Intra-ligand transitions

These are electronic transitions between orbitals that are largely localised on different atoms. In coordination compounds ML where M is a d block element and L represents the ligands, we can distinguish two types of charge transfer L —> M and M — L. These are depicted schematically in Fig. 2.2, along with d-d transitions and intra-ligand transitions. 

Transitions between MO s predominantly localized on the ligands. These are usually called ligand centered (LC) or intra-ligand transitions. 

The four processes are intra-ligand transitions, ligand-to-metal charge transfer, metal-to-ligand charge transfer and d- d transitions. Refer to Section 7.2.1 for the explanation of these terms. 

Intra-Ligand Transitions These are electronic transitions between two ligand orbitals, which are not involved in M—L bonding. The M—L bond is usually not affected by such a transition, but internal rearrange-... 

Figure 2.8 Top electronic absorption spectrum of Tp Mo 0(pyrrolo-S2BMOQO) in acetonitrile with solid state rR excitation profiles. Shown are the 1345 cm quinoxaline (red circles) and 1551 cm C=C + quinoxaline (blue circles) rR profiles. Bottom the electron density difference map (EDDM) for the intra-ligand transition. Red electron density lost in the intra-ligand CT transition, Green electron density gain in the transition. Adapted with permission from ref. 43. Copyright (2010) American Chemical Society.

Fig. 6.7 Allowed intra-ligand transitions from x to V -type ligand orbitals for tris-chelate complexes with D3 symmetry. The circular dichroism has a lower right-circularly polarized (rep) band and an upper left-circularly polarized Ocp) band. This gives the CD spectmm the appearance of the first derivative of a Gaussian curve, with a negative part at longer wavelength and a positive part at shorter wavelength...

Here, we used the Heisenberg commutator relation between the conjugate position and momentum operators x, px = ih. The magnetic moment matrix element of the intra-ligand transition with respect to the common origin of the coordinate system is given by ... 

Here, v is the frequency of the intra-ligand transition. The combination of this result with Eq. (6.117) yields ... 

Deng et al. demonstrated that transition metal ion-based MOFs can be used for the detection of anions [25]. The MOF of [Cd(p2-Cl)(p4-5MT)] exhibits an emission at around 370 nm owing to the intra-ligand transitions (MT = 5-methyl-l//-tetrazole). The decrease in the emission intensity caused by the addition of N02 is attributed to the energy transfer mechanism between the methyl groups and the nitrite ions. 

Three types of electronic transition can be distinguished among compounds of the d block transition elements d-d bands, charge transfer (or electron-transfer) bands and intra-ligand bands. Configuration interaction may make the distinctions rather hazy, however. 

LC transitions concern only ligands that are composed of two or more atoms, eg 2,2 -bipyridine, 1,10-phenantroline, or porphyrins. LC excitation entails a population of ligand-antibonding orbitals weakening the intra-ligand bonds, and consequently leading to bond cleavage ... 

Table 2 Comparison of intra-ligand vibrational energies as determined from emission spectra of the main sites of Ir(4,6-dFppy)2(acac) in CH2C12 (T=1.7 K, electronic 0-0 transition I — 0 at 21,025 cm4) and of Pt(4,6-dFppy)(acac) in CH2C12 (7=4.2 K, electronic 0-0 transition II/III — 0 at 21,867 cm4) [107] and in n-octane (7=4.2 K, electronic 0-0 transition II/III —> 0 at 21,461 cm4), respectively (compare Sect. 3.3). Corresponding vibrational energies of the free (4,6-dFppy) ligand, determined from a Raman spectrum (7= 298 K, neat ligand), are also given [53]...

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