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.
|Number of pages||3|
|Publication status||Published - 1986|
ASJC Scopus subject areas
- Inorganic Chemistry