Back-reactions in the photochemical disproportionation of Cp2Mo2(CO)6 (Cp = C5H4CH3) and the wavelength-dependent photochemistry of the Cp2Mo2(CO)6 complex with PPh3

Irradiation (λ > 525 nm) of Cp₂Mo₂(CO)₆ (Cp = η⁵-C₅H₅, η⁵-C₅H₄CH₃) with PPh₃ in CH₂Cl₂ proceeds as follows: Cp₂Mo₂(CO)₆ + PPh₃ →^hv CpMo(CO)₃PPh₃ ⁺ + CpMo(CO)₃ ^-. A similar disproportionation reaction does not occur in benzene or other nonpolar solvents for λ > 525 nm but does occur for λ = 290 nm. (The cationic product in this case is CpMo(CO)₂(PPh₃)₂ ⁺.) The dependence of the disproportionation reaction on solvent and radiation wavelength is shown to be a consequence of a facile back-reaction: CpMo(CO)₃PPh₃ ⁺ + CpMo(CO)₃ ^- → Cp₂Mo₂(CO)₆ + PPh₃. Thus, irradiation of Cp₂Mo₂(CO)₆ with PPh₃ at 525 nm in nonpolar solvents does disproportionate the dimer but the back-reaction occurs and there is no net disproportionation. The back-reaction is considerably slower in CH₂Cl₂, and net disproportionation is therefore observed in this solvent. Net disproportionation does occur at 290 nm in benzene because a secondary photochemical reaction of the initially formed CpMo-(CO)₃PPh₃ ⁺ product occurs: CpMo(CO)₃PPh₃ ⁺ + PPh₃ →^hv CpMo(CO)₂(PPh₃)₂ ⁺; this disubstituted cation and CpMo(CO)₃ ^- do not back-react. Several other unexplained observations from our previous studies of the disproportionation reactions can also be interpreted in terms of the occurrence of a back-reaction. The back-reactions can be prevented in several ways: a more polar solvent can be used; the cationic or anionic products can be reacted further; the disproportionation reaction can be done at low temperature. Each of these methods is discussed and demonstrated.