Synthesis and Reactions of Cobalt Complexes PDF

 

ory

1

Olson

Stanford University Stanford, Californ California ia 943 5

Synthesis Synt hesis and React Reactions ions o f

laboratory course in introductory college chemistry should provide experiments which give some insight into the spirit of che chemic mical al investigation as well as illustrate useful laboratory techniques and basic chemical principles. The experiment described here is intended t o fulfill these requirements by tthe he st study udy o off a series of reactions employed in th e synthesi synthesiss of a number of coordination compounds of cobalt(I cobalt(I1) 1) and cobalt(111 (111). ). Th e cobalt comp complexes lexes were chosen for this stud y because each ligand substitution alters the electronic distribution about the cobalt ion and thus creates significant color changes in the products. I n th e first of two three-h three-hour our laboratory periods, students prepare carbonatopentaammine cobalt (111) nitrate, I the precur precursor sor for subsequent reactions reactions..

A solution of cobaltous ni tr trat ate e and ammonium car-

bonate in aqueous ammonia is oxidized by passing oxygen gas gas thro ugh tthe he solution over a period of of 1.5 1.5-2 -2 hr.' Sin Since ce the reaction product, I has an absorption maximum at 510 mr, the progress of the reaction may be mon monit itor ored ed spectr ophot ~met rica lly~sing ~~ an aliquot of the reaction mixture diluted with concentrated ammonia (dilution with water causes precipitation of cobalt(I1) hydroxides). In 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 react reactions ions are performed. The products c comompared are produced by adding a source of carbonate ion and ammon ammonia ia separately, a nd then in the reve reverse rse order t o a small amount of cobaltous nit rate ra te solution. When these test s a re done carefu carefully, lly, students can observe the fo form rmat atio ion n of [C O ( N H ~ ) ~ ] Co(NH&I3+ ~+, (by aerial oxidation in the test tube), cobalt(I1) hydroxide, and cobaIt(I1) carbonate. The students wer were e asked asked to propose propo se structur es for these comp compounds ounds.. Th e im im-portant point which they must recognize in their proposalss is tha t they have evidence, and not pro posal proof of for the existence existe nce of certai cer tain n complexes. By considering possible alternatives, however, the most likely structure can be assigned assigned.. I n on one e re reaction action for examp example, le, a singl single e drop of of ammonia is added to t he cobaltous nitr ate at e sol soluution , and a solid immediately precip itates itates.. Upon adding more ammonia, the solid dissolves to give an amber solution. The most reasonable exp explanation lanation for these observations is that the solid which precipitates is Journal of Chemical Education

omplexes

A la b o ra to ry e x p e rim e n t

 

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obalt

cobalt(I1) hydroxide, formed because of th e change in p H of the solution. Fur Further ther addit addition ion of ammonia then produces a souble ammonia-cobalt(I1) com comple plex x which may be hexaammine, aquopentaamine, diaquotetraammine, ammin e, etc. Given the information4 th at 11 the peroxobridged species H3N

HaN'

NHs NH,

1

NHs

\

HaN HaN,,

0

4

,,NHa NHa

I\m NHa

I

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) comple complexes xes are unusually labi labile, le, th e solution probably contains an equilibrium mixture of th e various species. On th e b basis asis of all of thei r observations, stu dent s were then asked to suggest a structure for the darkcolored colore d complex the y were oxidizing on large scale, an d to write a pathway for its formation and its conversion to product. product. The students we were re given given the additional information that the decomposition of the peroxobridged compound, 11 to products is accelerated accelerated by the additio n of chloride ion or carbona carbonate te (Evident (Evidently ly these ligands increase the t he ra te o off 0-0 bond cleavage or assist in displacing hydroxide ion or water after 0-0 bond scissi scission. on.)) Th e success success with whic which h som some e stud ents assembled the information irito reasonable proposals showed th at they were capa capable ble of handling handling the problem with a minimum of theor etical introduc tion and a significant amount o off deductive reasoning. Several students even suggested isotopic labelling experiments which would would demonstr ate if a any ny oxygen in th e carbonate ligand of I could have come from the oxygen used in t he oxid oxidation. ation. I n th e second wee week k of the experiment, the com complex plex I is treated as diagrammed in Figure 1 Complex I is pink, pink, and t he formation of the darker iodide, 111 wit with h excess excess potassium iodide1illustr ates t he law of mass action (use of excess I- to force the equilibrium in th e desired direction) a nd the ideas of solu'T h e proeedme used here was adapted fr from om that given by WERNER and GOSLINGS, el.. 36 2380 1903), and Inorganic Syntheses, 4 p . 171. A Bausch and Lamb Spectronic 20 was used for this purpose. Following Followi ng the rate of ap appear pearance ance ooff the product iiss an in te r s 6 ing extension of the experiment, although unnecessary since the oxidation is complete in less than 2 hr. B n s o m , F. A N D J O H NS O N, . C., Coordination Chemistry, W. A . Ben ja ja m in in h e . , New Yo rk rk , 1964 pp. 01-92, and S C H A E F E R ,., ,., Imrg. Chem. 7, 725 1SGR).

 

C O B A LTO U S N I TR A TE

NHd dzC zC O O,,

SOLUTIONS I )

NH

SOLUTION 2

:

A

l dr op  J

Figure

1.

5

Reactions of the [Co NHalsC031+ omplex.

bility behavior (smaller anions form m more ore soluble salt s with th e same catio cation). n). The reaction of I with acid' liberates COz with th e overall overa ll substitution of HzO to form th e aquopentaammine IV. When the same reaction reaction is carried out in th e presence of nitr ite ion, th e two comple complexes xes VI a nd VI I are form formed. ed. These reacti reactions ons ssho how w th at under the conditions of t he experiment? t he stre strength ngth with which the ligands are bound decreases in the order NO2-, HiO, NO -. An interest interesting ing point is th at although the NHa igand to coba lt(II1) is not replaced in t he scheme scheme,, it is known from spectral evidence6 to be more weakly bound th an NO NO,,-.. Th e observed stabil stability ity is due t o the difficulty of remov removing ing th e second ligan ligand d to form the tetraammine. In the formation of the complexes VI and VII, two advanced ~r inci~les a an n be illustrated. illustra ted. Careful obobservation of the reaction mixture after the evolution of COZshows COZ shows that it contains a pale pink solid which becomes yellow upon standing about 10 min a t room temtemperature.. Thi s color change is due to the iso perature isomeriza meriza-tion tio n o off th e initia initially lly formed, oxygen bound lig ligand and of tthe he NOz- group to th e nitrogen bound li liga gand nd.' .' Thu s th e definition of isomers and isomerization can be introduced here, and a fai fairly rly detailed discussion of kinetic and thermodynamic thermodyna mic control of reaction pat pathways hways may be presented. I n th e last rea reacti ction, on, the aquo aquopentaa pentaammin mmine e IV is heated i n vacw to remove remove water. Th e vacant co co-ordination site is then occupied by a nitrate ion from the crystal lattice to form the nitratopentaammine, V. Experimental

ml

E

NH3 Figure

2.

Cobdtour nitrate

test

K 2C 03 SOLUTION 3

-

F

reactions,

Aquopenlaammine cobolt(I1I) nitrate, IV. Five grams of of carhonatopentaamine cohalt(II1) nitrate, I, is stirred in 12 ml of water da rin e the additio n of of 10 ml of of 6 N nitric acid. When the COS evolution is complete (10 min) th e mixtur e is diluted with 50 ml of me thanol a nd t he light-pink light-pink product, IV, collecte collected, d, washed washe d with acetone, and air dried. NilritoNilrit o- and nit ropa taam mine cobalt(ll1) ni(mles, ni(mles, VI and VII. Five grams of earbon atopenta amine cohalt( II1) nitra te, I and 5 g of so sodium dium ni nitr trit itee are mixed with 12 ml o off w ate aterr an d 5 ml of 6 N nit ric acid is added slowly slowly and cautiously with stirrin g. The reaction should be done in a. hood or with a n aspirator to remove remove the NOp fumes. A s soon as the CO. CO. evolution has stopped, t he suspended salid is the pink, oxygen bound nitrito complex VI which isomer isomeriaes iaes to the st able, nitrogen-hou nitrogen-hound nd nitro comp complex lex VII when allowed allowed to stand for 10-1 10-16 6 min a t room temp era tme. The suspension is diluted with 100 ml of methanol and the yellow solid VII collec collected ted as before. Nilratopenlaammine coball( lI1) coball(lI1) nilrat nilrate, e, isV.placed About g of aquopentaammine eobalt(II1) nitrate, IV, in a0.5heavywalled wal led te st tu he connec connected ted to aspirator vacuum 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 cons tan t weight weight is achieved. achieved. The color of the complex graduall y changes to a deep red as the salid V is formed and water is lost. Cobalt(I1) nitrate test readions (Fig. ). One gram of cobalto cobaltous us ni tra te is dissolv dissolved ed in 10 ml of of water and t he solution divided into equal portions portions in three test tubes (solutions I) . Ammonium Ammonium carb onat e (0.5 g) is dissolved in 5 ml o off w ate aterr a nd 5 ml of ammonia (solution (soluti on 11), and 0.5 g of potassium carbonate is dissolved in 10 ml ml o off wat er (solution 111). The reagents a re combined combined as indicated in Figure 2. A is a solotion having the light magenta color of t he preparative reaction mixture before oxidation; B is th e pale lavender solid, solid, cobalt cobalt (11) carbonat e; C is tho same as D A; is eobalt(I1) hydroxide, a blue precipita te 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, whi which ch has t he same appearance as C an d A.

Carbonatopentaammine cobalt(ll1) nitrate, I. Ammonium earbonate po bonate powd wder er (150 (150 g) and hot water water (150 d ) re mix mixed ed in t 1-1 erlenmeyer flask, 250 ml of of concent concentrated rated aq aqueous ueous ammonia ammon ia is added, and th e mixture swi swirled rled until dissolved. dissolved. To the solution is added 100 g of cobaltous nit rat e (Co(NOa )r6H. 0) dis dissolve solved d in 50 ml of hot ho t water. Oxygen gass gass is bubbled throug th rough h the solution using a gas dispersion tuhe at s moderate rate which permits both sat uration of t he solution with oxyg oxygen en and mild agitation. If th e reactiau cannot he carrie carried d o ut in a hood, ammonia vapon may h e carried carried aw ay by means of of a tuh e connected connected t o a water aspirator.* After 1.5 1.5-2 -2 hr, th e solution is transferr ed to an 800 ml beaker and placed in a refrigerator (near 0°C) until th e follow follow-ing laborato ry period. period. Th e crystall ine product is then co collected llected on a Biichner funnel, washed with 2.5 ml of ice water wa ter,, 50 ml of acetone, pressed dry with a. rubber dam, and partially dried in air. Abot Abotlt lt 45 g of da rk red crys tds are obtained. Carbonalopntaammine cobalt(II1) iodide, 111 Five grams of earhonatopentsammine eohalt(II1) nitrate, I, is dissolved in 20

Since water is the solvent for these reactions, the fact that the aquopentaammine aquopentaammine IV is formed in prefer preferenc encee to t he nit ratopentaammine V may reflect reflect t he effect of the 5 5 f caneentrrttion of water. Spectral studies see footnote 6) have shown shown that the aqua ligand is more strongly bound to cobalt(II1) than the nitrato ligand ligand.. COTTON,F F.. A,, A N D WILEKINSON,. Advanced Inorganic Chemistry, Chemistr y, Interscience Intersc ience Publishers Publishe rs (division of Jo John hn Wiley Sons Inc.). New York. 1962. D 579. B A ~ O L OA O A N D JOHNSON, p. 78. 8 Although air may be used for the oxidation (see footnote 1) th e reaction time is substanti ally increased to 10-1 10-12 2 hr. We

ml of (6.3 waterg) with heat ing in g tth o e8C 8C-9 -9O0 O0C. C. stirred Powdere Powd ered pota m iodide is ndded ndded and mixture untid l potassiu all of ossium f t he salid is dissolved. dissolved. The iodide salt, 111 crystallizes from the solution upon coal ingin ice for a. few minutes. The reddish brown produet is collected using a Bixhner funnel, washed with 20 of acetone. and air dried.

found t ha t 100 such oxidatiuns required five 200 200 it3 cylinders of of oxygen. 'The only time that an aspirator is inadequate is when the ammonia is measured and transferred to t he large fla flask. sk. o FIESER,., A N D FIESER, ., Reagenb for Organic Synthesis, thesi s, Joh n Wiley &Sons , Inc., New New York, 1967, p. 488.

onclusion

One of t he goals of modern mo dern chemis chemistry try is th e elucidation of of t he pathw pathway ay o off chemical che mical reacti reactions ons and th e dede-

iba.

Volume 46 Number 8

ugust 1969

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velopment of of q qualit ualitative ative relat relationships ionships upon which the th e behavior of mole molecule culess mav be nredicted. freshm freshman an chemistry laboratory course can provide the inspiration to pursue the field of chemical investigation only if aspects of curren currentt research problems and methods of scientif scie ntific ic reasonin reasoning g are demonstrated in its ex exper periiments and by its instruction. The Th e experiment described in this paper can fulfill these requirements by introducing first-year chemistry studen stu den ts to some of

51

Journal of o f Chemic Chemical al Educati Education on

th e thoug thought ht p processes rocesses technique and spirit o off modern che mic ali nve nvesti sti~at ~at ion. ion . Acknowledgmenl

would would like to than th an k Drs. R. H. Eastman P. F. Linde and C Hamilton for permission to include this experiment in the Stanf ord University general che chemmistry laboratory curriculum and for their helpful comments.