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3 years ago in Chemistry , Inorganic Chemistry By Pravin Patel
Why do absorption bands show a blue shift upon complex formation?
In my PhD work on spectroscopic characterization of metal-organic complexes, I keep encountering this spectral shift toward higher energy. I understand red shifts from charge transfer, but the blue shift is less intuitive. Is this primarily due to changes in the ligand field, stabilization of the ground state, or perhaps a restriction in molecular geometry upon binding?
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By Trisha Answered 2 years ago
This is a common and critical observation. From my experience, the blue shift often arises from ligand field stabilization. When a ligand coordinates to a metal ion, it typically raises the energy of the metal's d-orbitals. This increases the energy gap for d-d transitions, causing a shift to higher energy (blue shift). It can also occur if the complex formation rigidifies the molecule, reducing vibrational relaxation in the excited state, making the transition energetically "tighter" and therefore higher in energy.
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