Polarized electronic absorption spectra for potassium tetrabromoplatinate(II) dihydrate. Spectral effects of a platinum(IV) component

Crystals of the metastable compound K/sub 2/PtBr/sub 4/.2H/sub 2/O have an orthorhombic structure, space group Pbam, with a:b:c = 8.31 (2):13.73 (3):4:84 (1) A and Z = 2. The PtBr/sub 4//sup 2 -/ ions occupies a crytallographic site of C/sub 2h/ symmetry but retains essentially a D/sub 4h/ molecular symmetry with the z axis directed along c. The absorption spectra in the d reverse arrow d region were recorded for a, b, and c polarizations. Band maxima lie at 300 to 800 cm/sup -1/ higher energies than in anhydrous K/sub 2/PtBr/sub 4/. The /sup 1/A/sub 2g/ reverse arrow /sup 1/A/sub 1g/ transition, forbidden in z polarization, was not observed in polarization. Vibrational components were resolved in the /sup 3/B/sub 1g/ shoulder and in the /sup 1/A/sub 2g/ peak. Striking differences in intensities which occurred in the a and b polarization indicated that crystal effects were important in the vibronic excitation process. Red sections, in some crystals, were attributed to a dipole-allowed band, completely c polarized, for a mixed-valence electron-transfer transition involving a PtBr/sub 6//sup 2 -/ impurity defect. 6 figures, 3 tables.