Electronic Absorption and Emission Spectral and Photochemical Studies of Low-Spin d⁶ Metal Hexacarbonyl Anions

The electronic absorption spectra of various salts of M(CO)₆^- (M = V, Nb, Ta) have been measured in solution and in KBr pressed disks at 298 and 20 K. For M = Nb, Ta, but not V, we find strongly temperature-dependent optical luminescence upon electronic excitation of the pure solids or the solutions. Quantitative emission measurements (lifetimes and quantum yields) have been made for the pure solids in the range 22-100 K. Maximum emission yields (~0.2) and lifetimes (~10^-4 sec) are found at the lowest temperature and fall off rapidly, diminishing by nearly 2 orders of magnitude upon warming to 100 K. The overlapping emission and absorption bands and the central metal dependence support a ³T_lg(t_2g⁵e_g¹) ⟶ ¹A_1g(t₂g⁶) assignment for the emission. Except for the absolute energetic position, the lowest absorption bands in M(CO)^6- correspond closely to those found in the neutral and cationic low-spin d6 metal hexacarbonyls. Some photochemical studies have been carried out both in rigid media, KBr or glassy solvents at low temperature, and in solution at room temperature. Photosubstitution in solution is efficient with the formation quantum yields of V(CO)₅X- (X = CH₃CN, pyridine) equal to 0.5 ± 0.05 for 313-, 366-, or 436-nm irradiation of either [Ph₄As][V(CO)₆] or [Et₄N][V(CO)₆]. Spectral changes occurring in rigid media are consistent with dissociative loss of CO to yield coordinatively unsaturated or weakly solvated M(CO)₅^-.