Organometallic oxides: preparation and properties of the tetrahedral (Td) chromium cubane [(.eta.-C5(CH3)5)Cr(.mu.3-O)]4 and comparison with the D2 [(.eta.-C5H5)Cr(.mu.3-O)]4 and D2d [(.eta.-C5H4CH3)Cr(.mu.3-O)]4