The partially filled d orbitals of transition metals give rise to a cluster of characteristic properties: colour, paramagnetism, catalytic activity, complex formation, the ability to trap small atoms (interstitial compounds) and easy alloy formation. Two important compounds — potassium dichromate and potassium permanganate — bring many of these ideas together.
Colour and d–d transitions
Most transition-metal ions are coloured. When light falls on the ion, an electron is promoted from a lower-energy d orbital to a higher-energy d orbital (a d–d transition). The energy gap corresponds to part of the visible spectrum; the colour we see is the complement of the absorbed colour. Ions with empty ($d^0$, e.g. $\text{Sc}^{3+}$, $\text{Ti}^{4+}$) or full ($d^{10}$, e.g. $\text{Zn}^{2+}$) d subshells are colourless because no d–d transition is possible.
Magnetic properties
Species with unpaired electrons are paramagnetic (attracted into a magnetic field); those with all electrons paired are diamagnetic. For most first-row complexes the orbital contribution is quenched, so the magnetic moment is given by the spin-only formula $\mu=\sqrt{n(n+2)}$ BM, where $n$ is the number of unpaired electrons. Paramagnetism therefore rises and falls as $n$ changes across a series.
Catalytic properties
Transition metals and their compounds are excellent catalysts. They can adopt variable oxidation states and provide surfaces that adsorb reactants, lowering the activation energy. Examples: iron in the Haber process, $\text{V}_2\text{O}_5$ in the Contact process, and nickel in hydrogenation.
Complex formation and interstitial compounds
Their small, highly charged ions with vacant d orbitals readily accept lone pairs from ligands, forming coordination complexes such as $[\text{Fe}(\text{CN})_6]^{3-}$. Small atoms (H, C, N) can also occupy the holes in their metal lattices to give hard, chemically inert interstitial compounds (e.g. steel, which is iron with interstitial carbon).
Alloy formation
Because transition metals have similar atomic sizes, atoms of one can replace another in the lattice to give alloys — brass (Cu–Zn), bronze (Cu–Sn) and stainless steel are common examples.
Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$)
Prepared from chromite ore: it is fused with $\text{Na}_2\text{CO}_3$ in air to give sodium chromate, which is acidified to sodium dichromate and then treated with KCl. It is a powerful oxidising agent in acidic medium: $\text{Cr}_2\text{O}_7^{2-}+14\text{H}^++6e^-\rightarrow2\text{Cr}^{3+}+7\text{H}_2\text{O}$. The orange dichromate ion is in pH-dependent equilibrium with the yellow chromate ion: $\text{Cr}_2\text{O}_7^{2-}+2\text{OH}^-\rightleftharpoons2\text{CrO}_4^{2-}+\text{H}_2\text{O}$.
Potassium permanganate ($\text{KMnO}_4$)
Made by fusing pyrolusite ($\text{MnO}_2$) with KOH and an oxidant to give green manganate, $\text{K}_2\text{MnO}_4$, which is then oxidised to purple permanganate. In acidic medium it is a strong oxidant: $\text{MnO}_4^-+8\text{H}^++5e^-\rightarrow\text{Mn}^{2+}+4\text{H}_2\text{O}$. It oxidises ferrous ions, oxalate and iodide, and is widely used in volumetric analysis.