Bis(acetylide) complexes of iron(II) phosphines Fe(DMPE)2 (DMPE = (CH3)2PCH2CH2P(CH3)2) rearrange in the presence of mild acids to give cationic complexes of general formula [Fe(eta-3-C(C=CR)=CHR)(DMPE)2]+ containing an eta-3-bound but-1-en-3-yn-2-yl ligand. A number of iron complexes with coordinated butenyne ligands have been fully characterized as their hexafluorophosphate salts, and their synthesis and properties are reported.