23 Transition Elements

Question 1

Definition of Transition Elements

Answer 1

A d-block element which forms at least one stable ion with a partially filled d subshell

Question 2

Variation in atomic radii

Answer 2

Little variation because:
- Electrons are being added to inner 3d subshell
- Shielding effect increases
- Effect of increasing nuclear charge reduced
- Negligible increase in effective nuclear charge

Question 3

High melting and boiling point of transition elements

Answer 3

Much higher compared to other metals
- 4s and 3d electrons close in energy, and both can contribute to sea of delocalised electrons
- Ionic radii of transition metal cations are smaller (higher nuclear charge, poor shielding effect by remaining 3d electrons
- stronger metallic bonding -> higher boiling point

Question 4

Why do transition elements exhibit variable oxidation states?

Answer 4

• Closeness in energy of 3d and 4s electrons
• Both 3d and 4s electrons are available for bond formation

Question 5

Why are transition elements able to function as effective homogenous catalysts?

Answer 5

1. Ability to exist in different oxidation states
2. Relative ease with whihc the oxidation state can be converted between one another
   Facilitates the formation and decomposition of intermediates

Question 6

Why are transition elements able to function as effective heterogenous catalysts

Answer 6

1. Availability of energetically accessible vacant/partially filled 3d subshells which allow the ready exchange of electrons to and from reactants, facilitating the formation of weak bonds with reactants
2. Availability of 3d and 4s electrons for bond formation with reactant particles

Question 7

Complex definition

Answer 7

A central metal atom/ion bonded to one or more surrounding ligands

Question 8

Ligand definition

Answer 8

An ion/molecule which contains at least 1 atom containing 1 lone pair of electrons which can be donated into energetically accessible vacant orbital of the central metal atom/ion, forming a dative bond

Question 9

Ligand types

Answer 9

• Monodentate (1 dative bond with central atom/ion)
• Polydentate/chelating (>1 dative bond with central atom/ion)

Question 10

Why do polydentate ligands form more stable complexes than monodentate ligands?

Answer 10

• Claw-like grip of polydentate ligands, holding onto central atom/ion more securely

Question 11

Why are transition metal complexes coloured?

Answer 11

1. Presence of ligands cause 3d orbitals to split into 2 sets of non-degenerate orbitals at different energies
2. Difference in energies is small and falls within the energy range of the visible spectrum of light
3. Radiation absorbed when electron from lower energy d-orbital promoted to d-orbital of higher energy
4. Colour observed is the complement of the colour of the radiation absorbed

Question 12

Crystal field theory

Answer 12

1. dz^2 and dx^2-y^2 have lobes that project towards negative charges, while dxy, dxz, dyz orbitals have lobes that project between charges
2. Orbitals of the ligands repel dz^2 and dx^2-y^2 orbitals more
3. Repulsion results in the splitting of d orbitals
4. dz^2 and dx^2-y^2 orbitals are higher energy level, dxy, dxz, dyz orbitals have lower energy levels

Question 13

Factors affecting colour of complexes

Answer 13

Dependent on energy gap between d orbitals
1. Nature of central atom/ion
2. Nature of ligand

Question 14

Complexation stability constant

Answer 14

Ks, of a ligand exchange reaction
Higher Ks -> stronger binding ligand

Question 15

Why is CO poisonous?

Answer 15

CO is able to replace H2O ligand in the haem groups
CO ligand is strongly and irreversibly bonded to the site,
Hinders the haemoglobin’s O2 carrying capacity
Oxyhaemoglobin converted to Caeboxyhaemoglobin