Direct Observation of a Bent Carbonyl Ligand in a 19-Electron Transition Metal Complex

The photochemistry of [CpRu­(CO)2]2 in P­(OMe)3/CH2Cl2 solution has been studied using picosecond time-resolved infrared (TRIR) spectroscopy. Photolysis at 400 nm leads to the formation of 17-electron CpRu­(CO)2 radicals, which react on the picosecond time scale to form 19-electron CpRu­(CO)2P­(OMe)3 adducts. The TRIR spectra of this adduct display an unusually low CO stretching frequency for the antisymmetric CO stretching mode, suggesting that one carbonyl ligand adopts a bent configuration to avoid a 19-electron count at the metal center. This spectral assignment is supported by analogous experiments on [CpFe­(CO)2]2 in the same solvent, combined with DFT studies on the structures of the 19-electron adducts. The DFT results predict a bent CO ligand in CpRu­(CO)2P­(OMe)3, whereas approximately linear Fe–C–O bond angles are predicted for CpFe­(CO)2P­(OMe)3. The observation of a bent CO ligand in the 19-electron ruthenium adduct is a surprising result, and it provides new insight into the solution-phase behavior of 19-electron complexes. TRIR spectra were also collected for [CpRu­(CO)2]2 in neat CH2Cl2, and it is interesting to note that no singly bridged [CpRu­(CO)]2(μ-CO) photoproduct was observed to form following 400- or 267-nm excitation, despite previous observations of this species on longer time scales.