A new metal carbonyl cluster, [PPN]2[Fe6(CO)18C4], containing a C4 ligand forms upon the reaction of [Fe3(CO)9CCO]2- (I) with (CF3SO2)2O. We postulate that triflic anhydride attacks the ketenylidene unit, resulting in a short-lived intermediate, [Fe3(CO)9CCOSO2CF3]-, which undergoes C-C coupling with loss of SO3CF3- togenerate the unusual C4 ligand. X-ray crystallographic data indicate a C1-C2-C3 angle of 148.0(6)degrees and bond distances suggestive of a C=C-C=C linkage.