Two-electron reduction of Fe3(CO)9(.mu.3-CCH2R)(.mu.3-COC2H5) complexes: induction of an alkylidyne-alkylidyne coupling reaction and of a subsequent rearrangement to acetylide or allenyl ligands. X-ray structure of [PPh4][Fe3(CO)9(.mu.3-.eta.3-C(OC2H5):C:CHC(O)CH3)] at 100 K