Absorption Spectra of Tetrahedral Co(III) and Co(II) Complexes in Heteropolytungstates

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Abstract The absorption spectra of Co(III) and Co(II) in the single crystals of the Keggin-structure compounds, K5[CoIIIW12O40]·nH2O and K6[CoIIW12O40]·mH2O, have been measured at 77 K. The spectra of these complexes in aqueous solutions have also been observed. All the spectra have been interpreted using crystal-field theory by assuming that a Co(III) or Co(II) ion is in Td symmetry. The parameters, Dq, B, and C⁄B can be evaluated as 780 cm−1, 630 cm−1, and 4.7 respectively for the Co(III) complex, and 460 cm−1, 650 cm−1, and 4.5 respectively for the Co(II) complex.