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A thermal and structural study on Lanthanum Hexacyanocobaltate(III) pentahydrate, La[Co(CN)6]·5H2O. Part III

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Abstract

On dehydration of La[Co(CN)6]·5H2O, the color of the complex, changes from white to pale blue at around 230°C. Heating the pale blue specimen, the color changes to deep blue at around 290°C. This deep blue specimen is easily rehydrated to a pink one. As reported previously, in the pale blue specimen, Co3+ ions are situated in the center of the D4h crystal field formed by six CN- ions.

The deep blue specimen is due to the presence of [Co(CN)4]2- ions in which Co2+ was situated in a Td coordination field formed by four CN- ions and the Co-C bond length is 1.67 Ĺ. The pink species corresponded to trans-[Co(CN)4(H2O)2]2- and the bond lengths of Co-C and Co-O are 1.89 and 1.85 Ĺ, respectively.

The Raman spectra of the complex observed at 25°C displays two bands at 2157 and 2176 cm-1 associated with the vibration of C-N bond, and the band of 2157 cm-1 was split into two bands, 2150 and 2156 cm-1, at around 100°C. When the complex was heated to around 230°C, three new bands were observed at 2103, 2116 and 2141 cm-1. The bands of 2103 and 2116 cm-1 were assigned to the stretching vibration of C=N bonding to Co2+. The band of 2141 cm-1 was assigned to the stretching vibration of the inverted CN- as follows: Co-C=N-La→Co-N=C-La. The activation energy for the inversion of CN- was estimated as 67 kJ mol-1.

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Seto, Y., Nagao, S., Wang, X. et al. A thermal and structural study on Lanthanum Hexacyanocobaltate(III) pentahydrate, La[Co(CN)6]·5H2O. Part III. Journal of Thermal Analysis and Calorimetry 73, 755–762 (2003). https://doi.org/10.1023/A:1025874212520

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  • DOI: https://doi.org/10.1023/A:1025874212520

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