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Transition Metals and Color. Why are so many transition metal complexes colored? Usually only metals with d 0 or d 10 form colorless compounds (Zn, Ag). Colors in metal complexes (or any compound) is due to their absorption spectrum. Transition Metals and Color.

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Transition metals and color
Transition Metals and Color

  • Why are so many transition metal complexes colored?

  • Usually only metals with d0 or d10 form colorless compounds (Zn, Ag).

  • Colors in metal complexes (or any compound) is due to their absorption spectrum.


Transition metals and color1
Transition Metals and Color

  • When a metal complex absorbs light, an electron undergoes an electronic transition from a ground state to an excited state.

  • Remember:

    ΔE = hc/λ OR λ = hc/ΔE

  • We see the color which is NOT absorbed, but which was reflected or transmitted.

  • We see the complementary color of what was absorbed.



Crystal field theory
Crystal Field Theory

  • Why do metal complexes absorb light in the Vis light spectrum?

  • Crystal Field Theory tries to explain this.

  • When a ligand approaches a free metal atom or ion in order to form a bond, e-e repulsions occur between the metal’s d-electrons and the ligands electrons.

  • This causes the metal’s 5 degenerate d-orbitals to increase in energy AND to split.

  • So they are no longer degenerate.




Crystal field theory1
Crystal Field Theory

  • Different ligands cause more of an energy split in the d-orbitals.

  • Ligands which cause the d-orbitals to split more with a higher ΔE are called strong-field ligands.

  • Ligands which cause the d-orbitals to split less with a lower ΔE are called weak-field ligands.

  • Ligand Series from Weak to Strong:

    I-<Br-<Cl-<F-<H2O<NH3<en<CN-


Octahedral complexes weak field strong field ligands
Octahedral Complexes:Weak Field / Strong Field Ligands


Crystal field theory2
Crystal Field Theory

  • Ligand splitting of a metal’s d-orbitals also explains why some complexes are highly paramagnetic and others are diamagnetic or weakly paramagnetic.

  • Highly Paramagnetic: These are weak field complexes with a low ΔE, so the d-e are easily promoted. The result is a complex with many unpaired electrons, or a high-spin complex.



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