Synthesis, stereochemistry, and bonding of cobalt-cobalt multiple-bonded (pentamethylcyclopentadienyl)- and cyclopentadienylcobalt carbonyl dimers, [Co2(.eta.5-C5R5)2(.mu.-CO)2]n (R = Me, n = 0,1-; R = H, n = 1-): a comparative analysis of the antibonding dimetal nature of the unpaired electron in the monoanions and its structural effects upon oxidation