Mechanism of the Low-Energy Photochemical Disproportionation Reactions of (η5-C5H5)2Mo2(CO)6

Article abstract of DOI:10.1021/ja00357a012

The photochemical disproportionation reactions of the (RCp)2Mo2(CO)6 (R = H or CH3; Cp = η⁵-C5H4) complexes were investigated.The general disproportionation reaction can be written (RCp)2Mo2(CO)6 + 2L <*> (RCp)Mo(CO)3^- + (RCp)Mo(CO)2L2⁺ + CO.Control experiments showed that the previously reported analogous thermal disproportionation reactions are actually photochemical reactions.The properties of the ligand L are important in determining whether or nor the (RCp)2Mo2(CO)6 complex will disproportionate; the ligands cannot be sterically bulky and they must be good electron-donating ligands if disproportionation is to occur.If either of these criteria is not met, the irradiation of the (RCp)2Mo2CO)6 complex in the presence of the ligand leads only to substitution products of the type (RCp)2Mo2(CO)5L and/or (RCp)2Mo2(CO)4L2.The disproportionation reaction is wavelength dependent.For example, PPh3 will disproportionate the (RCp)2Mo2(CO)6 complex upon 290-nm irradiation but not upon 405-nm irradiation.Smaller ligands will disproportionate the dimer with 405-nm irradiation.The mechanism of lower energy pathway was investigated in detail, and it is proposed to be a radical chain pathway: (1) (RCp)2Mo2(CO)6->2(RCp)Mo(CO)3, (2) (RCp)Mo(CO)3+L->(RCp)Mo(CO)2L+CO, (3) (RCp)Mo(CO)2L+L->(RCp)Mo(CO)2L2, (4) (RCp)Mo(CO)2L2+(RCp)2Mo2(CO)6->(RCp)Mo(CO)2L2⁺+(RCp)2Mo2(CO)6^-, (5) (RCp)2Mo2(CO)6^-->(RCp)2Mo2(CO)3^-+(RCp)Mo(CO)3.The quantum yield data support this chain pathway; the quantum yields are greater than 1 and they are not reproducible from trial to trial.The key intermediate (CH3Cp)2Mo2(CO)6^- was generated by reacting (CH3Cp)2Mo2(CO)6 with Na; in the presence of appropriate L the disproportionation resulted.The key to the mechanism is the formation of the 19-electron intermediate (RCp)Mo(CO)2L2.Electron transfer from this electron rich species is the driving force for the reaction.Evidence is presented for the formation of this intermediate.