Oxidation States of Transition Metals

Transition metals characteristically show variable oxidation states -- often three, four, or even more distinct states for a single element -- in sharp contrast to the s-block metals, which almost always show one fixed oxidation state ($+1$ or $+2$), or a typical p-block nonmetal, which usually shows only two or three.

Why variable oxidation states occur. The underlying reason is that, for transition metal atoms, the energies of the $(n-1)d$ and $ns$ orbitals are close enough to one another that electrons from both sets can be involved in bond formation, rather than only the outermost $ns$ electrons. Because a comparatively small amount of extra energy is needed to remove a further $d$ electron beyond the one or two $4s$ electrons, several different numbers of electrons can be removed (or shared) with only a modest energy penalty, each giving rise to a distinct, isolable oxidation state -- so long as the resulting ion is stabilized in some way (e.g. by combination with oxygen or a suitable ligand).

How the range widens then narrows across the series. Scandium, at the very start of the series, shows only the $+3$ state, since removing all three outer electrons ($4s^2 3d^1$) leaves the stable, noble-gas-like $[\text{Ar}]$ core with no partially filled $d$ subshell left to offer any alternative bonding option. The number of accessible oxidation states then widens steadily on moving across the series -- titanium shows $+2, +3, +4$; vanadium $+2$ through $+5$; chromium $+2$ through $+6$ -- and reaches a maximum at manganese, which shows every oxidation state from $+2$ to $+7$, corresponding to the successive loss or sharing of each of its seven $3d^5 4s^2$ valence electrons. Beyond manganese, the range narrows again: iron and cobalt show mainly $+2$ and $+3$; nickel shows predominantly $+2$; copper shows $+1$ and $+2$; and zinc, having completely filled its $3d^{10}$ subshell and having only its $4s^2$ electrons left to lose, shows the single fixed oxidation state $+2$ -- behaving, in this one respect, much like a typical s-block metal. …