Photosubstitution Behavior of Dicarbonyl(η⁵-cyclopentadienyl)pyridinomanganese and -rhenium and Related Complexes

Electronic spectra and photosubstitution reactions of (η⁵-C₅H₅)M(CO)₂X (M = Mn, Re; X = CO, aliphatic amine, pyridine, or a substituted pyridine) have been investigated. The low-energy region of the electronic absorption spectra of the pyridine and substituted pyridine complexes is dominated by a M → π* X CT absorption (ε ≈ 5 × 10³ 1. mol^-1 cm^-1). The M → π* X CT absorption blue shifts in more polar or polarizable media, and the band position for a given M and solvent red shifts smoothly with increasing electron withdrawing power of the substituent(s) on the pyridine ring. For example, in isooctane at 25 °C the Mn → π* X CT maximum is at 385 nm for X = 3,4-dimethylpyridine and at 555 nm for X = 4-formylpyridine. For all complexes the exclusive photosubstitution is dissociative loss of X for irradiation into the lowest absorption band. However, the quantum yield is a function of M and X and spans a range of approximately four orders of magnitude. For = Mn all of the complexes are quite photosensitive with efficiencies in the range of 0.2-0.4, whereas for M = Re only those complexes not having low energy Re → X CT bands are reactive. The crucial fact is that for several Re complexes the relaxed M → X CT excited state is lowest in energy, and the complexes are photosubstitution inert (ϕ < 10^-4). A correlation of the spectroscopic results and the quantum efficiencies suggests that the CT state is unreactive with respect to ligand substitution, and the data are consistent with a substitutionally reactive ligand field excited state.