The Emission Spectra of Hexamminechromium(III) Hexacyanocobaltate(III), [Cr(NH3)6][Co(CN)6]

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Abstract
Spin-forbidden absorption bands of K3[Co(CN)6] were reexamined employing conventional transmission spectroscopy, diffuse reflectance spectroscopy, and microspectrophotometry. A small but very distinct shoulder (ε≈0.3–0.4) obtained at a low temperature at 26000 cm−1 was assigned to the 3T1g←1A1g transition. Another very small band (ε≈0.001) at 16300 cm−1 was probably due to an impurity. The emission spectra of K3[Co(CN)6] powder in the temperature range from room temperature to that of liquid hydrogen were observed by means of a modified Hitachi 203 fluorescence spectrophotometer. The broad emission band at room temperature showed some vibrational structures at low temperatures. The emission spectra of powders of [Cr(NH3)6](NO3)3 and [Cr(NH3)6][Co(CN)6] were also measured. The following results were obtained: i) Similar emission spectra, which seemed to originate from [Cr(NH3)6]3+, were observed with both complexes [Cr(NH3)6](NO3)3 and [Cr-(NH3)6][Co(CN)6] using mercury lines at 313, 365, and 435 nm for excitation, ii) With 254 nm excitation, such emission spectra as i) were not observed with [Cr(NH3)6](NO3)3, but the same emission spectra as in i) were observed with [Cr(NH3)6][Co(CN)6]. iii) The ratio of the emission intensities at 15000 cm−1 with 435 and 313 nm excitations was 1:0.25–0.30 for [Cr(NH3)6](NO3)3, but it was 1:1.5–1.7 for [Cr(NH3)6][Co(CN)6]. From these results, it was presumed that, in [Cr(NH3)6][Co(CN)6], there occurred an energy transfer from the complex anion to the complex cation. Furthermore, emission spectra were observed with [Cr(NH3)6](ClO4)3 and the equimolar mixture of complex salts [Cr(NH3)6](ClO4)3–K3[Co(CN)6] and [Cr(NH3)6](NO3)3–K3[Co(CN)6] for the purpose of confirming the energy transfer in [Cr(NH3)6][Co(CN)6]. The temperature dependence of the emission intensity was also discussed.