Laporte-forbidden ligand-field transitions

E20.22 The blue-green colour of the Cr ions in [Cr(H20) ] is caused by spin-allowed but Laporte-forbidden ligand field transitions. The relatively low-molar-absorption coefficient, , which is a manifestation of the Laporte-forbidden nature of the transitions, is the reason that the intensity of the colour is weak. The oxidation state of chromium in tetrahedral chromate dianion is CifVI), which is d . Therefore, no ligand field transitions are possible. Ilte intense yellow colour is due to LMCT transitions (i.e., electron transfer from the oxide ion ligands to the Cr(VI) metal centre). Charge transfer transitions are intense because they are both spin-allowed and Laporte-allowed. 

Note that the e for the bands at 9700 and 11,600 cm" (Fig. 7) are significantly lower owing to the fact that this complex has inversion symmetry and must overcome the Laporte forbiddenness of the ligand field transitions by vibronic coupling. 

Judd [8] and independently Ofelt [9] presented treatises about the calculation method for Laporte forbidden f-f transition intensities of Ln systems with using coupling scheme for angular momenta. The basic idea is that f-f transitions can be allowed because of the mixing of opposite parity configurations, such as 4f 5d or 4f - g states to 4f states due to the presence of odd parity crystal field generated by the ligands. In this theory [8, 9], the... 

Because of the orbital singlet nature of high-spin Fe111 there are no excited states of the same spin multiplicity and all d-d transitions are therefore spin forbidden as well as Laporte forbidden. The free-ion ground state 65 becomes 6A, in a cubic field while the first excited state 4G splits into two T states,4 Ti and 4 T2, and into a degenerate pair, At and 4E (Figure 2). The first four ligand field... 

Ligand Field Spectra. Ligand field (d,d) transitions are overlap forbidden and in centrosymmetric complexes also Laporte forbidden. For these reasons even the spin-allowed LF absorption bands are generally weak (10 < 10 )(16). There are many... 

Transitions occur mainly by an electric dipole mechanism. Such transitions are allowed if the initial and final states are made up of orbitals of opposite parity (A/ =1,3,... / orbital angular momentum quantum number) and if the spin remains unchanged AS = 0) (Laporte rules, see Ligand Field Theory Spectra. However parity-forbidden transitions can occur as a result of mixing with states of opposite parity. Mixing of states by the crystal field requires that the cationic site lacks an inversion center. If the site is centrosymmetric, transitions can nevertheless be observed owing to vibronic coupling. Their probability is low and increases with temperature. 

All of these electronic transitions are technically forbidden by Laporte s rule, which states that electronic transitions can only occur if the orbital angular momentum changes by 1 during the transition. Since this does not occur for transitions from one d state to another d state, or from one f state to another f state, no absorption should occur for these ions. Fortunately, Laporte s rule is relaxed in solids due to the lack of perfect spherical symmetry, which results from the presence of a limited number of point sources, so that electronic transitions can occur with a low probability between 3d or 4f levels which are split by the fields of the neighboring ligands. The low probability of these transitions, however, does reduce the intensity of the absorption. As a result, ligand field induced transitions are much weaker than the charge transfer effects which occur in the ultraviolet. 

The transitions within 5f shells are sharp, lanthanide-like lines for heavier actinides. They are considerably broadened and ligand-field dependent for earlier actinides where 5f orbitals contribute much more to the metal-ligand bonding. These transitions are Laporte-forbidden and their intensity (10 times greater than 4f transitions) is due to crystal field perturbations and vibronic coupling. 

Kotzian (1991) and Kotzian et al. (1995) reported INDO/S-CI results for transition energies and relative oscillator strengths of the 4f—>4f excitations in the [R(H20) ] (R=Pr, Nd, Tm, n=8,9) complexes. These excitations are parity-forbidden for the free ion (Laporte selection rule), but may gain intensity due to admixture of opposite-parity character in a non-centrosymmetric environment The field of the water ligands leads only to a small perturbation of the free ion energy levels (Pr 4f, Nd 4f, Tm 4f ). Due... 

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