9701 · 28.3
Colour of complexes flashcards
Revision flashcards for Cambridge 9701 Colour of complexes (syllabus 28.3). Flip, recall, then mark a real past-paper question.
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What are degenerate orbitals?
Orbitals that have the same energy level. In an isolated gaseous transition metal ion, all five d-orbitals are degenerate.
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What is Crystal Field Splitting?
The splitting of degenerate d-orbitals of a central metal ion into two or more energy levels by the electrostatic field of the surrounding ligands.
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What is a d-d transition?
The promotion of an electron from a lower energy d-orbital to a higher energy d-orbital by absorbing a photon of light with energy equal to the splitting energy (ΔE).
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What is the relationship between the colour absorbed and the colour observed?
The colour observed is the complementary colour to the colour absorbed. For example, if a complex absorbs orange light, it will appear blue.
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Why is the [Cu(H₂O)₆]²⁺ complex blue?
It absorbs light in the orange region of the visible spectrum (around 600 nm). The remaining light transmitted appears blue, which is the complementary colour to orange.
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Why are complexes of Zn²⁺ ions colourless?
Zn²⁺ has a d¹⁰ electron configuration (a full d-subshell). There are no empty d-orbitals for an electron to be promoted into, so d-d transitions cannot occur.
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Why are complexes of Sc³⁺ ions colourless?
Sc³⁺ has a d⁰ electron configuration (an empty d-subshell). There are no d-electrons to be promoted, so d-d transitions cannot occur.
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What is the crystal field splitting energy, ΔE?
The energy difference between the split d-orbitals. Its magnitude determines the frequency (and thus colour) of light absorbed.
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What is the spectrochemical series?
A list of ligands ordered by their ability to cause d-orbital splitting. Strong-field ligands (e.g., CN⁻) cause a large ΔE, while weak-field ligands (e.g., Cl⁻) cause a small ΔE.
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How does the oxidation state of the central metal ion affect the colour of a complex?
A higher oxidation state on the metal ion attracts the ligands more strongly, increasing the d-orbital splitting (ΔE). This causes absorption of higher energy (shorter wavelength) light, changing the observed colour.
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Name the two sets of split d-orbitals in an octahedral complex.
The lower energy t₂g set (d_xy, d_xz, d_yz) and the higher energy e_g set (d_x²-y², d_z²).