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.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
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.
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