Stereochemical and electrochemical analyses of the cyclopentadienyl and methylcyclopentadienyl dinitrosyl-capped tricobalt analogs, Co3(.eta.5-C5H5-xMex)3(.mu.3-NO)2 (where x = 0, 1), of the classical Fischer-Palm Ni3(.eta.5-C5H5)3(.mu.3-CO)2