A convenient source of hexaaquacobalt(III)

Article abstract of DOI:10.1016/S0020-1693(02)01236-7

A convenient preparation and full characterization, including a crystal structure, of green, crystalline [Co(NH₃)₆][Co(CO₃)₃] is reported. This salt can be treated at 0°C with aqueous solutions of HClO₄, HNO₃ or CF₃SO₃H to give CO₂(g), the corresponding insoluble salt of Co(NH₃)₆³⁺ and aqueous Co(OH₂)₆³⁺. This reaction provides ready access to solutions of Co(OH₂)₆³⁺ in a wide range of concentrations and acidities.

Full text of DOI:10.1016/S0020-1693(02)01236-7

Inorganica Chimica Acta 343 (2003) 347Á350
/
www.elsevier.com/locate/ica
Note
A convenient source of hexaaquacobalt(III)
Grant Wangila, R.B. Jordan *
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received 21 November 2001; accepted 12 July 2002
Abstract
A convenient preparation and full characterization, including a crystal structure, of green, crystalline [Co(NH3)6][Co(CO3)3] is
reported. This salt can be treated at 0 8C with aqueous solutions of HClO4, HNO3 or CF3SO3H to give CO2(g), the corresponding
3ꢀ
3ꢀ
insoluble salt of Co(NH3)6 and aqueous Co(OH2)63ꢀ. This reaction provides ready access to solutions of Co(OH2)6 in a wide
range of concentrations and acidities.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Oxidation; Cobalt(III); Perchloric acid
1. Introduction
Co(OH2)63ꢀ, but the resultant solution contains large
amounts of sodium or potassium ions and is of
uncertain acidity because of the large amount of
bicarbonate that must be neutralized. Solid sodium
Studies of the chemistry of hexaaquacobalt(III) ion [1]
have been severely limited by the lack of a convenient
source for solutions of this species. This has hampered
systematic studies on such fundamental properties as the
hydrolysis constant, the state of oligimerization and the
mechanism of ligand substitution. Early studies [2,3]
and potassium salts, formulated as M3[Co(CO3)3]×
/
3H2O or M3[Co(CO3H)3(OH)3], have been prepared
by Mori et al. [6] and Bauer and Drinkard [7]. However,
these solids are of uncertain stability and purity and
have not been used as sources of aquacobalt(III). In
3ꢀ
indicated that Co(OH2)6 is a far less reactive oxidant
some early studies [8,9], solid Co2(SO4)3×
treated with Ba(ClO4)2 in perchloric acid, but the solid is
inconvenient to prepare and store. The same is true of
/
18H2O was
than predicted by Marcus theory, but there has been no
further work to determine if this is a general feature that
might provide insight into the electron-transfer process.
3ꢀ
the alum CsCo(SO4)2×
/
12H2O, although its crystal struc-
In these ways, Co(OH2)6 has fallen far behind its
chromium(III) and iron(III) analogues.
ture has been determined [10].
The ideal source of aquacobalt(III) would be a stable,
easily prepared, well characterized solid that can be
dissolved rapidly to give the hexaaquacobalt(III) ion.
Such a source is described here.
There are two conventional preparative methods.
Electrolysis of strongly acidic solutions of cobalt(II)
salts [4] gives about 80% conversion to Co(III) in
competition with water reduction. This method is
tedious for routine work and yields rather modest
concentrations of Co(III) in strongly acidic solution.
The reaction of hydrogen peroxide with cobalt(II) in
concentrated sodium or potassium bicarbonate solu-
tions yields a green solution containing cobalt(III)
species of moderate stability and uncertain composition
[5]. These solutions can be acidified to obtain
2. Results and discussion
Although [Co(NH3)6][Co(CO3)3] was prepared many
years ago by McCutcheon and Schuele [11] by a
somewhat circuitous route, its potential as a source of
aquacobalt(III) seems to have gone unrecognized. The
present study reports a convenient synthesis of the salt
in crystalline form and its simple conversion to aqueous
* Corresponding author. Tel.: ꢀ
8231
E-mail address: bob.jordan@ualberta.ca (R.B. Jordan).
/1-780-492 5551; fax: ꢀ1-780-492
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 2 3 6 - 7

348
G. Wangila, R.B. Jordan / Inorganica Chimica Acta 343 (2003) 347Á350
/
Scheme 1.
solutions of cobalt(III). The essence of the synthesis and
conversion are described in Scheme 1.
The conditions for crystallization are given in Section
3. The composition of the green crystals has been
determined from the content of Co(NH3)63ꢀ, the
equivalent weight in the reaction with HX, and the X-
ray structure. The material is quite stable and has been
routinely stored under ambient conditions for several
months without any sign of change in composition. It is
insoluble in common organic solvents, and has very low
Fig. 1. Atom arrangement and average bond lengths and angles in
3ꢁ
Co(CO3)3
structure contains equal amounts of both.
. Only one diastereoisomer is shown although the
numerous variations in the procedure for mixing the
green crystals and the aqueous acid, a significantly
better yield has not been achieved. Grinding the crystals,
or using the powder gives lower and irreproducible
solubility in water. The yield of crystalline material is
3ꢀ
rather modest, but, if too much Co(NH3)6 is used in
the preparation, the product is a green powder that gives
yields of B
reduction of cobalt(III) during the dissolution/reaction
of the solid. The final acidity is routinely within 95% of
/80%. It appears that there is some inevitable
3ꢀ
substantially lower yields of Co(OH2)6
ified.
The X-ray structure reveals that the crystalline solid
when acid-
/
that calculated from the stoichiometry in Scheme 1.
There are several reports of the electronic spectrum of
3ꢁ
3ꢀ
contains the Co(CO3)3
ion, with three chelated
Co(OH2)6
in strongly acidic solution [1,9,10,13,14]
12H2O [13]. The previous
work is in agreement with the observations here that the
carbonato ligands. The essential features of the coordi-
nation and average bond lengths and angles are shown
in Fig. 1. Full details are given in Section 4. The
carbonato ligand is essentially planar with a dihedral
and in the solid CsCo(SO4)2×
/
main bands in solution are observed at 400 and 605 nm
with extinction coefficients (o) of 40.09
/
0.5 and 35.09
/
angle of 179.28 at C. The structural parameters for
0.4 Mꢁ1 cmꢁ1. Because the present method allows the
preparation of moderately concentrated solutions of
Co(OH2)63ꢀ, it also has been possible to resolve a weak
3ꢁ
Co(CO3)3 are quite similar to those of the chelated
mono-carbonatocobalt(III) complexes that have been
reported [12]. It is a general feature of these structures
transition at 795 nm with o of ꢀ
0.9 Mꢁ1 cmꢁ1, that
was tentatively identified [15] in the alum at 800 nm. In
/
that the OÃ
/
CoÃ
/
O and OÃ
/
CÃ
/
O angles are compressed
to ꢀ708 and ꢀ
/
/
1108, respectively, compared to the ideal
order of decreasing energy, these are assigned to
1
transitions from the ground state A1g to T2g, T1g,
1
1
values of 908 and 1208. It is noteworthy that the latter
could be attained with normal bond lengths by com-
3
and T2g, respectively. The crystal field analysis de-
scribed by Johnson and Nelson [16] gives the parameters
740 and Cꢂ
3360 cmꢁ1. Molecular
orbital calculations of Zerner and coworkers [17] and
Kai et al. [18] have shown modest success in calculating
these transition energies.
pressing the CoÃ
The preparation of solutions of Co(OH2)6
[Co(NH3)6][Co(CO3)3] is described in Section 3. Treat-
/
OÃ
/
C angle from ꢀ
/
908 to ꢀ
/
758.
3ꢀ
from
Dꢂ
/
16 130, Bꢂ
/
/
ꢁ
ment with a strong acid HX (Xꢂ
/
ClO4ꢁ, NO3
,
ꢁ
CF3SO3
)
gives solid [Co(NH3)6]X3, CO2 and
Co(OH2)63ꢀ. The solid is easily separated by centrifu-
gation, and the final cobalt(III) and Hꢀ concentrations
are controlled by the mass of [Co(NH3)6][Co(CO3)3]
A useful feature of the electronic spectrum is the
absorbance at 325 nm, the minimum before the peak at
400 nm. At the minimum, oꢂ
6.6 Mꢁ1 cmꢁ1 and any
/
increase in this value is indicative of hydrolysis/oligo-
merization. Below this wavelength, the absorbance rises
monotonically without any maximum down to 200 nm.
The extinction coefficient also has been determined at
used and the initial concentration of HX. Reproducible
3ꢀ
production of Co(OH2)6
requires crystalline rather
than powdered starting material.
The yield of cobalt(III) (ꢀ90%) is best if the reaction
is done at low temperature, routinely 0 8C here, and
with a final [Hꢀ]ꢁ
1.0 M (before dilution). Despite
/
250 nm as 3.0ꢃ
/
103 Mꢁ1 cmꢁ1, in agreement with
/
earlier results [9,14].

G. Wangila, R.B. Jordan / Inorganica Chimica Acta 343 (2003) 347Á
/350
349
3ꢀ
3. Experimental
Co(NH3)6
was calculated from the extinction coeffi-
cients of 56.7 and 3.24 Mꢁ1 cmꢁ1 for Co(NH3)6 and
3ꢀ
2ꢀ
3.1. Materials
Co(OH2)6
Co(NH3)6
,
respectively. Assuming
per mole of material, the average of six
1.8,
1
mole of
3ꢀ
Hexaamminecobalt(III) chloride was prepared by a
3ꢁ
determinations gave a molar mass of 400.39
/
standard method [19]. Solutions containing Co(CO3)3
were prepared by the method of Hoffman-Bang and
compared to the theoretical value of 400.09. The
equivalent weight was determined similarly by reacting
a known mass of sample with a known excess of HCl
and titrating the remaining Hꢀ with standardized
NaOH. The average of four determinations gave an
equivalent weight of 66.79
tical value of 400.09/6ꢂ66.7.
The extinction coefficients were determined in 2.0 M
HClO4 by dissolving various masses of the green
material as described and recording the spectrum on a
Cary 219 spectrophotometer. The cobalt(III) concentra-
tion was determined by adding NaI to an aliquot of the
solution and titrating the I2 with standardized Na2S2O3.
The reported values are the average of four determina-
tions.
Wulff [5] by mixing 2.9 g (1.0ꢃ
10ꢁ2 mole) of cobalt(II)
/
nitrate hexahydrate and 75.0 g (0.89 mole) of sodium
bicarbonate in 250 ml of water. Then 2.0 ml of 30%
hydrogen peroxide in 250 ml of water was added during
1 h, while stirring the mixture. The green solution was
diluted to 1.0 l with water and stirred overnight to
ensure dissolution of the NaHCO3.
/1.1, compared to the theore-
/
3.2. Preparation of [Co(NH3)6][Co(CO3)3]
A 3.1 g (1.15ꢃ
10ꢁ2 mole) sample of hexaammine-
cobalt(III) chloride was dissolved in 200 ml of water and
added to the 1.0 l of green solution described above. The
solution was stirred and then stored in an ice bath in a
/
refrigerator for ꢀ24 h. The dark green crystalline
/
product was collected by filtration, washed with water,
and dried by drawing air through the sample. Yield 1.16
g, or 29% based on the initial cobalt(II) nitrate. Anal.
Calc. for [Co(NH3)6][Co(CO3)3]: H, 4.5; N, 21.0; C, 9.0.
Found: H, 4.5; N, 20.5; C, 9.2%.
4. Supplementary material
Tables listing crystal data, atomic coordinates,
equivalent isotropic displacement parameters, selected
interatomic distances and angles for [Co(N-
H3)6][Co(CO3)3] are available from the author.
3.3. Preparation of hexaaquacobalt(III)
For a typical procedure, 0.25 g of crystalline [Co(N-
H3)6][Co(CO3)3] were added to 5.0 ml of 2.0 M HClO4
Acknowledgements
in an ice-bath, and the mixture was stirred for ꢀ60 min
/
The authors are pleased to acknowledge the Natural
Sciences and Engineering Research Council of Canada
for financial support, and Dr. R. McDonald of the X-
ray Crystallography Service Laboratory of the Univer-
sity of Alberta for the crystal structure.
until all the green crystals had dissolved and evolution
of CO2 had stopped. The mixture was centrifuged and
the supernatant blue solution was separated from the
yellow solid with a pipet or eye-dropper and transferred
to an appropriate container for dilution or direct use.
For the example given, the concentrations before
dilution should be 1.25 M Hꢀ, and 0.125 M Co(III),
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ꢀ
/
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