Alkyne-induced carbyne-carbyne coupling in the trinuclear bis(carbyne) iron cluster Fe3(CO)9(.mu.3-COC2H5)(.mu.3-CCH3). X-ray structures of Fe3(CO)6(.mu.-CO)2.mu.3-.eta.4[C(OC2H5)C(CH3)C(C6H5)C(C6H5)] and Fe3(CO)6(.mu.-CO)2.mu.3-.eta.4[C(CH3)C(OC2H5)C(Si(CH3)3)C(Si(CH3)3)]