Low-energy LMCT transitions

The unique electronic structure of these (L-A3)MoO(dithiolene) complexes arises from two basic factors. The first is the strong axial a- and Ji-donor properties of the terminal oxo ligand, which dominates the ligand field and predetermines the energy of the Mo-based dxz, dyz, and dzi acceptor orbitals. The second is the equatorial dithiolene sulfur donors, from which the low-energy LMCT transitions arise. Dithiolene covalency contributions to the electroactive C, or redox, orbital can be directly probed via the relative oscillator strengths of the / —> ixy and /fp —> (/", transitions (see above). These three wave functions may be expanded in terms of Mo- and dithiolene sulfur-based functions ... 

It is not clear why this luminescence does not appear in solution. Complexes of d metals of the second and third transition row do not seem to show an LF emission. Such metal ions exist only in low-spin configurations. In octahedral complexes they then possess an electron hole in their dn subshell. It follows that such compounds are often characterized by low-energy LMCT transitions [8]. Accordingly, LF emissions are not likely to occur. 

Since U(VI) has an empty valence shell with the electron configuration f ° only low-energy LMCT transitions are feasible [123]. Accordingly, the 0 —> U(VI) LMCT assignment to the emitting state of is unambiguous. 

Low-energy LMCT transitions occur if the metal is oxidizing and the ligand reducing. LMCT absorptions determine the color of many coordination compounds. LMCT bands can be classified according to the electron configuration at the metal. The most important configurations are included in our discussion. 

A combination of descent in symmetry (Ctv —> C3v —> Cs) and orbital overlap considerations have been utilized in order to assign the six lowest energy bands in the (L-/V )MoO(dithiolene) series. The inherently low oscillator strengths of the Mo ligand-field bands in oxo-molybdenum complexes (s < 100 M em ), coupled with the presence of low-energy LMCT excitations, result in ligand-field transitions that are difficult to observe in (L-/V3)MoO(dithiolene) compounds. As a result, LMCT bands are anticipated to dominate the absorption spectra of these complexes. 

Irradiation of (he trans isomer leads to a different isomeric mixture of the chloro aquo species (66% cis, 34% trans) demonstrating that the two photolabilizations do not proceed via a single, common intermediate. These reactions occur with small quantum yields [0.003 for Eq. (p)], the result of the low-energy LMCT excited states that dominate the absorption spectra. For the d case, the orbital character of the lower energy LMCT transitions is (7TL) (dt2g) - (7r ) (t2g) and the resulting ES should not be substitution labile. 

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