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 Ԑ_max values for electronic absorptions; a large  Ԑ_max 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.

مواضيع ذات صلة

ايجاد الصيغة التركيبية للمعقدات

معقدات الخارصين

معقدات النحاس

كيمياء ومعقدات بعض عناصر السلسلة الانتقالية الاولى

تعريف المركبات المعقدة

Crystal Field Stabilization Energies

Colors of Transition-Metal Complexes

The Nature of the Ligands

Principal Quantum Number of the Metal

Charge on the Metal Ion

Factors That Affect the Magnitude of Δo

Electronic Structures of Metal Complexes