Electronic Structure of Luminescent d(0) Niobium and Tantalum Imido Compounds cis,mer-M(NR)Cl(3)L(2).

Electronic absorption and emission spectra are reported for luminescent d(0) monoimido group 5 compounds M(NR)Cl(3)L(2) (M = Nb, Ta; R = alkyl, aryl; L = dme, Cl(-), py). These compounds display weak (epsilon < 200 M(-)(1) cm(-)(1)), well-resolved lowest-energy transitions in the high-energy visible and near-UV regions (20 000 < E(abs) < 29 000 cm(-)(1)). The energy of this absorption band depends strongly on the nature of the imido substituent, with a significant decrease observed when aryl groups are present. Excitation into this transition results in long-lived luminescent excited states. Long emission lifetimes (50 ns to 17 &mgr;s) and high quantum yields (0.001-0.24) are observed, decreasing primarily as a function of the alkyl substituent, being lowest in the aryl imidos. Good overlap is observed with absorption, excitation, and emission mirror spectra, indicating absorption into and emission from the same excited state. The data are consistent with absorption into and emission from a (3)(nb, pi) state, or d(xy)() <-- Ta-N pi. Semiempirical molecular orbital calculations are presented which suggest that the imido compounds may be considered as having highly mixed but localized Ta=N pi-bonding. A significant difference is noted in [Ta(NPh)Cl(5)](2)(-), in which there is appreciable aryl character in Ta=N pi-type orbitals. This accounts for the difference in electronic properties of the aryl imidos compared to the alkyl imidos. An analysis of radiative and nonradiative excited-state deactivation pathways is presented. Significantly, an energy gap law correlation is observed for nonradiative decay in the imido compounds as a group, but a corresponding correlation of radiative rates with emission energy is not observed when aryl and alkyl imidos are compared, evidence of electronic perturbation by the aryl substituent.