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Selection Rules
The Selection Rules governing transitions between electronic energy levels of transition metal complexes are:
- ΔS = 0 The Spin Rule
- Δl = +/- 1 The Orbital Rule (or Laporte)
The first rule says that allowed transitions must involve the promotion of electrons without a change in their spin. The second rule says that if the molecule has a center of symmetry, transitions within a given set of p or d orbitals (i.e. those which only involve a redistribution of electrons within a given subshell) are forbidden.
Relaxation of these rules can occur through:
- Spin-Orbit coupling: this gives rise to weak spin forbidden bands
- Vibronic coupling: an octahedral complex may have allowed vibrations where the molecule is asymmetric. Absorption of light at that moment is then possible.
- Mixing: π-acceptor and π-donor ligands can mix with the d-orbitals so transitions are no longer purely d-d.
Question for Selection Rules for Electronic Spectra of Transition Metal ComplexesTry yourself: Which type of electronic transition is often found in the UV region but may have a tail into the visible region?View Solution
Transition Types
- Charge transfer, either ligand to metal or metal to ligand. These are often extremely intense and are generally found in the UV but they may have a tail into the visible.
- d-d, these can occur in both the UV and visible region but since they are forbidden transitions have small intensities.
Expected intensities of electronic transitions
Expected Values
The expected values should be compared to the following rough guide.
- For M2+ complexes, expect Δ = 7,500 - 12,500 cm-1 or λ = 800 - 1,350 nm.
- For M3+ complexes, expect Δ= 14,000 - 25,000 cm-1 or λ = 400 - 720 nm.
For a typical spin-allowed, but Laporte (orbitally) forbidden transition in an octahedral complex, expect ε < 10 m2mol-1. Extinction coefficients for tetrahedral complexes are expected to be around 50-100 times larger than for octrahedral complexes. B for first-row transition metal free ions is around 1,000 cm-1. Depending on the position of the ligand in the nephelauxetic series, this can be reduced to as low as 60% in the complex.
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FAQs on Selection Rules for Electronic Spectra of Transition Metal Complexes - Chemistry Optional Notes for UPSC
| 1. What are selection rules for electronic spectra of transition metal complexes? |  |
Ans. Selection rules for electronic spectra of transition metal complexes are a set of guidelines that determine which transitions are allowed and which are forbidden in the absorption or emission of light by these complexes. These rules depend on factors such as spin, symmetry, and electric dipole moment. For example, transitions involving a change in spin (spin-forbidden) are typically not observed in electronic spectra, while transitions that maintain the same spin (spin-allowed) are more likely to occur.
| 2. What factors determine the transition types in electronic spectra of transition metal complexes? | |
Ans. The transition types in electronic spectra of transition metal complexes are determined by several factors. These include the energy difference between the initial and final states of the transition, the nature of the ligands attached to the metal ion, and the electronic configuration of the metal ion. These factors influence the electronic transitions that can occur, such as ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT) transitions.
| 3. What are some commonly observed transitions in electronic spectra of transition metal complexes? | |
Ans. Some commonly observed transitions in electronic spectra of transition metal complexes include d-d transitions, ligand-to-metal charge transfer (LMCT) transitions, and metal-to-ligand charge transfer (MLCT) transitions. In d-d transitions, electrons in the d orbitals of the metal ion are excited to higher energy levels within the d orbitals. LMCT transitions involve the transfer of an electron from a ligand orbital to a metal orbital, while MLCT transitions involve the transfer of an electron from a metal orbital to a ligand orbital.
| 4. How do selection rules affect the absorption or emission of light by transition metal complexes? | |
Ans. Selection rules determine which transitions are allowed and which are forbidden in the absorption or emission of light by transition metal complexes. If a transition is allowed according to the selection rules, it has a higher probability of occurring and can be observed in the electronic spectrum. On the other hand, if a transition is forbidden, it has a lower probability of occurring and is typically not observed. By considering the selection rules, one can predict the types of transitions that are likely to be observed in the electronic spectra of transition metal complexes.
| 5. How do selection rules contribute to the understanding of the electronic structure of transition metal complexes? | |
Ans. Selection rules provide valuable insights into the electronic structure of transition metal complexes. By analyzing the allowed transitions in the electronic spectra, one can determine the symmetry properties of the orbitals involved, the nature of the ligand-metal interactions, and the electronic configuration of the metal ion. This information helps in understanding the bonding and electronic properties of transition metal complexes, which in turn influences their reactivity, catalytic activity, and optical properties. Therefore, selection rules play a crucial role in elucidating the electronic structure of transition metal complexes.
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Oct 31, 2024 Last updated |
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