Ligand-field transitions

The Fe(II) complex of this ligand shows crossover behavior both in solution and in the solid state. The complex has a distinct green color derived from the ligand field transition 2=620 nm) of the low-spin form of... 

These three types of clusters all involve one electron orbital. They provide a basis for the description of the d-d (i. e. ligand field) transitions and the ligand to metal charge transfer transitions which are responsible for most of the UV-visible spectra and opti-... 

The charge transfer transitions - involving Fe -0 or Fe"-Fe" - are mainly responsible for absorption of visible light. They produce an absorption band centered in the near UV, one side of which (the absorption edge) extends into the visible region. This intense absorption is overlain by bands due to the ligand field transitions (between 550-900 nm) and double excitation processes at ca. 450 nm. The d-d transitions contribute more to the colour of iron oxides than would be expected, owing to the interactions between the Fe -Fe pairs. 

Silica makes up 12.6 mass-% of the Earth s crust as crystalline and amorphous forms. It was found that both modifications show similar main luminescence bands, namely a blue one centered at 450 nm ascribed to which substitutes for Si, red centered at 650 nm linked with non-bridge O, and dark-red at 700-730 nm linked with Fe. Time-resolved luminescence of hydrous volcanic glasses with different colors and different Fe, Mn, and H2O contents were measured and interpreted (Zotov et al. 2002). The blue band with a short decay time of 40 ns was connected with T2( D)- Ai ( S) and Ai C G)- Ai ( S) ligand field transitions of Fe " ", the green band with a decay time of approximately 250 ps with a Ti( G)- Ai( S) transition in tetrahedrally coordinated Mn ", while the red band with a much longer decay time of several ms with T1 (4G)- Ai( S) transitions in tetrahedrally coordinated Fe ". We detected Fe " " in the time-resolved luminescence spectrum of black obsidian glass (Fig. 4.43d). 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

Figure 4.75 Schematic representation of the charge transfer in various excited states of a metal complex. M is the metal centre and L stands for a ligand. LF is a ligand field transition, CTs are the charge transfer transitions, LL is an intraligand transition, and CTTS is a charge transfer to solvent...

Fig. 10. High resolution single crystal absorption spectrum of Ir4+ doped in K2SnCl6 exhibiting progressions of ligand field transitions in the absorption gap between the first (27 ) and second (2T2u) charge transfer bands (region B)...

Ligand field transitions, and photochemistry, 1, 240 Ligand fragmentation, as metal vapor synthesis milestone,... 

In the same samples, a second absorption feature was detected that is associated with the dopant ions themselves. These ligand-field transitions allow distinction among various octahedral and tetrahedral Co2+ species and are discussed in more detail in Section III.C. The three distinct spectra observed in Fig. 4(b) correspond to octahedral precursor (initial spectrum), tetrahedral surface-bound Co2+ (broad intermediate spectrum), and tetrahedral substitutional Co2+ in ZnO (intense structured spectrum). Plotting the tetrahedral substitutional Co2+ absorption intensity as a function of added base yields the data shown as triangles in Fig. 4(b). Again, no change in Co2+ absorption is observed until sufficient base is added to reach critical supersaturation of the precursors, after which base addition causes the conversion of solvated octahedral Co2+ into tetrahedral Co2+ substitutionally doped into ZnO. Importantly, a plot of the substitutional Co2+ absorption intensity versus added base shows the same nucleation point but does not show any jump in intensity that would correspond with the jump in ZnO intensity. Instead, extrapolation of the tetrahedral Co2+ intensities to zero shows intersection at the base concentration where ZnO first nucleates, demonstrating the need for crystalline ZnO to be... 

Geometric Dependence of Spin-Allowed Ligand Field Transitions Ligand field theory quantitates the splittings of the one-electron d-orbitals due to their repulsion/antibonding interactions with the ligands. 

We now consider the spin-allowed ligand field transitions of optically active Cu(II) complexes. The table below gives the effect of the L, operator on electrons in d-orbitals.3... 

The Fe3+ ligand field transitions are spin-forbidden since... 

Figure 6. Visible to near-infrared spectra of several nontronites showing the Fe3+ ligand field transitions (Adapted from reference 4).

The ligand field transitions of tetrahedrally coordinated Fe3 are Laporte-allowed. Consequently, small amounts of tetrahedrally coordinated Fe3 may have a large effect on the spectra of iron-bearing clays. In the optical spectrum of nontronite (4), the small amount of tetrahedral Fe3 gives an intense absorption band near 23,000 cm 1 that is assigned to the 6A- - Aj, E transition. 

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