The transition elements have characteristic properties; these properties are related to atoms and ions having incomplete d sub-levels.

Electron configuration of d-block elements

  • Transition metals have partially filled d orbitals in their atoms or ions.
  • $\rm Zn$ is not a transition element because it has a full $\rm d$ sub-level in its atoms and ions.
  • Characteristic properties include: variable oxidation number, complex ion formation with ligands, existence of coloured compounds, and catalytic and magnetic properties.
  • All electrons are always removed from the $\rm 4s$ level before the $\rm 3d$ level $\rm (e.g.~ Mn: [Ar]3d^54s^2, Mn^{2+}: [Ar]3d^5)$.
  • Multiple oxidation states arise because the $\rm 3d$ and $\rm 4s$ sub-levels are close in energy and both are involved in bonding.
  • All $\rm d-$block elements except $\rm Sc$ show an oxidation state of $+2$.
  • All $\rm d-$block elements except $\rm Zn$ show an oxidation state of $+3$.

Complex ions

  • The bonding in transition metal compounds involves coordination bonding, with both electrons in the shared pair of a covalent bond originating from the same species. A ligand is a molecule or negative ion that donates a pair of electrons to a central metal ion to form a covalent (coordinate) bond. They are Lewis bases. Examples include $\rm NH_3$, $\rm H_2O$, $\rm Cl^-$, $\rm CN^-$.
  • Complex ions are formed when a central metal ion is bonded to a ligand with a coordinate bond. Examples include $\rm [Fe(H_2O)_6]^{2+}$, $\rm [Fe(CN)_6]^{3-}$, $\rm [CuCl_4]^{2-}$ and $\rm [Ag(NH_3)_2]^+$.
  • The charge on a complex ion is the sum of the charges of the metal ion and the ligands.
  • The coordination number is the number of ligands around the central ion.

Properties of Transition Metals

  • Transition metals act as heterogeneous catalysts as they can provide a surface for reaction: they use the $\rm 3d$ and $\rm 4s$ electrons to form weak bonds to reactant molecules.
    Examples of elements include: $\rm Fe$ in the Haber process: $\rm N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$
    $\rm Ni$ in the conversion of alkenes to alkanes. $\rm Pd$ and $\rm Pt$ in catalytic converters. Examples of compounds include $\rm V_2O_5$ in the Contact process: $\rm 2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, $\rm MnO_2$ in the decomposition of hydrogen peroxide $\rm 2H_2O_2(aq) \rightarrow 2H_2O_2(l) + O_2(g)$
  • Transition metals ions act as homogenous catalysts due to their ability to show variable oxidation states.
    Examples include: $\rm Fe^{2+}$ in heme. $\rm Co^{3+}$ in vitamin $\bf B12$.
  • Transition metal catalysis has important industrial and biological significance.
  • Magnetic properties are a result of unpaired electrons in the transition metal atom or ion.