Nineteen-electron Adducts In The Photochemistry of Cp₂Fe₂(CO)₄(Cp = η⁵-C₅H₅)

Cp₂Fe₂(CO)₄is 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_„L_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 CH₂C1₂solution, 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)₆³“, 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.