Preparation, properties and structures of the clusters hexakis(.mu.3-oxo)pentakis(.eta.5-cyclopentadienylvanadium), [(.eta.5-C5H5)5V5O6], and tetrakis(.mu.3-oxo)tetrakis(.eta.5-cyclopentadienylchromium), [(.eta.5-C5H5)4Cr4O4]