Selection rules
المؤلف:
CATHERINE E. HOUSECROFT AND ALAN G. SHARPE
المصدر:
INORGANIC CHEMISTRY
الجزء والصفحة:
2th ed p 571
23-8-2016
1246
Selection rules
Electronic transitions obey the following selection rules. Spin selection rule: ΔS = 0 Transitions may occur from singlet to singlet, or triplet to triplet states and so on, but a change in spin multiplicity is forbidden. Laporte selection rule: There must be a change in parity: allowed transitions: g ↔ u forbidden transitions:
g↔g u↔ u
This leads to the selection rule: Δl = _1 and, thus, allowed transitions are s →p, p → d, d → f ; forbidden transitions are s → s, p → p, d → d, f → f , s → d, p → f etc. Since these selection rules must be strictly obeyed, why do many d-block metal complexes exhibit ‘d–d’ bands in their electronic spectra?
A spin-forbidden transition becomes ‘allowed’ if, for example, a singlet state mixes to some extent with a triplet state. This is possible by spin–orbit coupling but for first row metals, the degree of mixing is small and so bands associated with ‘spin-forbidden’ transitions are very weak (Table 1.1).
Table 1.1 Typical Ԑmax values for electronic absorptions; a large Ԑmax corresponds to an intense absorption and, if the absorption is in the visible region, a highly coloured complex.
Spin-allowed ‘d–d’ transitions remain Laporte-forbidden and their observation is explained by a mechanism called ‘vibronic coupling’. An octahedral complex possesses a centre of symmetry, but molecular vibrations result in its temporary loss. At an instant when the molecule does not possess a centre of symmetry, mixing
of d and p orbitals can occur. Since the lifetime of the vibration (≈10-13 s) is longer than that of an electronic transition (≈10-18 s), a ‘d–d’ transition involving an orbital of mixed pd character can occur although the absorption is still relatively weak (Table 1.1). In a molecule which is noncentrosymmetric (e.g. tetrahedral), p–d mixing can occur to a greater extent and so the probability of ‘d–d’ transitions is greater than in a centrosymmetric complex. This leads to tetrahedral complexes being more intensely coloured than octahedral complexes.
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