Optical and ESR spectra for high-spin tetrahedral complexes of Co(III) and Co(II) in Keggin-structure heteropolytungstate.

The ligand-field spectra for the cobalt complexes in the single crystals of K3H2[CoIIIW12O40]·16H2O and K4H2[(COII, ZnII)W12O40]·nH2O have been observed at several temperatures above 1.4 K and assigned under D2 site symmetry. In the spectrum of the elongated CoIIIO4 complex, two absorption bands of 8250 and 8953 cm−1 strengthen with increasing temperature by vibronic mechanism. The X-band ESR spectra of both complexes have also been measured at 77 K. The signal from the Co(III) complex is not observed. The signal of the high-spin Co(II) complex of tetrahedral symmetry is unequivocally observed even at relatively high temperature of 77 K. From the analysis of the ESR spectrum, it is concluded that the CoIIO4 tetrahedron of D2 symmetry is flattened and deviated slightly from axial symmetry.