Trends in Stability of Higher Oxidation States

Halides: fluorine as a stabiliser of high oxidation states

Among the stable halides of the 3d metals, the highest oxidation numbers achieved are found in the fluorides: titanium reaches its highest halide as the tetrahalide $\text{TiX}_4$, while vanadium and chromium go further still, forming $\text{VF}_5$ and $\text{CrF}_6$ respectively. Manganese's characteristic +7 state is not represented among the simple halides at all — there is no $\text{MnX}_7$ — though it does appear in combination with oxygen, as $\text{MnO}_3\text{F}$. Past manganese, the ability of a halide to sustain a high oxidation state falls away sharply: no metal beyond Mn forms a trihalide, with the sole exceptions of $\text{FeX}_3$ and $\text{CoF}_3$.

Fluorine's particular capacity to stabilise a metal's highest oxidation state has two possible sources, depending on the character of the compound. For a largely ionic fluoride such as $\text{CoF}_3$, it is the high lattice energy of the solid that provides the stabilisation. For the more covalent higher fluorides, such as $\text{VF}_5$ and $\text{CrF}_6$, it is instead the strength of the metal–fluorine bond itself that matters.

Even where a high-valent fluoride does form, it need not survive contact with water. $\text{VF}_5$ is the only halide representing the $\text{V}^{5+}$ state; the vanadium halides more generally hydrolyse readily to oxohalides of the form $\text{VOX}_3$. Halides also become unstable at the low end of the oxidation-state range: simple dihalides such as $\text{VX}_2$ ($\text{X} = \text{Cl, Br, I}$) are unstable, and the same pattern holds for the copper(I) halides, $\text{CuX}$.

Copper illustrates this asymmetry particularly well. Every copper(II) halide is known except the iodide, because $\text{Cu}^{2+}$ is itself oxidising enough to convert iodide ion to iodine rather than form a stable $\text{CuI}_2$: $2\text{Cu}^{2+} + 4\text{I}^- \rightarrow \text{Cu}_2\text{I}_2(s) + \text{I}_2$. Conversely, many copper(I) compounds are unstable once in aqueous solution and disproportionate: $2\text{Cu}^+ \rightarrow \text{Cu}^{2+} + \text{Cu}$. The +2 state wins out over +1 in solution because the hydration enthalpy of $\text{Cu}^{2+}$ is far more negative than that of $\text{Cu}^+$ — negative enough to outweigh the extra ionisation energy needed to remove copper's second electron.

Oxides: oxygen as an even stronger stabiliser

Oxygen stabilises high oxidation states more effectively than fluorine does. Across the oxides of the 3d metals, the highest oxidation number reached coincides with the metal's own group number, running from $\text{Sc}_2\text{O}_3$ in group 3 all the way to $\text{Mn}_2\text{O}_7$ in group 7. Beyond group 7, no oxide of iron higher than $\text{Fe}_2\text{O}_3$ is known — the ferrate(VI) ion, $\text{FeO}_4^{2-}$, can be generated in alkaline media but readily decomposes back to $\text{Fe}_2\text{O}_3$ and $\text{O}_2$ rather than persisting as a stable species. Some high oxidation states show up instead as oxocations rather than neutral oxides: $\text{V}^{5+}$ appears as $\text{VO}_2^+$, $\text{V}^{4+}$ as $\text{VO}^{2+}$, and $\text{Ti}^{4+}$ as $\text{TiO}^{2+}$. …