Electronic structures, (d-p)π conjugation effects, and spectroscopic properties of polyoxometalates:: M6O192- (M = Cr, Mo, W)

The electronic structures and bonding of isopoly oxometalates M6O192- (M=Cr, Mo, W) have been investigated by using ab initio and relativistic density functional methods. We have discussed the role of the central oxygen atom and the (d-p)pi conjugation interactions between the metal and bridging oxygen atoms. It is found that there exist 12 three-centered two-electron (d-p-d)pi bonds for the three M-4(mu-O)(4) planar rings in M6O192- ions and these hexametalates are considered to have quasi-aromaticity. The (d-p) pi conjugation effects play essential role in stabilizing these cluster compounds, and the reduced (d-p)pi conjugation effects account for the instability of the isopoly oxochromate ion, Cr6O192-. The vibrational spectra and electronic spectra of M6O192- ions are evaluated and assigned theoretically and the calculated spectra are in fairly good agreement with the measured experimental results.