Alkoxy and aryloxy derivatives of (pentamethylcyclopentadienyl)ruthenium. X-ray crystal structures of [(.eta.5-C5Me5)Ru(.mu.-OMe)]2, [(.eta.5-C5Me5)(CO)Ru(.mu.-OEt)]2, and (.eta.5-C5Me5)Ru(.eta.5-2,6-tBu2C6H3O) and molecular orbital analysis of [(.eta.5-C5H5)Ru(.mu.-OMe)]2