Chromate-Dichromate Interconversion, Structure and Colour

In basic or neutral solution, chromium(VI) exists as the yellow tetrahedral chromate ion, CrO4²-. Lowering the pH (adding acid) protonates chromate and drives two chromate ions to condense, releasing water and forming the orange dichromate ion, Cr₂O7²-, in which two CrO₄ tetrahedra share one bridging oxygen (2 CrO4²- + 2H⁺ <=> Cr₂O7²- + H₂O). Raising the pH again (adding base) reverses this, splitting dichromate back into two chromate ions. Because chromium stays at +6 throughout, this entire interconversion is NOT a disproportionation or any other redox process — it is exactly why chromate is the one common d-block oxoanion in this cluster that does NOT undergo disproportionation, unlike manganate (MnO4²-, which does disproportionate in acid) or other species where the central atom genuinely changes oxidation state. Any weak acid is enough to shift the equilibrium toward dichromate: dilute nitric acid protonates chromate directly, and even passing CO₂ gas (which forms the weak acid H₂CO₃ in solution) is acidic enough to convert yellow Na₂CrO₄ into orange-red Na₂Cr₂O₇, well short of any redox chemistry.

Both chromate and dichromate get their colour from charge-transfer transitions: in a d₀ ion like Cr(VI) (chromium has lost all its d-electrons at the +6 oxidation state, so there is no d-d transition available), the colour instead comes from an electron being promoted from a filled oxygen-based orbital to an empty chromium-based orbital (a ligand-to-metal charge-transfer, LMCT). Because dichromate has a different, more extended bonding arrangement across its two bridged chromium centres than the single isolated tetrahedron of chromate, its charge-transfer transition occurs at a different energy, shifting the observed colour from the yellow of chromate to the orange of dichromate.

Dichromate, Cr₂O7²-, consists of two CrO₄ tetrahedra sharing exactly one oxygen atom (the bridging oxygen) between them. This bridging oxygen forms one Cr-O bond to each chromium (two bonds total, both slightly different from the rest by virtue of being a bridge), while each chromium additionally carries three TERMINAL oxygens that are not shared. The six terminal Cr-O bonds (three on each chromium) are equivalent to each other, but the two Cr-O(bridge) bonds are distinct from those six terminal bonds — so the correct description is that six Cr-O bonds are equivalent (the terminal ones), not that all seven Cr-O bonds in the ion are identical.