28 Chemistry of Transition elements

28.1 General Physical and Chemical Properties of the First Row of Transition Elements (Titanium to Copper):

  1. Transition Element: Transition elements are d-block elements that have one or more stable ions with incomplete d orbitals. These elements include titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu).
  2. Shape of 3d Orbitals: The 3d orbitals have different shapes. The 3dxy orbital has a dumbbell shape, while the 3dz2 orbital has a shape similar to a double dumbbell or a doughnut shape.
  3. Properties of Transition Elements:

(a) Variable Oxidation States: Transition elements exhibit variable oxidation states. This is because the energy difference between the 3d and 4s sub-shells is small, allowing electrons to be easily gained or lost from the 3d orbitals.

(b) Catalysts: Transition elements act as catalysts due to their ability to change oxidation states and form intermediate compounds during chemical reactions.

(c) Formation of Complex Ions: Transition elements have vacant d orbitals that are energetically accessible, allowing them to form complex ions by coordinating with ligands.

(d) Coloured Compounds: Transition elements form coloured compounds due to the presence of partially filled d orbitals that can absorb visible light and undergo electronic transitions.

  1. Variable Oxidation States: The similarity in energy between the 3d and 4s sub-shells allows transition elements to easily gain or lose electrons from the 3d orbitals, leading to the existence of multiple oxidation states.
  2. Catalysts: Transition elements can act as catalysts because they can undergo changes in oxidation states and provide suitable active sites for reactions to occur. The presence of vacant d orbitals allows them to form dative bonds with ligands, facilitating the reaction.
  3. Formation of Complex Ions: Transition elements have vacant d orbitals that can accept electron pairs from ligands, forming coordinate bonds and resulting in the formation of complex ions. The presence of these vacant orbitals makes the formation of complexes favorable.

28.2 General Characteristic Chemical Properties of the First Row of Transition Elements:

  1. Formation of Complexes: Transition elements can react with ligands to form complex ions. Ligands are species that contain a lone pair of electrons and form dative covalent bonds with the central metal atom/ion. Examples of ligands include H2O, NH3, Cl-, and CN-.
  2. Types of Ligands:

(a) Monodentate Ligands: Ligands that can form only one bond with the metal ion, such as H2O, NH3, Cl-, and CN-.

(b) Bidentate Ligands: Ligands that can form two bonds with the metal ion simultaneously, such as ethylenediamine (en) and the ethanedioate ion (C2O42-).

(c) Polydentate Ligands: Ligands that can form multiple bonds with the metal ion, such as ethylenediaminetetraacetate (EDTA4-).

  1. Complexes: Complexes are molecules or ions formed by a central metal atom/ion surrounded by one or more ligands.
  2. Geometry of Transition Element Complexes: Transition element complexes can have various geometries based on the coordination number, which is the number of ligands bonded to the central metal ion. Common geometries include linear, square planar, tetrahedral, and octahedral.
  3. Coordination Number: The coordination number represents the total number of ligands bonded to the central metal ion in a complex.
  4. Ligand Exchange: Ligand exchange can occur in complex ions, where one ligand is replaced by another. The complexes of copper(II) and cobalt(II) ions with water, ammonia, hydroxide, and chloride ions are examples of ligand exchange reactions.
  5. Redox Reactions: Transition elements can undergo redox reactions due to the presence of multiple oxidation states. The feasibility of redox reactions involving transition elements can be predicted using standard electrode potentials (E°) values.
  6. Redox Reactions Examples:

(a) MnO4- / C2O42- in acid solution.

(b) MnO4- / Fe2+ in acid solution.

(c) Cu2+ / I-.

28.3 Colour of Complexes:

  1. Degenerate and Non-degenerate d Orbitals: Degenerate d orbitals have equal energy, while non-degenerate d orbitals have different energies.
  2. Splitting of d Orbitals: In the presence of ligands, the degenerate d orbitals split into two sets of non-degenerate orbitals with different energies. This splitting is known as crystal field splitting. In octahedral complexes, the splitting results in two higher energy orbitals and three lower energy orbitals.
  3. Absorption of Light: Transition elements form colored compounds because they can absorb visible light due to the electronic transitions between two non-degenerate d orbitals.
  4. Effects of Ligands on Colour: Different ligands cause different degrees of splitting (ΔE) of the d orbitals, leading to the absorption of light at different frequencies. The absorbed frequencies determine the complementary color observed.
  5. Ligand Exchange and Color: Ligand exchange reactions can result in changes in the color of transition metal complexes. For example, the complexes of copper(II) and cobalt(II) ions with water, ammonia, hydroxide, and chloride ions exhibit different colors due to ligand exchange.

28.4 Stereoisomerism in Transition Element Complexes:

  1. Types of Stereoisomerism: Transition element complexes can exhibit two types of stereoisomerism:

(a) Geometrical (Cis-Trans) Isomerism: It occurs in square planar and octahedral complexes. In square planar complexes, geometric isomers can arise when two ligands are located in either a cis (adjacent) or trans (opposite) configuration. In octahedral complexes, geometric isomers can arise when two sets of ligands are located either in a cis or trans arrangement.

(b) Optical Isomerism: It occurs when a complex possesses chiral centers, resulting in the existence of enantiomers. These enantiomers rotate plane-polarized light in opposite directions.

  1. Overall Polarity: The presence of geometric or optical isomerism in complexes influences their overall polarity.

28.5 Stability Constants (Kstab):

  1. Stability Constant: The stability constant (Kstab) of a complex is the equilibrium constant for the formation of the complex ion from its constituent ions or molecules in a solvent.
  2. Expression for Kstab: The expression for a stability constant does not include the concentration of water ([H2O]).
  3. Calculations: Stability constants can be used to perform calculations related to the formation of complex ions and ligand exchanges. Large stability constants indicate the formation of stable complex ions.