Photosubstitution Reactivity of a Series of Tetracarbonylbis(substituted pyridine)tungsten(0) Complexes Having Ligand Field or Charge-Transfer Lowest Excited States

Electronic absorption and emission spectra and photosubstitution efficiencies are reported for cis-W(CO)4X2 (X = 3,4-dimethylpyridine, 4-ethylpyridine, pyridine, 3-benzoylpyridine, 4-phenylpyridine, 3,5-dichloropyridine, 3,5-dibromopyridine, 4-benzoylpyridine, 4-cyanopyridine, 4-formylpyridine). The spectral data show that the tungsten -→ pyridine charge-transfer (CT) state moves smoothly to lower energy with more electron-withdrawing substituents on the pyridine, while the ligand field (LF) states are essentially insensitive to these changes. Photosubstitution of X occurs in all cases upon irradiation into the lowest energy absorption system, but the quantum efficiency for the substitution is very dependent on whether the lowest energy excited state is LF or W → XCT in character. The W → XCT state is virtually unreactive; e.g., cis-W(CO)4(4-form-py)2 undergoes photosubstitution with a 436-nm quantum yield of ~0,0007. Complexes having LF lowest excited states are very photosubstitution labile; e.g., for X = 3,4-dimethylpyridine, 4-ethylpyridine, or pyridine the photosubstitution of X in cis-W(CO)4X2 occurs with a 436-nm quantum yield of ~0.4.