Photochemical disproportionation of Mn2(CO)10. Nineteen-electron intermediates and ligand and intensity dependence

The photochemical disproportionation of Mn₂(CO)₁₀ proceeds as Mn₂(CO)¹⁰ 366nm → +L, -2CO Mn(CO)₅ ^- + Mn(CO)₃L₃ ⁺ where L is a nitrogen or oxygen donor ligand. With many ligands, but not CH₃CN, a secondary disproportionation of Mn(CO)₃L₃ ⁺ occurs: 3Mn(CO)₃L₃ ⁺ 366nm → +L 2MnL₆ ²⁺ + Mn(CO)₅ ^- + 4CO The net reaction is thus 3Mn₂(CO)₁₀ + 12L hv → 2MnL₆ ²⁺ + 4Mn(CO)₅ ^- + 10CO The stoichiometry of Mn(CO)₅ ^- formation in CH₃CN solution was measured and found to be as described in the initial equation above. Disproportionation of Mn₂(CO)₁₀ occurs with nitrogen- and oxygen-donor ligands but not with monodentate phosphines and phosphites; disproportionation does result, however, with the multidentate 1,2-bis(dimethylphosphino)ethane (dmpe), bis-(2-(diphenylphosphino)ethyl)phenylphosphine (triphos), and 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (tetraphos) ligands. These results are interpreted in terms of the previously proposed radical-chain pathway for Mn₂(CO)₁₀ disproportionation involving 19-electron Mn(CO)₃L₃ intermediates. It is proposed that steric bulk and electron-donating ability are the dominant factors in determining whether or not the dimer will photochemically disproportionate with a particular ligand. Disproportionation occurs with the chelating ligands because these ligands effectively increase the concentration of the key 19-electron intermediate. Two experiments provide additional evidence for the 19-electron intermediate: (1) Reaction of PMe₃ with Mn(CO)₃depe (depe = 1,2-bis(diethylphosphino)ethane) in the presence of Mn₂(CO)₁₀ in the dark gives disproportionation products. It is proposed that PMe₃ attacks Mn(CO)₃depe, giving the 19-electron complex Mn(CO)₃(depe)(PMe₃), which then reduces Mn₂(CO)₁₀. (2) The cationic product from the reaction of Mn₂(CO)₁₀ with tetraphos is Mn(CO)₃(tetraphos-P,P′,P″)⁺. The formation of this product and not Mn(CO)₂(tetraphos-P,P′,P″,P‴)⁺ supports the proposal that the chain reaction involves electron transfer from a 19-electron Mn(CO)₃L₃ species rather than from a 17-electron Mn(CO)₂L₃ intermediate; the latter would yield the Mn(CO)₂L₄ ⁺ cation product. The dependence of the disproportionation quantum yields on the exciting light intensity was investigated. In agreement with the proposed radical-chain mechanism, the quantum yields are linearly proportional to I^-1/2 (I = the absorbed intensity).