Ligand localized transitions

Transitions between MO s having major contributions from ligand orbitals. These are termed Intrallgand, Internal ligand (IL), or ligand localized transitions. 

The parent complex, cls-IrCl9(phen) , has an essentially pure MLCT emitting level. The decay constants of this state are similar to those of the MLCT emitting states of the bipy series. The luminescent state of IrCl2(5,6-dlmethylphen)2 , however, has almost complete IL character. As expected for ligand localized transitions, the radiative and nonradlative rate constants of this state are much smaller than those of the MLCT emitting states in the bipy system (Table 9). The emitting levels of the other substituted phen complexes are proposed to have fairly equal contributions from the zero-order IL and MLCT states (194). 

These tru 5-dioxoruthenium(VI) complexes have characteristic UV-vis absorption spectra. The fj-saturated nature of the macrocyclic tertiary amine ligands enables the high-energy metal-localized transition to be observed. " The weak vibronic structured band at 370-400nm has been assigned to (0 ) —> Ru charge transfer transition that is vibronically coupled to the... 

Robin and Day presented a particularly helpful molecular orbital description of the hypothetical Cu Y2X2Cu X3 mixed valence system (Figure 3). This model indicates that the difference in energy between the mixed valence transitions 1 and 2 is equal in energy to the lowest energy ligand field transition in Cu. Unlike, the localized square-planar Cu complexes where three transitions occur, four are expected, 1-4, for the mixed valence system. The intensities of these... 

Acetylacetone (acac) and related ) -diketones continue to be extensively used as ligands for transition metal complexes. This section deals with tris(acac) complexes followed by a discussion of the spectra of adducts of bis(acac) complexes. Cramer and Chudyk (75) have studied [Ni(acac)3]2C104. INDO calculations show that the major spin delocalization is into the highest filled ligand orbital which possesses cr-symmetry plus a minor contribution from delocalization into the lowest empty 7i-orbital. This is in contrast to the spin delocalization in Ni(acac)3 and Ni(acac)py2 where the major de-localization is into the highest filled ligand orbital of Ti-symmetry. 

The lowest absorption (and the emission) transition is ligand localized on the right-hand side and charge transfer on the left-hand side of Fig. 22. In between is an intermediate region. Since the excited states arc open shell, spin-singlets and spin-triplets appear. This simple approximation can be extended by including more excited configurations. 

In a few cases, one observes narrow absorption bands caused by the co-excitation of solvent 36) or aqua ligand 85) vibrations. It is worth emphasizing how relatively rare this phenomenon is—again showing a considerable isolation of the individual chromophore. In the case of crystalline solids, the energy band description encounters insurmountable difficulties in the case of fairly localized transitions 50, 53, 57). 

The emissions in these cases have been assigned to spin-forbidden ligand field transitions (d -> d) (179-181). The short lifetimes (relative to the ligand localized emitters) are due to the stronger perturbing effect of the metal in such transitions where metal orbitals are directly involved (180,... 

Rh(phen) + compounds (cf. Table 1). This is indicative that the excitation in the mixed ligand compound is ligand localized, be it bpy or phen. This is further supported by means of PMDR measurements. For instance, Fig. 7 shows the PMDR spectra obtained for the 5.0 GHz transition of [Rh(phen)3] (0104)3 and [Rh(phen)2 (bpy) 1(0104)3, respectively. 

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