Patrick Sharrock Universite de Sherbrooks Sherbrooke, Quebec,JIK 2R1
I
I
Speeding Up the Inorganic Lab How fast is too fast?
The standard procedure for the preparation of [Co(en)zelz]CI involves mixing 10% ethylenediamine and aaueous CoCL. -. followed bv air oxidation of no more than 10 to 12 hours, to avoid side reactions (I). If this is carried out overnight, then by concentrating the solution over a steam bath, the following day, a good crop of green crystals can he obtained. This traditional preparation method is still used, even in recent research work ( 2 ) ,but popular modifications, used to speed up the preparation of cobalt(II1) complexes in undergraduate laboratory experiments have been described (3-5) involving activated charcoal, pure oxygen gas (63or , hydrogen peroxide (7),and these work well, even though problems arise occasionally (8). A recent note describes the three hour synthesis of both Co(en)a" and [Co(en)zClz]Clusing H202 and rotary evaporator techniques (9).However, this procedure in the hands of our students, repeatedly failed to synthesize [Co(en)~Cl~]Cl, producing instead some blue cobalt(I1) complexes and ethylenediamine dihydrochloride. This is not so surprising, if one examines the reduction potentials (10) which give a clue as to the reactions occurring under standard conditions. Under basic conditions, with CoN6 standing for cobalt hexacoordinated to nitrogen donor ligands, one has CoNs2+ HOz- + H20 + 2e 2CoNs2++ HOn- + H20
a CoNs3++ e -
a 30Hs 2CoNs3++ 30H-
E" -0.11V +0.88V +0.77V
However, the C o N P produced can oxidize the remaining peroxide according to EO
+O.O8V t 0% + H20 + 2eHOZ- + OHt CON?+ +O.llV CoNs3++ e +0.19V 2CoNs3++ H20- +OH- e 2CoNe2++ Oz + HzO On the other hand, under acidic conditions, hydrogen peroxide will oxidize cobalt amine complexes
and the resulting cobalt(II1) complex will not decompose remaining peroxide
but if the solution is too acidic, C O ( H ~ O )will ~ ~be + produced, a strongeroxidizing agent than H ~ itself! O ~ ColH20)s3++ e - a Co(H20)e2+ E n = +1.84V In practice, excess HC1 prevents the formation of the green cohdt(II1) complex. Under basic conditions, one can visualize the oxidation readily, hut the rotary evaporator work-up yields a mixture of green and blue complexes. Presumably the Co(I1)-Co(II1)equilibrium is also temperature dependent, and some heating is required to obtain the desired products. This could also be due to the lower than stoichiometric amounts of ethylenediamine used to avoid formation of the tris-chelate compound and the relativelv - high - HCI concentrations used to obtain [Co(en)2C12]C1. Our experimental procedure now uses concentrated solutions (WARNING: fuming ethylenediamine and concentrated HC1 must be handled with precaution in an efficient hood). Thus, llg, CoCl2 .6H20 and 6 ml ethylenediamine are mixed in water (total volume 125 ml) and neutralized with couc. HCI (less than 5 ml). This heats up the solution. Very slowly, 5 ml of 30% Hz02 is added in 1-mlportions (Caution: this may cause frothing. Avoid splashing Hz02 on your skin), and the solution is placed on a warm hot plate. Thirty minutes after Hz02 additioi, 30 ml conc. HC1 k i d d e d gradually and the solution concentrated. When the total volume reaches 60 ml, green crystals of the hydrochloride of trans- [Co(en)zCl$l can he obtained by cooling. The product is washed with 10 ml cold water and 50 ml ethanol. The yield varies depending on ~crops ~of the length of tirne used for crystallization. ~ large crystals are usually obtained by cooling the filtered mother liquor in a refrigerator overnight. By similar procedure, better than 87%yields can he obtained for Co(en)sCla, if ethanol is added to the crystallizing solution.' In conclusion, hydrogen peroxide can he used for speeding u p the preparation of cobalt(II1) complexes if the solutions are heated to complete the reaction. Our ~rocedureallows students to synthc4ze and rhararterize rhese complex~iin one 6-hr laboratory period. Cohult and nickel diarnine complexes have also be& prepared by a nonaqueous route (12). ~~
~~
Literature Cited (11 B ~ ~ IJ.~ c. ,. . d auuinson, L. L.. inor8.syn, 2 . 2 2 2 ( 1 ~ 6 1 A .
I ~ A ~ ~M.,s"~ ~ ~ . D . Raynor. J. B.,"Aduanced Practical Incxganie Chemistry." J. Wiley & Sam, New York. 1967, p. 67 & 141 and Brwmhead. J. A , hvyor,F. P,mdHuprth. J. W., Inor#. Syn.
6.183119601.
(21 Ball, S., Broman, J.. andde Kieuiet, W., J. Inorg. N u c l Chrm., 41.331 119791. Also. House. D.A.,Co~rd.Chem. R r u . 23.223 (1977). 11) Anxeiici. R.J., "Synthesis and Technique in Inorganic Chemistry; Saunders. Toronto, l a . " .
