Nineteen-electron adducts in the photochemistry of Cp2Fe2(CO)4 (Cp = η5-C5H5)

Cp₂Fe₂(CO)₄ is apparently different from the other metal-metal-bonded carbonyl dimers in that irradiation of this dimer in the presence of ligands does not lead to disproportionation: Cp₂Fe₂(CO)₄ + L + hv » CpFe(CO)₂ ^- + CpFe(CO)_3-nL_n ⁺. However, it was demonstrated that with most ligands the disproportionation reaction does occur but the back-reaction of the products is facile. The back-reaction can be prevented by reacting further one of the products. For example, in CH₂Cl₂ solution, CpFe(CO)₂ ^- reacts with the solvent to give CpFe(CO)₂CH₂Cl and the back-reaction is prevented. The mechanism of the disproportionation reaction is proposed to be a chain mechanism involving 19-electron adducts formed by the reaction of a 17-electron metal radical with a ligand, e.g. CpFe(CO)₂ + L → CpFe(CO)₂L. It is demonstrated that the 19-electron adducts are powerful reductants. The following organometallic, organic, and inorganic substrates were reduced by the 19-electron species, thereby demonstrating the versatility of these species as reducing agents: CpMo(CO)₃Cl, Fe(CN)₆ ^3-, Mn₂(CO)₁₀, Re₂(CO)₁₀, Ru₃(CO)₁₂, Fe(CO)₅, N-n-butylpyridinium, and Cp₂Co⁺. Experiments involving the reduction of Cp₂Co⁺ demonstrated several mechanistic points concerning the reactivity of 19-electron species.