抄録
Anhydrous phosphates of transition metals exhibit a surprising variety of colours. Even phosphates of the same cation show significantly different electronic absorption spectra. Using calculations within the framework of the angular overlap model a detailed correlation can be obtained between the observed electronic transitions and the particular coordination geometry of M3+ in a series of anhydrous phosphates (M(PO3)3, MPO4, M4(P2O7)3, and M4P6Si2O25) of trivalent vanadium and chromium. Our investigation gives evidence for the strong influence of the coordination number of oxygen on its coordination behaviour towards M3+. While isotropic π-bonding is observed for such O2- coordinated by P5+ and one M3+, oxygen with C. N.=3 (P5+ and 2 M3+) exhibits anisotropic π-bonding towards M3+. Thus the π-interaction M3+-O2- within the plane through the two M3+ and the bridging O2- is significantly reduced compared to the interaction perpendicular to this plane.
