Aqueous $\mathrm{Fe}{}^{2+}$ reacts with $[\mathrm{Fe}(\mathrm{CN}){}_6]{}^{3-}$ to give the dark blue pigment Turnbull's blue, which is the same as Prussian blue, $\mathrm{KFe}[\mathrm{Fe}(\mathrm{CN}){}_6]$. In this solid the carbon-bonded Fe is low-spin Fe(II) and the nitrogen-bonded Fe is high-spin Fe(III). The colour of the pigment and the spin-only magnetic moment per formula unit are respectively:

1. Aintervalence charge transfer; 1.73 BM
2. Bligand-to-metal charge transfer; 0 BM
3. Cd-d transition; 5.92 BM
4. Dintervalence (metal-to-metal) charge transfer; 5.92 BM