where AA is a hidentate chelating agent capable of spanning only cis-positions, 2 optical isomers should be possible. I t was not until 1911 that Werner and the American, King (3), eventually succeeded in resolving a coordination compound, and even then his views still were accepted only partially. The generally accepted view at that time always associated optical activity with the presence of chiral carbon atoms and a number of Werner's contemporaries argued that optical activity was somehow due to the organic groups present, such as 1,2-diaminoethane, even though these ligands contained no chiral carbon atoms. Finally, in 1914, when Werner (4) resolved the completely inorganic compound, I,
r
dodecaammine-p-hexol-tetracobalt(II1)bromide, even his most skeptical opponents were satisfied; and the octahedral configuration of the cobalt(II1) ion was established unequivocallv. Ironicallv. this comnound. which daved such an important role in-the development of ~ e i n e r ' scoordination theorv. was first ~ r e ~ a r bv e dJorzensen (5). the foremost exponent of the rival ~ l o m s t r a n dchain theory! In this experiment, the student is asked to prepare the compound now associated with Werner's success. (Note, however, that he received the Nobel Prize in Chemistry 1913, before this work was published.) The experiment was designed with several factors in mind. These include: (.8 .) eivine ..an historical introduction to coordination chemistrv:.. (I,) drawing i t ~ ~ d e n t s ' ~ t r m r i ~ ~f~w~r ~that t o ropti~alactivilv hr nrcd nut unly hr n s r u ~atrdwith organic rnolwulci, dr5pitr rhr nurnhrr of examples used in textbooks; (c) con~iderstionsof cost and availability of materials. The students carry out a sequence of preparations where the final yield depends on the success of each step. The sequence we have used is ~~~
0
--
-
CoS04 [Co(NH3)&0&30~.3HzO cis-[Ca(NH~)n(HzO)z]~(SO~)3.3H~O "hexol'' ((SO&. 9Hz0 The oxidation of cobalt sulfate to [Co(NH3)&03]zS04. 3Hz0 can be done by sucking air through the solution for 2 hr, or more conveniently, using HzOz. (Although this is more costly, i t allows the experiment to he completed easily in one session.) The conversion of the his-aqua complex to "hexol" given here is the method originally used by Jorgensen (5). Werner et al. ( 6 )added pyridine to a hot, dilute acetic acid solution of the his- aqua complex. "Hexol" exists in several hydrated forms (7), all of them dark purple-black crystals. Air-drying the crystals has been found to result in the eunea-hydrate, whereas drying a t 9S0 or over sulfuric acid results in the tetrahydrate. We include the use of infrared spectroscopy to establish that [Co(NH3)&03]+ is not produced during the pathway since the method of its production is similar to that of [Co(NH3)&03]+.This serves to introduce the idea of different modes of coordination of COi-. We also ask our students to examine their products under a microscope to test for homogeneity and crystal habit. I t is also possible to use [Co(NH&C03]+ as starting material for other cobalt(II1) complexes. Thus, we have prepared [Co(NH3)&1I2+from which 0 and N bonded linkage isomers of nitrito and nitropentaammine cohalt(II1) can he obtained (8).These complexes serve as an interesting and instructive general chemistry experiment showing linkage isomerism. (The different modes of coordination can be distinguished readily using infrared spectroscopy, and incomplete conversion is observed by the appearance of absorption bands due to hoth O N 0 and NO*.) We have found that the preparation and infrared studies take a full 6 hr. A second laboratory session can he used fruitfully for additional experiments such as: (a) resolution of "hexol" using potassium antimony1 (+) tartrate (9) and recording the circular dichroism (10)and/or the optical rotatory dispersion spectrum of the resolved product, and (h) chemical analvsis for Co andlor SO?-. At U. W. I. our Advanced inorganic Course is divided into non-transition metal chemistrv in the 2nd vear and transition metal chemistry in the final year, both compulsory for students majoring in chemistry. This experiment has been run with our graduating class of approximately 100 students divided into 3 laboratory groups. We feel that this experiment is a valuable introduction to coordination chemistry, because it incorporates: history, optical activity, differing modes of coordination, and a synthetic pathway of reasonable length. In addition, i t is of sufficient versatility to allow for the introduction of several physical techniques, some of which will he available in most Chemistry Departments throughout the world. Experimental Carbonatotetraamminecobalt(IIZ) sulfate.trihydrate [Co(NHd&OalzSO&.3Hz0 Dissolve 7 g of ammonium carbonate in 20 ml of H20 and add 20 Volume 59
Number 5
May 1982
419
ml of concentrated aqueous NH3. While stirring, pour this solution into a solution containing 5 g of CaSOa in 10 ml H20. Then slowly add 3 ml of a30% Hz02solution. (HandleHzOz carefully. If spilled on t h e skin, wash t h e affected a r e a immediately with water; hydrogen peroxide c a n cause severe skin burns.) Pour the solution into an evaporating dish and concentrate i t over a gas burner in a fumehood to 30-35 ml. Do not allow the solution to boil. Durine the evaooration time add.. in small oortions.. 2 ,e. of (NHainCOs. . - . . .. suction tilrer (uirh uatrrmpirnuw, rhe hot s c h r m ~ n n dthen mol rhet~lrrnce in an ice water hnth. Under sumon, filter off the red product crystals of I(:~I(~~H~)~CO~IZSO~~IHZO. Wash the product in the fiitratlm apparatus using a saturated solution prepared from a small portion of the precipitate. Record the yield and show the product t o a demonstrator. cis-Diaquotetraamminecobalt(1II)sulfate t r i h y d r a t e
~~.
~
~
Theclear solution is then treatid with 20-25 ml of ethanol. added in small pmions. The precipilnte is filter~doff, wnshed w t h X P r erha n d water until free of acid. (Check with l ~ t m wpaper., Finally, ~t
420
Journal of Chemical Education
is dried in air. Record the yield and show the sample t o a demonstrator. Dodecaammine-fi-hexol-tetracobalt(II1) sulfate.enneahydrate [ C O [ ( O H ) Z C O ( N H ~( )S~o~&I . 9Hz0 Two grams of the previously prepared cis-[Co(NH3).,. (HZ0)2]2(S01)3.3H20is dissolved in 25-30 ml of 0.2 M ammonia solution. After 24 hr (leave till next session) purple-black crystals precipitate. These are suction filtered. Record the yield and show the sample t o a demonstrator. Literature Cited (1) Werner, A,, 2.Anorg Chem., 3,267:(1693).trandated by G. B. KaufEman in "Classics in Coordination Chemistry Part 1 : Dover Publications,Ine.. N.Y., 1968, pp. 488. (2) PieiIRr, P., J.CHEM.EDUC.5,1090,(1923). (3) Werner, A. and King, V. L., Chem Berichte, 44, 1887, (19111.Trsnslsted by G . B. Kauffmanin"C1agliesin CmrdinsfionChemisiryPart 1."Dover Publications,Inc., N.Y., 1968, pp. 15913. (4) Werner, A., Chem. Beriehfe, 41.3087, (1914). (51 Jorgen%en.S. M . , Z Anorg Chem.. 16,184,(1898). (6) Werner,A. and Bed, E.,Chem.Berichte,4O,2103, (1907). (7) Kenifman, G. B. and Pinnell. R. P., Inorganic Syntheses V I , 176,(196Ql. (81 Pssr, G, and SuWiffe. H.."Plsctid Inorganic Chemktry,"Chapman and Hall, Second Edition, 1974. (9) Goodwin,H.A..Gyarfas,E.C.,andMellor,D.P.. A u l . J. Cham., 11,426,(19581. (10) Mason,S. F,and Wood,J. W., J. Chsm Sac. Chsm. Comm..209.(1967).
