Transformation of the C2H ligand in Fp*C.tplbond.CH [Fp* = (.eta.5-C5Me5)Fe(CO)2] into various C2 functional groups via an iron-substituted vinylidene complex, (.eta.5-C5H4Me)Mn(CO)2[:C:C(H)Fp*]: its amphoteric reactivities, structural comparisons relevant to 1-alkyne-to-vinylidene rearrangements, and electronic influences on structures of heterobimetallic bridging alkynyl complexes [(.eta.5-C5R5)M(CO)2]2(.mu.-C2R)