298
RICHARD G. YALMAN AND THEODORE KUWANA
VOl. 59
PROPERTIES OF ACIDOCOBALTAMMINES. 111. c~s-NITRITOTETRAMMINES~ BY RICHARDG. YALMAN AND THEODORE KUWANA Contributions from the Chemistry Laboratory of Anttoch College, Yellow Springs, Ohio Received November bb, 1064
cis-Nitritoaquo-, nitroaquo, and dinitritotetramminecobalt(II1) nitrates were prepared, and the rates of their conversion to cis-nitroaquo-, trans-nitroaquo- and cis-dinitrotetramminecobalt( 111),respectively, were studied. The formation of chdinitrotetramminecobalt(II1) salts from cobalt(I1) is discussed.
,
Although trans-dinit rotetramminecobalt (111) salts can be prepared directly from those of cobalt (11),2 salts of the cis-dinitrotetramminecobalt (111) series are only obtained from carbonatotetrammine~obalt(III).~~~ The purpose of the present paper is to examine more closely the formation of the cis-dinitrotetramminecobalt(II1) ion in order to discover why it cannot be made directly from cobalt(I1) in large quantities.
conversion of the cis-diaquo complex into the trans-form, for the conversion of the cis-nitritoaquo ion to the more stable cis-nitroaquo ion and for the intramolecular rearrangement of the cisdinitrito ion to form the stable cis-dinitro complex. Although the latter reaction may take place in two steps, only one was observed a t this time. These results are listed in Table I. Each figure is an average of four measurements. The equilibrium constant for the reaction
Results
L cis-[(Co(NHa)((H20)2If++4 trans-[( CoNHs)d(HzO)p] + + +
Three new complex nitrates in the cis-tetramminecobalt(III)6 series were prepared. Two of these, cis-nitritoaquo and cis-dinitrito nitrate, were prepared directly from carbonato and cisdiaquo nitrate. Attempts to obtain these cisnitrito complex ions from trans-diaquo nitrate were unsuccessful. The third compound, cis-nitroaquo nitrate, was obtained when cis-nitritoaquo nitrate was allowed to stand in the dry state. Upon standing in aqueous solution both of these compounds were converted into trans nitroaquo nitrate. However, either in the dry state or in aqueous solution cis-dinitrito nitrate forms cis-dinitro nitrate. The theoretical intermediate compound, cis-nitritonitro nitrate, could not be identified by means of its absorption spectrum during the conversion of the dinitrito to the dinitro complex. Attempts t o prepare cis-nitritonitro nitrate from both cisand trans-nitroaquo nitrate were also unsuccessful. The chemical properties of the cis-nitritoaquo, cis-nitroaquo, cis-dinitrito and of trans-diaquo nitrate were determined. Each of these ions reacted with concentrated hydrochloric acid a t room temperature to form trans-dichloro chloride and with 50% nitric acid at 80" to form nitratoaquo nitrate, The cis-nitritoaquo nitrate, and cis-nitroaquo nitrate as well as cis-diaquo nitrate react with weakly acid solutions of sodium nitrite a t room temperature t o form cis-dinitro nitrate. However trans-diaquo nitrate reacts with weakly acid solutions of sodium nitrite to form transdinitro nitrate. First-order reaction rates were observed for the (1) The work on this paper was supported by a Cottrell grant from the Research Corporation of America. (2) 8.M. Jorgensen, 2.anorg. Chen.. 5, 4G9 (1894). (3) S. M.Jorgensen, ibid., 5, 163 (1894); 17,473 (1898). (4) C. Duval, Compt. rend., 182, 636 (1926),obtained a mixture of nitrocobaltammines including cia-dinitrotetramminecobalt(II1) directly from cobalt(I1) without isolating the intermediate carbonatotetramminecobalt(II1). ( 5 ) Unless otherwise noted all of the compounds to be discussed belong to the tetramminecobalt(II1) series. For purposes of abbreviation this part of the name of the compounds will be eliminated.
BI
was found to be 6.0 = 0.5ia t both 20 and 30". No equilibrium was observed for the cis-trans conversion of the nitroaquo ion. TABLE I RATESOF REACTION IN
THE Cis-TETRAMMINE
Reactant
Product
Diaquo Nitritoaquo Dinitro Nitroaquo
trans-Diaquo cis-Nitroaquo cis-Dinitro trans-Nitroaquo
SERIES
k X 102,min.-l 20° 30°
E kc./l$ole
3.3 2.3 1.9 0.05
25,000
0.72 1.1
8,300
Discussion The configuration of nitroaquotetramminecobalt(111)nitrate reported here was determined by comparing its chemical properties with those of transnitroaquo nitrate0 and cis- and trans-dinitro nitrate. The reaction products of these four salts with sodium nitrite and concentrated HC1 a t 30" and with 50% " 0 , at 80" are given in Table 11. The similarity in the properties of cis-dinitro nitrate and the nitroaquo nitrate obtained here as well as their generic relationship indicate that the latter has the cis configuration. Attempts to determine the configuration of this compound by comparing its absorption spectrum in the ultraviolet region with that of the trans isomer were inconclusive. TABLE I1 REACTIONSOF NITROTETRAMMINECOBALT(III) NITRATES Compound
Nitroaquo Dinitro Nitroaquo Dinitro
Reagent HC1
"01
NaNOz
A. Products of the cis Series Dichloro Diaquo cis-Dinitro Dichloro Diaquo B. Products of the trans Series Chloronitro Nitroaquo trans-Dinitro Chloronitro Nitroaquo
(6) 8.M. Jorgensen, Z.anorg. Chen., 7, 289 (1894): J. Meyer and R. Grohler, ibid., 155,99 (1926).
April, 1955
ACIDOCOBALTAMMINES : Cis-NITRITOTETRAMMINES
Because the nitritocobaltammines are more sensitive to mineral acids than the nitro isomers' these reactions cannot be used to establish their configuration. It has been assumed, however, that the nitrito-nitro conversion does not involve a change in the configuration of the complex ion.* If this assumption is correct, then the nitritoaquo and dinitrito ions will have the same configuration as the cis-nitroaquo and cis-dinitro ions, respectively. In addition to cis-nitritoaquotetramminecobalt(111) nitrate and dinitritotetramminecobalt (111) nitrate, salts of cis and trans-dinitrito-bis-(ethylenediamine)-cobalt (111) and dinitritodipyridiamminecobalt (III)981L' as well as nitritopentamminecobalt(III)11 are well known. The occurrence of nitritonitro-bis-(ethylenediamine)-cobalt(II1) has also been reported.12 The formation of these compounds which are unstable and undergo rearrangements in the dry state to form the corresponding stable nitro complexes, indicates that the preparation of all nitro complexes from aquo ions in aqueous solution may involve the formation of intermediate nitrito complexes. The stepwise formation of cis-dinitrotetrammine cobalt(II1) and the apparent greater stability of the trans isomers of the nitroaquo and trans-diaquo complex ions provided added information as to why the cis-dinitro salts are not prepared directly from cobalt(I1). I n the preparation of transdinitrotetramminecobalt(II1) salts2 air is passed through a mixture of cobalt(I1) and ammonium salts, sodium nitrite and aqueous ammonia for four hours. During this time the pH of the mixture decreases from approximately 9 to 7. The solution is then allowed to stand for 12 hours before the reaction product is filtered, washed free of nitropentamminecobalt(II1) and a little of the more soluble cis-dinitro salts and then recrystallized. During the initial period of oxidation the only mononuclear cobaltammines present will be hydroxopentamminecobalt (111) and some dihydroxotetramminecobalt (III)? As the ammonia is expelled and the concentration of the aquo ions increases, they will begin to react at a measurable rate with sodium nitrite a t a pH of 7.14 Any nitropentamminecobalt(II1) which is formed can react further with sodium nitrite to form transdinitrotetramminecobalt(III),l6 while trans-diaquotetramminecobalt(II1) also reacts with sodium nitrite to form the trans-dinitro complex ion. Furthermore, because of the slow conversion of cisnitroaquotetrammine cobalt(II1) to the more stable (7) R. Weinland, "Complex Compounds," Stuttgart, 1924,pp,44,64. (8) A. Werner, Ber., 40, 769, 779 (1907). (9) A. Werner, ibid., 40, 769, 779, 785 (1907). (10) B. Adell. Acta Chem. Scand., 4 , 1 (1950). (11) S. M. Jorgensen, 2. anorg. Chem., 6 , 169 (1893): B. Adell, Compt. rend., 26Z, 272 (1944). Although C. Duval and J. Lecompte, BUZZ. 8oc. chim., [5] 19, 678 (1045), question the formation of this compound, the work of Adell has been duplicated in this Laboratory, while that of Duval has been shown to be in error. (12) F. Basolo, B. D. Stone, J. C. Bergmann and R. G. Pearson, J . A m . Chem. Soc., 76, 3079 (1954); R. G.Pearson, P. M. Henry, J . G. Bergmann and F. Basolo, ibid., 7 6 , 5920 (1954). (13) J. Bjerrum, "Metal Ammine Formation in Aqueous Solution," P. Haase, Copenhagen, 1941, p. 253. (14) R. G . Yalman and T. Kuwana, THIS JOURNAL, in press. (15) N. R . Dhar, Akad. Amsterdam. Veral., 28, 551 (1920).
299
trans form, some of the reaction product of the cis-diaquo ion with sodium nitrite will be found as trans-dinitrotetramminecobalt(II1). As a result of these various reactions very little cis-dinitrotetramminecobalt(II1) will be obtained from cobalt(I1). Experimental Preparation of Compounds.-Carbonatotetramminocobalt(II1) nitrate was used as the starting material for all of the other cobaltammines. cis-Diaquo and cis-dinitro nitrates were prepared in the usual way. trans-Dinitro nitrate was prepared from trans-diaquo nitrate by means of the procedure used for the preparation of cis-dinitro nitrate from carbonato and cis-diaquo nitrates. A. trans-[Co(NH~)4(HzO)z](NOa)8.-This compound was prepared by the partial precipitation of the equilibrium mixture of cis- and trans-diaquo nitrates by the addition of an equal volume of 50% nitric acid at room temperature and then chilling the reaction mixture. The red product was filtered, washed with alcohol and ether, and dried in a desiccator. The product obtained was recrystallized by dissolving in water a t 5" and reprecipitating with nitric acid until a constant absorption spectrum was obtained. The final product may still contain a small percentage of the cis isomer. Anal. Calcd. for trans-[Co(NH,)~(HzO)2](NO&: Co, 17.28. Found: Co. 17.19. 17.24. Absorption spectrum maximum at 306, 357, 512 min. a t 285,326 and 417 mp. B. C~~-[CO(NH~)~H~O(ONO)](NO~)~.-A solution of 5 g. of carbonatotetraamminocobalt( 111) nitrate and 10 g. of sodium nitrite in 100 ml. of water is cooled in an ice-salt bath a t 0". Twenty-five ml. of 10 N nitri: acid is now added and the solution maintained at 0-2 After five minutes, a carmine red precipitate is transferred to a sintered glass filter with the aid of suction and washed with two 5-ml. portions of cold 2 N nitric acid, followed by cold alcohol and ether. The product must be stored in a refrigerated desiccator: vield 2.4 E. or 48%. Anal. Calcd. ' ;or [CO
