NaBH 4 gives addition at the bridging vinyliminium ligand in the diiron complexes Fe 2{μ-η 1:η 3-C γ(R′) C β (R″)C α N(Me)(R)}(μ-CO)(CO)(Cp) 2 SO 3CF 3 (R′ and R″ = alkyl or aryl substituents, R = Me, CH 2Ph or Xyl). Depending on the nature of these substituents and on the geometry of the parent complexes cis or trans, H − addition occurs selectively at the iminium carbon (C α) or at the adjacent C β position, affording complexes of the type 5d or 6a, respectively. New μ-vinylalkylidene complexes cis-Fe 2{μ-η 1:η 3-C γ(R′)C β(R″) C αHN(Me)(R)}(μ-CO)(CO)(Cp) 2 (R = Me, R′ = R″ = Me, 3a; R = Me, R′ = R″ = Et, 3b; R = Me, R′ = R″ = Ph, 3c; R = CH 2Ph, R′ = R″ = Me, 3d; R = CH 2Ph, R′ = R″ = COOMe, 3e; R = CH 2 Ph, R′ = SiMe 3, R″ = Me, 3f) have been obtained b yreacting the corresponding vinyliminium complexes Fe 2{μ-η 1:η 3-C γ(R′) C β(R″)C α N(Me)(R)}(μ-CO)(CO)(Cp) 2SO 3CF 3 ( 2a– f) with NaBH 4. The formation of 3a– f occurs via selective hydride addition at the iminium carbon (C α) of the precursors 2a– f. By contrast, the vinyliminium cis-Fe 2{μ-η 1:η 3-C γ (R′) = C β(R″)C α = N(Me)(Xyl)}(μ-CO)(CO)(Cp) 2SO 3CF 3 (R′ = R″ = COOMe, 4a; R′ = R″ = Me, 4b; R′ = Pr n , R″ = Me, 4c; Pr n = CH 2CH 2CH 3, Xyl = 2,6-Me 2C 6H 3) undergo H − addition at the adjacent C β, affording the bis-alkylidene complexes cis-Fe 2{μ-η 1:η 2-C(R′)C(H)(R″)CN(Me)(Xyl)}(μ-CO)(CO)(Cp) 2, ( 5a– c). The cis and trans isomers of Fe 2{μ-η 1:η 3-C γ(Et) C β(Et)C α N(Me)(Xyl)}(μ-CO)(CO)(Cp) 2SO 3CF 3 ( 4d) react differently with NaBH 4: the former reacts at C α yielding cis-Fe 2{μ-η 1:η 3-C γ(Et)C β(Et) C αHN(Me)(Xyl)}(μ-CO)(CO)(Cp) 2, 6a, whereas the hydride attack occurs at C β of the latter, leading to the formation of the bis alkylidene trans-Fe 2{μ-η 1:η 2-C(Et)C(H)(Et)CN(Me)(Xyl)}(μ-CO)(CO)(Cp) 2 ( 5d). The structure of 5d has been determined by an X-ray diffraction study. Other μ-vinylalkylidene complexes cis-Fe 2{μ-η 1:η 3-C γ(R′)C β(R″) C αHN(Me)(Xyl)}(μ-CO)(CO)(Cp) 2, (R′ = R″ = Ph, 6b; R′ = R″ = Me, 6c) have been prepared, and the structure of 6c has been determined by X-ray diffraction. Compound 6b results from treatment of cis-Fe 2{μ-η 1:η 3-C γ(Ph) C β(Ph)C α N(Me)(Xyl)}(μ-CO)(CO)(Cp) 2SO 3CF 3 ( 4e) with NaBH 4, whereas 6c has been obtained by reacting 4b with LiHBEt 3. Both cis- 4d and trans- 4d react with LiHBEt 3 affording cis- 6a.