Alkynes (R ′CCR ″) insert into the μ-carbon–metal bond of diiron complexes affording μ-σ:η 3 vinyliminium species. Several isomers have been found arising from: (i) the iminium moiety configuration ( E, Z), (ii) mutual Cp position ( cis– trans isomers), (iii) head–head or head–tail insertion mode of R ′CCR ″. These isomeric forms have been investigated by NMR and X-ray diffraction. Terminal alkynes (HCCR ′) (R ′=COOMe, CH 2OH) insert into the metal–carbyne bond of the diiron complexes Fe 2{μ-CN(Me)(R)}(μ-CO)(CO)(NCMe)(Cp) 2SO 3CF 3 (R=Xyl, 1a; CH 2Ph, 1b; Me, 1c; Xyl=2,6-Me 2C 6H 3), affording the corresponding μ-vinyliminium complexes Fe 2{μ-σ:η 3-C(R ′)CHCN(Me)(R)}(μ-CO)(CO)(Cp) 2SO 3CF 3 (R=Xyl, R ′=COOMe, 2; R=CH 2Ph, R ′=COOMe, 3; R=Me, R ′=COOMe, 4; R=Xyl, R ′=CH 2OH, 5; R=Me, R ′=CH 2OH, 6). The insertion is regiospecific and C–C bond formation selectively occurs between the carbyne carbon and the CH moiety of the alkyne. Disubstituted alkynes (R ′CCR ′) also insert into the metal–carbyne bond leading to the formation of Fe 2{μ-σ:η 3-C(R ′)C(R ′)CN(Me)(R)}(μ-CO)(CO)(Cp) 2SO 3CF 3 (R ′=Me, R=Xyl, 8; R ′=Et, R=Xyl, 9; R ′=COOMe, R=Xyl, 10; R ′=COOMe, R=CH 2Ph, 11; R ′=COOMe, R=Me, 12). Complexes 2, 3, 5, 8, 9 and 11, in which the iminium nitrogen is unsymmetrically substituted, give rise to E and/or Z isomers. When iminium substituents are Me and Xyl, the NMR and structural investigations (X-ray structure analysis of 2 and 8) indicate that complexes obtained from terminal alkynes preferentially adopt the E configuration, whereas those derived from internal alkynes are exclusively Z. In complexes 8 and 9, trans and cis isomers have been observed, by NMR spectroscopy, and the structures of trans- 8 and cis- 8 have been determined by X-ray diffraction studies. Trans to cis isomerization occurs upon heating in THF at reflux temperature. In contrast to the case of HCCR ′, the insertion of 2-hexyne is not regiospecific: both Fe 2{μ-σ:η 3-C(CH 2CH 2CH 3)C(Me)CN(Me)(R)}(μ-CO)(CO)(Cp) 2SO 3CF 3 (R=Xyl, 13; R=Me, 15) and Fe 2{μ-σ:η 3-C(Me)C(CH 2CH 2CH 3)CN(Me)(R)}(μ-CO)(CO)(Cp) 2SO 3CF 3 (R=Xyl, 14, R=Me, 16) are obtained and these compounds are present in solution as a mixture of cis and trans isomers, with predominance of the former.