Oxidation States

A Defining Feature: Variable Oxidation States

One of the most distinctive features of the transition elements is the sheer variety of oxidation states they display in their compounds — a range far wider than what is typically seen among $s$-block or most $p$-block elements. The common oxidation states of the first-row transition metals, with the most frequently encountered one for each element highlighted, are tabulated for the series; a few patterns in that data are worth drawing out explicitly.

Where the Widest Range Occurs

The elements showing the greatest number of oxidation states are found at or near the middle of the series — manganese is the standout example, exhibiting every oxidation state from $+2$ right up to $+7$. The range narrows sharply at the two ends of the series, for two different reasons:

• Early in the series (Sc, Ti), there are simply too few electrons available to lose or share for a wide range of states to be accessible — so, for instance, scandium(II) is virtually unknown, and titanium(IV) is markedly more stable than either Ti(III) or Ti(II).
• Late in the series (Cu, Zn), there are too many $d$ electrons — leaving too few empty $d$ orbitals available to share electrons with other atoms and so support higher valence states. Zinc, in fact, shows only the single oxidation state $+2$, since no $d$ electrons participate at all.

The Maximum Oxidation State and Where Stability Drops Off

Up to manganese, the maximum oxidation state of reasonable stability for each element corresponds to the sum of its $s$ and $d$ electrons — for example $\text{Ti}^{\text{IV}}\text{O}_2$, $\text{V}^{\text{V}}\text{O}_2^{+}$, $\text{Cr}^{\text{VI}}\text{O}_4^{2-}$ and $\text{Mn}^{\text{VII}}\text{O}_4^{-}$. Past manganese, however, the stability of these higher oxidation states falls off rather abruptly, so that the oxidation states typically encountered for the remaining elements of the series are much lower: $\text{Fe}^{\text{II,III}}$, $\text{Co}^{\text{II,III}}$, $\text{Ni}^{\text{II}}$, $\text{Cu}^{\text{I,II}}$ and $\text{Zn}^{\text{II}}$.

Why the States Differ by Unity

The variability itself arises from the incomplete filling of the $d$ orbitals, and it shows up in a characteristic way: successive accessible oxidation states of a given transition element typically differ from one another by a single unit — vanadium, for instance, is found as $\text{V}^{\text{II}}$, $\text{V}^{\text{III}}$, $\text{V}^{\text{IV}}$ and $\text{V}^{\text{V}}$. This is a clear point of contrast with non-transition elements, whose accessible oxidation states normally differ from each other by a unit of two.

Down a Group: Heavier Members Favour the Higher State …