Thermal decomposition of metal complexes—VI some ammine cobalt (II) complexes

J. Inorg. Nucl. Chem., 1963, Vol. 25, pp. 985 to 993. PergamonPress Ltd. Printedin NorthernIreland

T H E R M A L D E C O M P O S I T I O N OF M E T A L C O M P L E X E S - - V I SOME A M M I N E COBALT (II) C O M P L E X E S ~a~ W . W . WENDLANDT a n d J. P. SMITH AFOSR Center for Molecular Research, Department of Chemistry, Texas Technological College, Lubbock, Texas

(Received 27 December 1962; in revised form 15 February 1963) Abstraet~The thermal dissociation of a number of cobalt (II) ammine complexes of the type, [Co(NH3)dX2 (X -- CI, Br, I, and NO3), and [Co(NHa)dSOa, were studied by thermogravimetry, differential thermal analysis, and gas evolution analysis. The thermal dissociation of these complexes involved intermediate ammine complexes containing lower ammonia to cobalt ratios. For the chloride and bromide complexes, the dissociation sequence was : [Co(NH3)6]X2 --~ [Co(NH3)z]X2 [Co(NH3)]X~ -~ COX2. The stoichiometry of the dissociation of [Co(NH3)6](NO3)~ was: 9[Co(NHa)6 (NOa)2 --~ 3CoaO4 ÷ 30NH3 ÷ 36HzO ÷ 17N2 + 4NO ÷ 2N20.

THE thermal dissociation of the hexammines of the cobalt (II) halides and sulphate has been the subject of a number of investigations. The hexamminecobalt (II) chloride and bromide lose four moles of ammonia per mole of complex when heated, to yield the diammines, [Co(NH3)2X2](X -----C1, Br). (2-6) Two forms of the diammines were obtained; a thermally unstable purple coloured fl-form (trans-) and a stable pink coloured s-form (cis-). Upon further heating, the diammines yielded the respective monoammines and then the anhydrous cobalt (II) halides, t5) Similarly, hexamminecobalt (II) iodide thermally decomposes to the diammine which exists in an unstable malachite green coloured fl-form and a stable blue coloured ~-form.13, 5-7) Further heating gives cobalt (II) iodide rather than the monoammine as in the case of the other halide complexes. The thermal dissociation of the pentamminecobalt (II) sulphate gives [Co(NH3)4]SO4 at 106-116 °, [Co(NH3)2]SO 4 at 254 °, and [Co(NH3)0.5]SO 4 between 254 and 257°. t5,8~ There is also reported that a triammine can be prepared by heating [Co(NH3)5]SO4 at 133°. c8~ In connection with our studies on the thermal dissociation of the [Co(NH3)6]X3 tgl and [Co(NH3)5X]X2 II°) complexes, it was of interest to investigate the thermal tl~ For Part (V) see: W. W. WENDLANDT and J. P. SMITH, J. Inorg. Nucl. Chem. 25, 843 (1963). t21 F. ROSE, Untersuchungen uber ammoniakalische kobalt verbindungen, Heidelberg, 26, (1871). lal W. BILTZ and B. FETKENHEUER,Z. Anorg. chem. 89, 97 (1914). 14) A. NAUMANNand J. RILL, Bet. Chem. Dtsch. Get. 42, 3792 (1909). 151 G. L. CLARK, A. J. QUICK and W. D. HARKINS,J. Amer. Chem. Soc. 42, 2483 (1920). t6) G. L. CLARK and H. K. BUCKNER,J. Amer. Chem. Soc. 44, 230 (1922). t7) C. F. RAMMELSBERG,Pogg. Ann. 48, 155 (1839). ~s~F. EPHRIAM,Z. Phys. Chem. 83, 196 (1915). tg~ W. W. WENDLANDT,J. Inorg. Nucl. Chem. 25, 545 (1963). tl0~ W. W. WENDLANDT and J. P. SMITH, J. Inorg. Nucl. Chem., In press. 6

985

986

W . W . WENDLANDTand J. P. SMITH

d i s s o c i a t i o n o f t h e [ C o ( N H a ) 6 ] X 2 ( X = C1, Br, I, a n d N O a ) a n d [ C o ( N H a ) 6 ] S O 4 c o m p l e x e s . T h e c o m p o u n d s w e r e s t u d i e d b y the t e c h n i q u e s o f differential t h e r m a l analysis ( D T A ) , t h e r m o g r a v i m e t r i c analysis ( T G A ) , a n d gas e v o l u t i o n analysis ( G E ) . EXPERIMENTAL

Thermobalance. The automatic recording thermobalance has previously been described, c11~ Sample sizes ranged in weight from 50 to 100 mg. The complexes were pyrolysed in a static air atmosphere at an atmospheric pressure of about 680 ram. The furnace heating rate was about 5°C. per min. Differential thermal analysis- gas evolution apparatus. The apparatus used has previously been described. ~12~ Sample sizes ranged in weight from 50-60 mg. The heating rate was about 10°C per min. TABLE 1 .--ANALYSESOF COBALT(II) AMMINECOMPLEXES

Compound [CofNH3)dC12

Cobalt ( ~ ) Theor. Found

Ammonia (~o) Theor. Found

25"40 35-96 35"96 40"13

25.9 35"8 35-5 42"0

44.04 20-78 20"78 11"59

44.1 20"3 21.0 10"3

18-36 23-31 23.31 25.00

18"5 23'4 23'0 25.8

31-84 13.47 13'47 7'22

31"7 13"8 13-0 6'84

cis-[Co(NHa)~]I2 trans-[Co(NH3)2]I~

14"20 16"99 16'99

14"2 16'8 16"0

24"62 9"82 9"82

24"8 9'9 10.0

[Co(NHa)n](NO3)2 [Co(NHa)4](NOz)~

20'67 23"47

20"8 23"0

35"84 27"13

36'1 27"3

[Co(NHa)6]SO4 [Co(NH3)4]SO4 [Co(NH3)0.5]SO4

22"92 26'41 36"05

23"3 25"8 36'3

39"73 30"53 5"21

41 "3 30"2 5"3

cis-[Co(NH3)2]Cl~ trans-[Co(NHa)~]Clz [Co(NHa)]CI2 [Co(NH3)6]Br2

cis-[Co(NH3)~]Br2 trans-[Co(NHa)~]Br~ [Co(NH3)]Br2 [Co(NH3)dI2

Preparation of complexes. The complexes, [Co(NH3)dX2 (X = C1, NO3) and [Co0NH3)dSO4, were prepared by reacting gaseous ammonia with methanol suspensions o f the salts as previously described, tS~ The rose coloured precipitates were filtered off and dried at 80 ° in an atmosphere of ammonia. The complexes, [Co(NHs)6]X2 (X = Br and I) were prepared by adding concentrated HBr and HI to a concentrated aqueous solution of cobalt (II) chloride, neutralizing with ammonium hydroxide, and cooling. The crystals were collected and dried as described al:ove. The intermediate ammine complexes, fl-[Co(NHa)~]Xz (X = CI, Br, I,), [Co(NHa)4]SO4, and [Co(NH3)d(NO~)2 were prepared by heating the hexammines in vaeuo at 100 ° for 24 hr. The complex, [Co(NH3)]o.sSO~, was prepared by heating the hexammine in vacuo at 275 ° for 24 hr. The nitrate complex was analyzed for cobalt by ignition in air at 800 ° and weighing as Co304. The other complexes were analyzed for cobalt by ignition of the compounds in helium and weighing as the anhydrous salts. The ammonia was determined by the Kjeldahl method. The results of the analytical determinations are given in Table 1. (11~ W. W. WENDLANDT,Z. D. GEORGE,K. V. KRISHNAMURTY,.L Inorff. NucL Chem. 21, 69 (1961). ~1~ W. W. WENDLANDT,Analyt. Chim. Acta 27, 309 (1962).

Thermal decomposition of metal complexes--VI

RESULTS

AND

987

DISCUSSION

Weight-loss studies.

The weight-loss curves for the cobalt (II) ammine complexes a r e g i v e n i n Fig. 1 w h i l e t h e w e i g h t - l o s s d a t a a r e g i v e n i n T a b l e 2. T h e w e i g h t - l o s s c u r v e s i n d i c a t e d t h a t all o f t h e c o m p o u n d s b e g a n t o e v o l v e a m m o n i a i n t h e 8 0 - 9 0 ° t e m p e r a t u r e r a n g e . I n t e r m e d i a t e a m m i n e s , as i n d i c a t e d f r o m

2

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FIG. 1.--Weight-loss curves of ammine complexes. TABLE 2.--WEIGHT-LOSS DATA FOR COBALT (11) AMMINECOMPLEXES Weight-loss ( ~ ) Compound

[Co(NH3)6]Clz [Co(NH3)6]Br2 [Co(NH3)6]I~

[Co(NHa)2]X2 Theor. Found 180°C. 28"2 178 ° 21.22 20.4 192 ° 16.42 16'5

29"36

[Co(NHa)]X2 Theor. Found

CoX2 Theor. Found

220 ° 36"70

302 ° 36"0

44"04

219 ° 26.53

26.2

31.84

31.8 315 °

24.62

[Co(NHa)4](NOa)~

80.7 CoaO4

125 ° [Co(NH3)6](NOa)2

11 "95

185 ° 11 '9

[Co(NHz)~]SO4

71 "85 [Co(NH3)o.5]SO~

113 ° [Co(NH3)dSO4

13"24

43'8 300 °

14"1

36"42

303 ° 35"6

71 '0

COSO4

39"73

321 ° 41"5

988

W.W. WENDLANDTand J. P. SMITH

breaks in the weight-loss curve, were found for [Co(NH3)6]CI~, [Co(NHa)o]Br2, [Co(NHa)6]I2, and [Co(NHa)6](NO3) 2. However, the first two complexes gave three breaks in the curve while the last two had only two. The nitrate complex, due to the presence of reducing (ammonia) and oxidizing (nitrate) groups, exploded at about 180 °. The weight-loss curves for [Co(NH3)s]C12 and [Co(NH3)6]Br2 were similar to each other. Curve breaks at 178-180 ° indicated the compositions, [Co(NHa)2]Br2 and [Co(NHa)2]C12, respectively. Other curve breaks at 219-200 ° indicated the presence of the monoammines, [Co(NHa)]Br2 and [Co(NH3)]CI~, while the anhydrous salt horizontal weight levels were obtained in the 300-302 ° temperature range. Thus, the thermal dissociation sequence appears to be (where X = C1 and Br): 80 °

[Co(NHz)6]X2

)- [Co(NH3)z]X2 + 4NH3 180 °

[Co(NH3)z]Xz

) [Co(NHa)]X 2 + NHa 220 °

[Co(NH3)]X2

• CoX 2 + NHa

The weight-loss curve for [Co(NHa)n]I2 had only a rather indefinite break at 195 °. The stoichiometry of the compound formed at this temperature, as calculated from weight-loss data, was that of [Co(NH3)2]Ie. Above this temperature there was no evidence for the formation of any other intermediate ammines nor of the anhydrous CoI2. Instead of CoI 2. as shown by Table 1, the oxide, Co304, was obtained at 315 °, This is probably due to the oxidation reaction: 195 °

3[Co(NHa)2]I2 + 202

• Co304 + 312 + 6NHa

This reaction is substantiated by the data for the weight-loss to the oxide: 80.66 per cent theor., 80-7 per cent found. The weight-loss curve for [Co(NHa)6](NOz)z showed that two moles of ammonia per mole of complex were evolved, giving a break in the curve at 125 ° which corresponded to the stoicheiometry, [Co(NHa)4](NOa)2. On further heating, the tetrammine exploded at 180 °, giving a residue of Co304. The weight-loss curve for [Co(NHz)n]SO4 gave no well defined breaks as was found for the other compounds. The curve break at 113° approximated the composition, [Co(NHa)4]SO 4, while the break at 303 ° approximated that for [Co(NH3)0.5]SOa; however, these compositions might be fortuitous. The anhydrous salt, CoSO~, was obtained at 340 °. Differential thermal analysis- gas evolution studies. The DTA curves of the ammine complexes are given in Figs. 2-4 while the GE curves are given in Figs. 5 and 6. The DTA curves for the various [Co(NHz)n]C1z complexes, Fig. 2, showed evidence for the formation of the diammines and monoammines, as previously indicated by the weight-loss curves. The origin of the various endothermic peaks was determined by obtaining the DTA curves of authentic lower ammine complexes. The DTA curve for [Co(NH3)6]C1~ had three endothermic peaks, with peak maxima temperatures, henceforth referred to as peak temperatures, of 143 °, 231 °,

Thermal decomposition of metal complexes--VI

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FIG. 2.--Differential thermal analysis curves of ammine complexes.

A. [Co(NH3)dCI~; B. [Co(NH3)]C1,; C. ¢is-[Co(NH3)JCI2; D. trans-[Co(NH3)~]C12

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FIG. 3.--Differential thermal analysis curves of ammine complexes. A. [Co(NH3)dBr2; B. cis-[Co(NH3)2]Br2; C. cis-[Co(NH3)2]Br2 and AlcOa; D. [Co(NH3)]Br2.

990

W . W . WENDLANDTand J. P. SMITH

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FIG. 4.--Differential thermal analysis curves of ammine complexes.

A. cis-[Co(NH3)2]I~; B. trans-[Co(NH3)2]I~; C. [Co(NH3)dI~; D. [Co(NH~)dSO4; E. [Co(NH~)d(NO3)v

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FIG. 5.--Gas evolution curves of ammine complexes A. trans-[Co(NH3)~]C12; B. cis-[Co(NH3)z]C12; C. [Co(NH3)]CI~; D. [Co(NHz)]Brz; E. cis-[Co(NH3)2]Br2 and Al~O3; F. cis-[Co(NH3)~]Br~; G. [Co(NHs)e]CI~; H. [Co(NH3)8]Br2; L Co[(NH3)6]I2.

Thermal decomposition of metal complexes--VI

991

and 286 °, respectively. On the basis of the D T A curves for cis- and trans[Co(NH3)2]C12 and [Co(NHa)]C1 ~, the assignment for these peaks is as follows: (a) 143 ° endothermic peak: [Co(NH3)6]C12 --~ [Co(NH3)2]C12 -k 4NHz (b) 231 ° endothermic peak: [Co(NHa)2]CI2 --~ [Co(NH3)]CI2 -k NH3 (c) 286 ° endothermic peak: [Co(NH3)]C12 --~ CoCI~ q- N H 3 There was a slight shift in peak temperature for the first peak in the curves for

cis- and trans-[Co(NHa)2]C12. The cis- peak temperature was 234 ° in contrast to the trans-peak temperature of 227 °. The second peak temperatures, however, were much closer together, at 293 ° and 290 ° , respectively.

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FIG. 6.--Gas evolution curves of ammine complexes A. [Co(NH3)~](NOa)~; B. [Co(NH3)6SO4. All of the G E curves for the cobalt (II) chloride ammines had peaks at the same temperatures as the peaks found in the D T A curves. This indicated that all of the reactions found in the D T A curve resulted in the evolution of a gaseous product. The D T A curves for the [Co(NHz)~]Br~, complexes, Fig. 3, were similar to those obtained for the [Co(NH3)n]C12 complexes. The curves were complicated by the fusion of the ammine complexes at temperatures above 200 °. This resulted in a system containing liquid---~gaseous state equilibria which gave many small endothermic peaks in the curve.

992

W.W. WENDLANDTand J. P. SMITH

The DTA curve of [Co(NHa)6]Br2 gave three major endothermic peaks with peak temperatures of 168 °, 233 °, and 308 °, respectively. Also present was a smaller shoulder peak at about 265 °. From the curves for cis-[Co(NHa)2]Br ~ and [Co(NH3)]Br2, the assignment for these peaks is as follows: (a) 168 ° endothermic peak: [Co(NHa)6]Br2 --+ [Co(NH3)~]Br2 + 4NH3 (b) 233 ° endothermic peak: [Co(NH3)2]Br2 ~ [Co(NH3)]Br2 ÷ NH3 (c) 308 ° endothermic peak: [Co(NHs)]Br 2 ~ CoBr 2 + NH8 From the erratic behaviour of the curve above 260 °, the sample must be in the liquid state above this temperature. This behaviour is illustrated more clearly in the curve for cis-[Co(NH3)2]Br ~. Very sharp pronounced peaks were obtained above 260 ° due to the decomposition of the fused [Co(NH3)]Br2. On mixing the diammine with an equivalent amount of aluminium oxide, curve C was obtained which had a broad endothermic peak at 295 ° instead of the series of small peaks. Apparently, the aluminium oxide aids in the dissociation of the monoammine by forming a larger surface area. The curve for the monoammine, curve D, showed that the compound fused between 170-210 °. A capillary tube melting point determination of this compound gave a melting point of 201-203 °, which is much lower than the 260 ° previously reported. (3) Again, the erratic behaviour of the curve was due to the decomposition of the fused monoammine. The peak at 455 ° must also be related to the dissociation of this compound although the peak temperature was much higher than that found in the other curves. The decomposition of the fused monoammine gave a series of small sharp peaks in the G E curves of these compounds also as shown in Fig. 5. The DTA curves of the [Co(NHa)n]I 2 complexes, as given in Fig. 4, were also similar to those obtained for the [Co(NH3),]C12 complexes. The complex, [Co(NHa)6]I2, gave DTA curve peak temperatures of 178 °, 210 ° and about 300 °, respectively. On the basis of the curves for cis- and trans-[Co(NH3)2]I2, the assignment for these peaks is as follows: (a) 178 ° endothermic peak: [Co(Nga)6]I2 --+ [Co(NH3)2]I2 + 4NH3 (b) 210 ° endothermic peak: [Co(NHz)2]Iz(s) --+ [Co(NHa)2JIz(1 ) (c) 300 ° endothermic peak: [Co(NHz)e]I2 ~ CoI 2 q- 2NH z The two compounds, cis- and trans-[Co(NHz)o]I 2 gave practically identical DTA curves. The DTA curve for [Co(NHz)o]SO4 as shown in Fig. 4, D, gave two rather broad

Thermal decomposition of metal complexes--VI

993

endothermic peaks with peak temperatures of about 130 ° and 240 °, respectively. It was not possible to assign definite chemical reactions to these peaks. The DTA curve for [Co(NHa)6](NO3)z, as shown in Fig. 4, E, gave two endothermic peaks, with peak temperatures of 137 ° and 180 °, respectively, and an exothermic peak at 210 ° . The 137 ° peak is due to the reaction: [Co(NH3)6](NOa)2 --~ [Co(NHs)4](NOs)2 + 2NHs while the 180 ° peak is apparently due to a phase transition since a GE peak, Fig. 6, was not observed in this temperature region. It is obvious that the 210 ° exothermic peak was due to the explosive decomposition of the tetrammine complex. Dissociation of [Co(NHs)6(NO3) 2. The stoichiometry of the thermal dissociation of [Co(NH3)6](NO3) 2 was of interest because of previous studies on the analogous Co(III) compound, [Co(NHs)6](NOs) s. Mass spectrometric analysis studies on the decomposition gases revealed the presence of N~O, NHa, NO, N2, and H20. From the gas absorption train and the modified Dumas nitrogen apparatus, the following stoichiometry data were obtained: NHs/Co, 3-17 found, 3-33 theor.; H~O/Co, 3"87 found, 4"00 theor.; NHs/N ~, 1.8 found, 1.77 theor. Although not all of the reaction products were determined in the analyses, the data suggest that the thermal dissociation reaction is as follows: 9[Co(NH3)6](NO3)2 --~ 3CosO ~ + 30NH s + 36H20 + 17N 2 + 4NO + 2N20 In the thermal dissociation of [Co(NHs)6](NO3)s, ammonium nitrate was also found in the reaction products but nitrogen (II) oxide was absent. For the corresponding nitro-complex, [Co(NHs)3(NO2)3], only nitrogen, water, and CosO 4 were obtained. 113) Acknowledgement--This work was supported by the U.S. Air Force, Office of Scientific Research,

through Grant No. AF-AFOSR-23-63. t13~G. L. CLARK,A. J. Qu1¢K, and W. D. HARKINS,J. Amer. Chem. Soc. 42, 2483 (1920).