Transition metals show variable oxidation states because ns and (n-1)d electrons have similar energies. Maximum oxidation state = total (ns + nd) electrons, up to Mn: Sc(+3), Ti(+4), V(+5), Cr(+6), Mn(+7). Beyond Mn, the maximum decreases: Fe(+6), Co(+5), Ni(+4), Cu(+3), Zn(+2). The +2 state is universal ($ns^2$ removal). Stability of +2 increases left to right. $Mn^{2+}$ ($d^5$) and $Zn^{2+}$ ($d^{10}$) are exceptionally stable. Higher oxidation states are stabilised by electronegative ligands ($F^-$, $O^{2-}$): $CrO4^{2-}$ ($Cr^{+6}$), $MnO4^-$ ($Mn^{+7}$). Lower states stabilised by pi-acceptor ligands (CO, $CN^-$).