Structural Features of Green Cobalt(III) Hydroxide
Structural Features of Green Cobalt(III) Hydroxide. D. A. Pankratov, A. A. Veligzhanin, Y. V. Zubavichus //Russian Journal of Inorganic Chemistry, 2013, Vol. 58, No. 1, pp. 67–73
Emission Mössbauer and X-ray absorption XANES/EXAFS spectroscopic techniques are applied to elucidate the structural features of green cobalt(III) hydroxide. A comparative analysis of structurally characterized cobalt(II) and cobalt(III) oxo-compounds shows that the parameters of the local environment of cobalt atoms in green cobalt(III) hydroxide differ substantially from those of its analogues.
According to several reports of the late 50's of the twentieth century, the low-temperature reaction of cobalt(II) chloride with hydrogen peroxide in alcoholic media in the presence of sodium hydroxide affords dark green cobalt(II) peroxide. Although this synthesis protocol is broadly recited (in particular, it is described in all editions of the classical manual on inorganic synthesis by N.G. Klyuchnikov starting from 1965), the formation of such a simple cobalt(II) peroxide compound seems hardly possible.
Indeed, although there are many compounds with a cobalt atom coordinated to a dioxygen moiety, this always occurs in mixed-ligand coordination compounds. A dioxygen ligand can be coordinated to either one or two cobalt atoms thus acting as a bridging group in a binuclear compound. In all cases depending on the specific pattern of the electron density redistribution between the anti-bonding orbitals of the dioxygen ligand and the cobalt π-orbitals, these compounds are classified as cobalt(III) and cobalt(III, III) or cobalt(III, IV) peroxo- or superoxo complexes in the cases of mono- and binuclear compounds, respectively. The binding of a peroxo group to a cobalt(II) atom with the d7 electronic configuration is energetically unfavorable and, thus, is hardly possible, taking into account the pronounced polarizability and donor properties of the electron pair residing in the anti-bonding orbital of the peroxide group. This suggestion is indirectly supported by our earlier results on similar heavy transition metal, such as platinum and iridium dioxygen complexes.
We tried to reproduce the described protocol for the synthesis of dark green cobalt(II) peroxide and found it to be incorrect. A detailed study using emission Mössbauer spectroscopy unambiguously demonstrated that the reaction product contained cobalt in the oxidation state +3 with no apparent coordination to a peroxo group. We suggested that this compound was actually a new green modification of cobalt(III) hydroxide. Therefore, the present paper is aimed at the further structural characterization of this compound and in particular at a comparative analysis of structural parameters of the closest coordination environment of the cobalt atoms in structurally characterized cobalt(II) and cobalt(III) hydroxo compounds.
In summary, a consistent comparative analysis of emission Mössbauer and EXAFS/XANES spectroscopy data for all known cobalt(II) and cobalt(III) hydroxo compounds show a substantial difference between the structural parameters of the closest coordination spheres of Co atoms in green cobalt(III) hydroxide from those of its analogues. The distinctive structural features of this compound include: shortened Co–O interatomic distances within the first coordination sphere; a larger electric field gradient and higher electron density on Co atoms with respect to brownish-black Co(OH)3; the splitting of the Co–Co second coordination sphere into two components. In overall, green cobalt(III) hydroxide is less ordered than brownish-black cobalt(III) hydroxide.