# Why is chromate coloured but tungstate colourless?

I have to try and work this out using the tetrahedral molecular orbital diagram for $$\ce{[WO4]^2-}$$ but I am confused as both are $$\mathrm d^0$$ right?

• I think this is a great question about CT complexes $\ce{[M^{VI}O4]^2-}$ and can be further extended with examples for sulfate $\ce{SO4^2-}$ and molybdate $\ce{MoO4^2-}$ (both colorless) and uranate $\ce{UO4^2-}$ (yellow). Jan 8 '20 at 13:42
• @andselisk why is this the case? Is it to do with LMCT? Jan 8 '20 at 14:10
• What doesn't? The majority of metal complexes are colored precisely because of LMCT. Jan 8 '20 at 14:15
• @andselisk - This is getting off topic of the question, but I don't think it's accurate to say that the "majority" of metal complexes are colored because of LMCT. Intensely colored ones like dichromate and permanganate are, but many transition metal complexes are weakly colored because of $d \rightarrow d$ transitions, not LMCT. I think the majority of common complexes that students encounter are likely $d \rightarrow d$. Jan 8 '20 at 15:17
• Jan 8 '20 at 23:17

The intense colour of permanganate and chromate is due to LMCT transitions: ligand-to-metal charge transfer. Essentially, an electron from the HOMO (a $$\mathrm{t_1}$$ orbital that can be thought of a ligand-centred p-type orbital) is excited to the LUMO (an $$\mathrm{e}$$-type orbital that can be thought of one of the metal’s empty d orbitals) by absorbing a photon from the visible range; we then see the complementary colour, meaning that chromate absorbs blue light and looks yellow to our eyes.
Without computer calculations, it is probably impossible to accurately predict the ultimate wavelength but note that such an absorption is practically always present if the corresponding orbitals are available although usually in the ultraviolet range. An obvious counterexample would be any zinc complex in which the metal has a $$\mathrm d^{10}$$ configuration and the LUMO is thus the much further removed $$\mathrm a_1$$ orbital; therefore, zinc complexes are typically colourless and do not display such an LMCT band.