Alkylidyne-alkylidyne coupling induced by carbon monoxide addition to the bis(alkylidyne) cluster Fe3(CO)9(.mu.3-CCH3)(.mu.3-COC2H5): an unexpected reversible reaction. X-ray structure of Fe3(CO)9[P(C6H5)3](.mu.3-.eta.2-CH3C.tplbond.COC2H5)