The acetylido methyl iron(II) complexes, cis/trans-[Fe(dmpe)(2)(Cequivalent toCR)(CH3)] (1) and trans-[Fe(depe)(2)(Cequivalent toCR)(CH3)] (2) (dmpe = 1,2-dimethylphoshinoethane; depe = 1,2-diethylphosphinoethane), were synthesized by transmetalation from the corresponding alkyl halide complexes. Acetylido methyl iron(II) complexes were also formed by transmetalation from the chloride complexes, trans-[Fe(dmpe)(2)(Cequivalent toCR)(Cl)] or trans-[Fe(depe)(2)(Cequivalent toCR)(Cl)]. The structure of trans-(Fe(dmpe)(2)(Cequivalent toCC(6)H(5)) (CH3)] (1a) was determined by single-crystal X-ray diffraction. The methyl acetylido iron complexes, [Fe(dmpe)(2)(Cequivalent toCR)(CH3)] (1), are thermally stable in the presence of acetylenes; however, under UV irradiation, methane is lost with the formation of a metal bisacetylide. Photochemical metathesis of cis- or trans-(Fe(dmpe)(2)(CH3)(C Cequivalent toCR)] (R = C6H5 (1a), 4-C6H4OCH3 (1b)) with terminal acetylenes was used to selectively synthesize unsymmetrically substituted iron(II) bisacetylide complexes of the type trans-[Fe(dmpe)(2)(Cequivalent toC R)(Cequivalent toCR')] [R = Ph, R' = Ph (6a), 4-CH3OC6H4 (6b), Bu (6c), Si(CH3)(3) (6d), (CH2)(4)Cequivalent toCH (6e); R = 4-CH3OC6H4, R' = 4-CH3OC6H4, (6g), Bu-t (6h), (CH2)(4)Cequivalent toCH (6i), adamantyl (6i)]. The structure of the unsymmetrical iron(II) bisacetylide complex trans-[Fe(dmpe)(2)(Cequivalent toCC(6)H(5))(C=CC6H4OCH3)] (6b) was determined by single-crystal diffraction. The photochemical metathesis of the bis-acetylene, 1,7-octadiyne, with trans-[Fe(dmpe)(2)(CH3)(Cequivalent toCPh)] (1a), was utilized to synthesize the bridged binuclear species trans, trans-[(C6H5C=C) Fe(dmpe)(2)(mu-Cequivalent toC(CH2)(4)Cequivalent toC)Fe(dmpe)(2)(Cequivalent toCC(6)H(5))] (11). The trinuclear species trans, trans, trans-[(C(6)H(5)Cequivalent toC)Fe(dmpe)(2)(mu-Cequivalent toC(CH2)(4)Cequivalent toC)Fe(dmpe)(2)(mu-Cequivalent toC(CH2)(4)Cequivalent toC)Fe(dmpe)(2)(Cequivalent toCC(6)H(5))](12) was synthesized by the photochemical reaction of Fe(dmpe)(2)(Cequivalent toCPh)(Cequivalent toC(CH2)(4)Cequivalent toCH) (6e) with Fe(dmpe)(2)(CH3)(2.) Extended irradiation of the bisacetylide complexes with phenylacetylene resulted in insertion of the terminal alkyne into one of the metal acetylide bonds to give acetylide butenyne complexes. The structure of the acetylide butenyne complex, trans-[Fe(dmpe)(2)(Cequivalent toCC(6)H(4)OCH(3))(eta(1)-C(C6H5)=CH(Cequivalent toCC(6)H(4)OCH(3)))] (9a) was determined by single-crystal X-ray diffraction.