Chemical and electrochemical reduction of .eta.5-cyclopentadienyldicarbonylcobalt(I) and .eta.5-cyclopentadienyl(triphenylphosphine)carbonylcobalt(I). Synthesis, crystal structure, and chemistry of sodium and bis(triphenylphosphine)iminium bis(.eta.5-cyclopentadienyl)di-.mu.-carbonyl-dicobaltate