Read Why Some Complexes Are Coloured and Others Aren't
Colour in coordination compounds needs a partially-filled d-subshell so an electron can jump between split d-orbitals when it absorbs visible light — no partial filling, no d-d transition, no colour from that mechanism.
Colour of Coordination Compounds is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
d-d transitions need d-electrons AND a gap to jump into
But [Zn(H₂O)₆]2+ (d₁₀, fully filled) and [Sc(H₂O)₆]3+ (d₀, empty) show no d-d transition at all — one has no empty orbital to jump into, the other has no electron to jump. Choose B.
Some colour comes from charge transfer, not d-d transitions
MnO₄⁻ and CrO4²- are intensely coloured despite the metal being d₀ (Mn⁷⁺, Cr⁶⁺ have no d-electrons at all) — the colour here comes from ligand-to-metal charge transfer, a completely different mechanism from d-d transitions, which is why 'd-d transition doesn't account for the colour' is correct for this whole family of oxoanions.
Predicting coloured vs colourless from electron count
[Al(OH)₄]- is colourless: Al³⁺ has no d-electrons at all (it isn't even a transition metal), so no orbital-splitting mechanism is available. [Ni(DMG)₂], [Co(SCN)₄]2- and [Fe(H₂O)5(SCN)]2+ are all coloured because their metal centres carry partially-filled d-subshells capable of d-d transitions.
OlympIQ exam lens
- ✓Colour from d-d transitions requires a PARTIALLY filled d-subshell — d₀ and d₁₀ configurations show no d-d transition.
- ✓d₀ metal oxoanions (MnO₄⁻, CrO4²-) can still be intensely coloured via charge transfer, a separate mechanism.
- ✓Al³⁺ and other non-transition-metal ions are colourless because they have no d-electrons to split at all.
- ✓Bigger Δ (stronger field ligand) shifts absorption to shorter wavelength, which shifts the observed (complementary) colour.
Don't assume 'coloured' always means d-d transition — d₀ complexes like permanganate and chromate are coloured via charge transfer, not d-d transitions, precisely because they have zero d-electrons available.
Ready to try the pattern?
100% coverage across the 6 linked questions.
Practise 6 linked questions