Gory 1. Olson
Stanford University Stanford, California 94305
Synthesis and Reactions of Cobalt Complexes A laboratory experiment
A
laboratory course in introductory college chemistry should provide experiments which give some insight into the spirit of chemical investigation as well as illustrate useful laboratory techniques and basic chemical principles. The experiment described here is intended to fulfill these requirements by the study of a series of reactions employed in the synthesis of a number of coordination compounds of cobalt(I1) and cobalt(111). The cobalt complexes were chosen for this study because each ligand substitution alters the electronic distribution about the cobalt ion and thus creates significant color changes in the products. I n the first of two three-hour laboratory periods, students prepare carbonatopentaammine cobalt (111) nitrate, I, the precursor for subsequent reactions.
A solution of cobaltous nitrate and ammonium carbonate in aqueous ammonia is oxidized by passing oxygen gas through the solution over a period of 1.5-2 hr.' Since the reaction product, I, has an absorption maximum at 510 mr, the progress of the reaction may be monitored spectrophot~metrically~~~ using an aliquot of the reaction mixture diluted with concentrated ammonia (dilution with water causes precipitation of cobalt(I1) hydroxides). I n an attempt to understand something about the deeply colored complex which is formed in the solution prior to oxidation, a series of test tube color comparison reactions are performed. The products compared are produced by adding a source of carbonate ion and ammonia separately, and then in the reverse order to a small amount of cobaltous nitrate solution. When these tests are done carefully, students can observe the formation of [ C O ( N H ~ ) ~ ][Co(NH&I3+ ~+, (by aerial oxidation in the test tube), cobalt(I1) hydroxide, and cobaIt(I1) carbonate. The students were asked to propose structures for these compounds. The important point which they must recognize in their proposals is that they have evidence, and not proof for the existence of certain complexes. By considering possible alternatives, however, the most likely structure can be assigned. I n one reaction for example, a single drop of ammonia is added to the cobaltous nitrate solution, and a solid immediately precipitates. Upon adding more ammonia, the solid dissolves to give an amber solution. The most reasonable explanation for these observations is that the solid which precipitates is 508
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cobalt(I1) hydroxide, formed because of the change in pH of the solution. Further addition of ammonia then produces a souble ammonia-cobalt(I1) complex which may be hexaammine, aquopentaamine, diaquotetraammine, etc. Given the information4 that 11, the peroxobridged species H3N
NHs I NH,
HaN,
I
,NHa
HaN' 1 \ 0 4 " I\m. NHs NHa
I1
is an intermediate in the oxidation of cobalt(I1) to cobalt(II1) in ammonia solution, the most likely complexes which can be suggested are the hexaammine and the aquopentaammine. Since cobalt(I1) complexes are unusually labile," the solution probably contains an equilibrium mixture of the various species. On the basis of all of their observations, students were then asked to suggest a structure for the darkcolored complex they were oxidizing on large scale, and to write a pathway for its formation and its conversion to product. The students were given the additional information that the decomposition of the peroxobridged compound, 11,to products is accelerated by the addition of chloride ion or carbonate (Evidently these ligands increase the rate of 0-0 bond cleavage or assist in displacing hydroxide ion or water after 0-0 bond scission.) The success with which some students assembled the information irito reasonable proposals showed that they were capable of handling the problem with a minimum of theoretical introduction and a significant amount of deductive reasoning. Several students even suggested isotopic labelling experiments which would demonstrate if any oxygen in the carbonate ligand of I could have come from the oxygen used in the oxidation. I n the second week of the experiment, the complex I is treated as diagrammed in Figure 1. Complex I is pink, and the formation of the darker iodide, 111,with excess potassium iodide1 illustrates the law of mass action (use of excess I- to force the equilibrium in the desired direction) and the ideas of solu'The proeedme used here was adapted from that given by WERNER and GOSLINGS, Bel.., 36, 2380 (1903), and Inorganic Syntheses, 4, p. 171. A Bausch and Lamb Spectronic 20 was used for this purpose. * Following the rate of appearance of the product is an inters6 ing extension of the experiment, although unnecessary since the oxidation is complete in less than 2 hr. R . C., "Coordination ChemisBnsom, F., AND JOHNSON, try," W. A. Benjamin h e . , New York, 1964, pp. 01-92, and SCHAEFER, W., Imrg. Chem., 7, 725 (1SGR).
COBALTOUS NITRATE
(NHdzCO,
(SOLUTIONS I)
*
NH3
(SOLUTION 2)
:
A
l drop
IJI Figure 1.
5 ml Reactions of the [Co(NHalsC031+complex.
bility behavior (smaller anions form more soluble salts with the same cation). The reaction of I with acid' liberates COz with the overall substitution of HzO to form the aquopentaammine IV. When the same reaction is carried out in the presence of nitrite ion, the two complexes VI and VII are formed. These reactions show that under the conditions of the experiment? the strength with which the ligands are bound decreases in the order NO2-, HiO, NO%-. An interesting point is that although the NHa ligand to cobalt(II1) is not replaced in the scheme, it is known from spectral evidence6 to be more weakly bound than NO,-. The observed stability is due to the difficulty of removing the second ligand to form the tetraammine. I n the formation of the complexes VI and VII, two advanced ~ r i n c i ~ l ecan s be illustrated. Careful observation of the reaction mixture after the evolution of COZshows that it contains a pale pink solid which becomes yellow upon standing about 10 min a t room temperature. This color change is due to the isomerization of the initially formed, oxygen bound ligand of the NOz- group to the nitrogen bound ligand.' Thus the definition of isomers and isomerization can be introduced here, and a fairly detailed discussion of kinetic and thermodynamic control of reaction pathways may be presented. I n the last reaction, the aquopentaammine IV is heated in vacw to remove water. The vacant coordination site is then occupied by a nitrate ion from the crystal lattice to form the nitratopentaammine, V. Experimental Carbonatopentaammine cobalt(ll1) nitrate, I. Ammonium earbonate powder (150 g) and hot water (150 d ) are mixed in rt 1-1 erlenmeyer flask, 250 ml of concentrated aqueous ammonia is added, and the mixture swirled until dissolved. To the solution is added 100 g of cobaltous nitrate (Co(NOa)r6H.0) dissolved in 50 ml of hot water. Oxygen gass is bubbled through the solution using a gas dispersion tuhe a t s. moderate rate which permits both saturation of the solution with oxygen and mild agitation. If the reactiau cannot he carried out in a hood, ammonia vapon may he carried away by means of a tuhe connected to a water aspirator.* After 1.5-2 hr, the solution is transferred to an 800 ml beaker and placed in a refrigerator (near 0°C) until the following laboratory period. The crystalline product is then collected on a Biichner funnel, washed with 2.5 ml of ice water, 50 ml of acetone, pressed dry with a. rubber dam," and partially dried in air. Abotlt 45 g of dark red crystds are obtained. Carbonalopntaammine cobalt(II1) iodide, 111. Five grams of earhonatopentsammine eohalt(II1) nitrate, I , is dissolved in 20 ml of water with heating to 8C-9O0C. Powdered potassium iodide (6.3 g) is ndded and the mixture stirred until all of the salid is dissolved. The iodide salt, 111, crystallizes from the solution upon coalingin ice for a. few minutes. The reddish brown produet is collected using a Bixhner funnel, washed with 20 d of acetone. and air dried.
E
K2C03
NH3 (SOLUTION 3 ) Figure 2. Cobdtour nitrate test reactions,
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F
Aquopenlaammine cobolt(I1I) nitrate, IV. Five grams of carhonatopentaamine cohalt(II1) nitrate, I, is stirred in 12 ml of water darine the addition of 10 ml of 6 N nitric acid. When the COS evolution is complete (10 min) the mixture is diluted with 50 ml of methanol and the light-pink product, IV, collected, washed with acetone, and air dried. Nilrito- and nitropataammine cobalt(ll1) ni(mles, VI and VII. Five grams of earbonatopentaamine cohalt(II1) nitrate, I, and 5 g of sodium nitrite are mixed with 12 ml of water and 5 ml of 6 N nitric acid is added slowly and cautiously with stirring. The reaction should be done in a. hood or with an aspirator to remove the NOp fumes. As soon as the CO. evolution has stopped, the suspended salid is the pink, oxygen bound nitrito complex VI which isomeriaes to the stable, nitrogen-hound nitro complex VII when allowed to stand for 10-16 min a t room temperatme. The suspension is diluted with 100 ml of methanol and the yellow solid VII collected as before. Nilratopenlaammine coball(lI1) nilrate, V. About 0.5 g of aquopentaammine eobalt(II1) nitrate, IV, is placed in a heavywalled test tuhe connected to aspirator vacuum through a trap. The tube is heated under good vacuum in a beaker of boiling water with occasional shaking over a period of 20 min, or until constant weight is achieved. The color of the complex gradually changes to a deep red as the salid V is formed and water is lost. Cobalt(I1) nitrate test readions (Fig. &). One gram of cobaltous nitrate is dissolved in 10 ml of water and the solution divided into equal portions in three test tubes (solutions I). Ammonium carbonate (0.5 g) is dissolved in 5 ml of water and 5 ml of ammonia (solution 11), and 0.5 g of potassium carbonate is dissolved in 10 ml of water (solution 111). The reagents are combined as indicated in Figure 2. A is a solotion having the light magenta color of the preparative reaction mixture before oxidation; B is the pale lavender solid, cobalt (11) carbonate; C is tho same as A; D is eobalt(I1) hydroxide, a blue precipitate which dissolvm in excess ammonia to form E, the labile eobalt(I1) ammine eomplex. E undergoes a slight color change upon adding carbonate and producing F, which has the same appearance as C and A.
Conclusion
One of the goals of modern chemistry is the elucidation of the pathway of chemical reactions and the deSince water is the solvent for these reactions, the fact that the aquopentaammine IV is formed in preference to the nitratopentaammine V may reflect the effect of the 55 Af caneentrrttion of water. Spectral studies (see footnote 6) have shown that the aqua ligand is more strongly bound to cobalt(II1) than the nitrato ligand. ' COTTON,F. A,, A N D WILEKINSON, G., "Advanced Inorganic Chemistry," Interscience Publishers (division of John Wiley & Sons Inc.). New York. 1962. D. 579. B A ~ O L OAND JOHNSON, p. 78. 8 Although air may be used for the oxidation (see footnote 1) the reaction time is substantially increased to 10-12 hr. We found that 100 such oxidatiuns required five 200 it3 cylinders of oxygen. ' T h e only time that an aspirator is inadequate is when the ammonia is measured and transferred to the large flask. lo FIESER, L., AND FIESER, M., "Reagenb for Organic Synthesis," John Wiley &Sons, Inc., New York, 1967, p. 488.
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velopment of qualitative relationships upon which the behavior of molecules mav be nredicted. A freshman chemistry laboratory course can provide the inspiration to pursue the field of chemical investigation only if aspects of current research problems and methods of scientific reasoning are demonstrated in its experiments and by its instruction. The experiment described in this paper can fulfill these requirements by introducing first-year chemistry students to some of
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the thought processes, technique, and spirit of modern chemicalinvesti~ation.
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Acknowledgmenl
I would like to thank Drs. R. H. Eastman, P. F. Linde, and C. Hamilton for permission to include this experiment in the Stanford University general chemistry laboratory curriculum and for their helpful comments.
