Chapter 8: The d-block and the transition element

General Properties of Transition Metals

  • A transition element is a d-block element which has at least one stable ion with an incomplete d-subshell
  • Transition metal ions form colored compounds because of the movement of electrons in partially filled d-orbitals. Particular wavelengths of light are absorbed, so the remaining wavelengths are the color perceived.
  • All the transition metals have more than one oxidation state.
  • The highest oxidation states will readily accept electrons (be reduced), which makes them powerful oxidizing agents.
  • Transition metals and their compounds act as catalysts by:
    • Providing a surface for the reaction to take place
    • Binding to reactants to form intermediates
  • Transition metals often form complexes. A complex is a central metal atom or ion surrounded by ligands.
  • A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a lone pair of electrons.

 

Configuration Period

Sc 1s22s22p63s23p6 4s23d1Sc 3+ [Ar] 4s03d0

Ti 1s22s22p63s23p6 4s23d2Ti 3+ [Ar] 4s03d1

V 1s22s22p63s23p6 4s23d3 V 3+ [Ar] 4s03d2

Cr 1s22s22p63s23p6 4s13d5Cr 3+ [Ar] 4s03d3

Mn 1s22s22p63s23p6 4s23d5Mn 2+ [Ar] 4s03d5

Fe 1s22s22p63s23p6 4s23d6 Fe 3+ [Ar] 4s03d5

Co 1s22s22p63s23p6 4s23d7 Co 2+ [Ar] 4s03d7

Ni 1s22s22p63s23p6 4s23d8  Ni 2+ [Ar] 4s03d8

Cu 1s22s22p63s23p6 4s13d10Cu 2+ [Ar] 4s03d9

Zn 1s22s22p63s23p6 4s23d10 Zn 2+ [Ar] 4s03d10

when forming, ions lose 4s before 4d

Substitution Reactions

All ligands contain at least one lone pair of electrons in their outer shell. Monodentate ligands can donate just one lone pair of electrons each and form one coordinate bond. E.g. H2O, NH3 and Cl

The ligands NH3 and H2O are similar in size and are uncharged. Therefore, exchange of a H2O ligand for a NH3 occurs without change of co-ordination number.

        [Cu (H2O) 6]+2 (aq) + 4NH3 (aq) ⇌ [Cu (NH3)4(H2O)2]2+ (aq) + 4H2O (l)
        This ligand substitution is incomplete

        [Co (H2 O)6]2+ (aq) + 6NH3 (aq) ⇌ [Co (NH3)6]2+ (aq) + 6H2O (l)
        This ligand substitution is complete

 

  • The Cl ligand is larger than the uncharged ligands NH3 and H2 Therefore, exchange of the ligand H2O by Cl– can involve a change of co-ordination number

[Cu (H2O)6]2+ (aq) + 4Cl (aq) ⇌ [Cu(Cl)4]2- (aq) + 6H2O (l)
[Co(H2O)6]2+ (aq) + 4Cl (aq) ⇌ [CoCl4]2- (aq) + 6H2O (l)
[Fe(H2O)6]3+ (aq) + 4Cl (aq) ⇌ [FeCl4] (aq) + 6H2O (l)​

  • Bidentate ligands have 2 lone pairs it can donate e.g., H2NCH2CH2NH2 and C2O4^2–

  • Multidentate ligands can donate multiple lone pairs of electrons from different atoms within the ligands to form multiple coordinate bonds e.g. EDTA^4–

  • Chelation is when multidentate ligands replace monodentate ligands in complexes 

[Cu (H2O) 6]^2+ (aq) + EDTA^4- (aq) → [Cu (EDTA)] ^2- (aq) + 6H2O (l)

  • For the reaction to be thermodynamically feasible, the Gibbs free energy change must be negative

Shape of complex ions

Transition metals ions commonly form octahedral complex with small ligands (e.g. H2O and NH3). [Co (NH3)6]2 [Cu (H2O)62+.

Transition metal ions commonly form tetrahedral complexes with larger ligands (e.g. Cl). [CoCl4]2-

Square planar complexes are also formed, e.g. cisplatin

Ag+ commonly forms linear complexes e.g. [Ag (NH3)2]+ used as Tollen’s Reagent.

Catalysis

Catalysts increase reaction rates without getting used up. They do this by providing an alternative route with a lower activation energy. Transition metals and their compounds can act as heterogeneous and homogeneous catalysts. A heterogeneous catalyst is in a different phase from the reactants. A homogeneous catalyst is in the same phase as the reactant.

Transition metals and their compounds can act as heterogeneous and homogeneous catalysts. Heterogeneous catalysts are spread onto a support medium to maximize their surface area and minimize the cost. Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency. They can be expensive to replace.

In the Contact process V2O5 acts as a heterogeneous catalyst 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

  • Sulphur dioxide adsorbs onto vanadium(V) oxide
  • V2O5 (s) + SO2 (g) → V2O4 (s) + SO3 (g)
  • V2O4 (s) + 1/2O2 (g) → V2O5 (s)

The reaction between the per-sulphate ion (S2O82-) and the iodide ion is catalyzed by the homogenous catalyst Fe2+.

  • S2O82- (aq) + 2I (aq) → 2SO42- (aq) + I2 (aq)
    • S2O82-(aq) + 2Fe2+ (aq) → 2SO42- (aq) + 2Fe3+ (aq)
    • 2I^- (aq) + 2Fe^3+ (aq) → I2 (aq) + 2Fe2+ (aq)

The reaction between ethanedioate ions and potassium manganate (VII) which is catalyzed by Mn2+ (autocatalysis).

  • 2MNO4(aq)+5C2O42-(aq)+16H+ (aq) → 2Mn2+(aq) + 10CO2 (g) + 8H2O(I)
    • 2MnO4 (aq) + 16H+ (aq) + 8Mn2+ (aq) → 8H2O (l) + 10Mn3+ (aq)
    • 5C2O4 (aq) + 10Mn3+ (aq) → 10CO2 (g) + 10Mn2+ (aq)