Electrophilic Attack on a Metal Alkynyl by Carbon Disulfide: Preparation and Structure of [Fe(C₂MeCS₂Me)(dppe)(Cp)]I·MeOH, a Cationic Vinylidene Complex

The electron-rich metal alkynyl complex [Fe(C≡≡CMe)(dppe)(Cp)] (Fp′C≡CMe, I) undergoes (2 + 2) cycloaddition with carbon disulfide to form [formula omitted] (II) containing a 2H-thiete-2-thione functional group. This CS₂ adduct is subsequently alkylated by iodomethane to yield [Fe(C₂MeCS₂Me)(dppe)(Cp)]I·MeOH (III), a cationic vinylidene complex. The products were investigated by spectroscopic means and, in the case of III, by an X-ray crystal structure analysis: space P2₁/c, a = 11.625 (3) Å, b = 20.609 (4) Å, c = 15.490 (2) Å, β = 103.25 (2)°, V = 3612.4 ų, Z = 4, ρ(calcd) = 1.487 g·cm⁻³. Refinement on 2552 reflections with F² ≥ 3σ(F²) yielded the final discrepency indices R = 0.058 and R_w = 0.070. The substituted vinylidene ligand of complex III is bonded to the iron atom by a very short FeC bond, 1.74 (2) Å, and the FeClC2 bond angle is nearly linear, 176 (1)°. Spectroscopic and structural evidence suggest that 2-(dithiocarbomethoxy)propenylidene is a very large strong π-acid ligand.