Aromaticity in transition metal oxide structures

The concept of aromaticity is useful for understanding the properties of so me polyoxometalates containing transition metals such as vanadium, molybden um, and tungsten having structures based on metal macropolygons and macropo lyhedra with M-O-M edges. Thus, the aromatic macrocuboctahedral Keggin ions readily undergo one-electron reductions to highly colored mixed-valence "b lues" (e.g., molybdenum blue), whereas the macroicosahedral Silverton ions, (MMo12O428-)-Mo-IV (M-IV = Ce, Th, U), which, like cyclohexane, do not hav e vertex valence orbitals available for delocalization, do not undergo anal ogous reduction reactions. A macrohexagon of d(1) vanadium(IV) atoms as V-O -V units has been imbedded into an electronically inactive berate matrix in the ion [V6B20O50H8](8-). The small beta unit for the V-O-V interactions i n this Vg macrohexagon leads to an unprecedented example of high spin aroma ticity with a paramagnetism corresponding to four unpaired electrons per V- 6 Unit in contrast to benzene, which is diamagnetic and hence exhibits low spin aromaticity. The M-O-M interactions in these aromatic metal oxides are closely related to the Cu-O-Cu interactions in the high critical temperatu re superconducting copper oxides which are essential to the electron transp ort in these systems.