ELECTRONIC-STRUCTURE OF LUMINESCENT D(0) NIOBIUM AND TANTALUM IMIDO COMPOUNDS CIS,MER-M(NR)CL3L2

Electronic absorption and emission spectra are reported for luminescen t d(0) monoimido group 5 compounds M(NR)Cl3L2 (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 ene rgy of this absorption band depends strongly on the nature of the imid o substituent, with a significant cm decrease observed when aryl group s are present. Excitation into this transition results in long-lived l uminescent excited states. Long emission lifetimes (50 us to 17 mu s) and high quantum yields (0.001-0.24) are observed, decreasing primaril y as a function of the alkyl substituent, being lowest in the aryl imi dos. Good overlap is observed with absorption, excitation, and emissio n mirror Spectra, indicating absorption into and emission from the sam e excited state; The data are-consistent with absorption into and emis sion from a (3)(nb, pi) state, or d(xy) <--Ta-N pi. Semiempirical mole cular 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. T his accounts for the difference in electronic properties of the aryl i midos compared to the alkyl imidos. An analysis of radiative and nonra diative excited-state deactivation pathways is presented. Significantl y, an energy gap law correlation is observed for nonradiative decay in the imido compounds as a group, but a corresponding correlation of ra diative rates with emission energy is not observed when aryl and alkyl imidos are compared, evidence of electronic perturbation by the aryl substituent.