L4 - Properties of Transition Metals

Magnetic Properties

• Paramagnetism is a weak magnetic force produced by unpaired electrons and is only detectable in the presence of an external magnetic field.
• The strength of the force is directly proportional to the number of unpaired electrons in the atom or ion.
• As the transition metals DO NOT USE the electrons in their 3d shells for bonding this leaves them available to display paramagnetism if unpaired.
• Take the following example:

• In the ground state Chromium has 6 unpaired electrons whereas the Chromium 3+ ion only has 3 unpaired electrons
• This means that the Chromium atom has twice as many unpaired electrons when compared to it's ion
• Resulting in the chromium atom having twice the magnetic force.
• This whole example is more easily represented by using the orbital diagrams
• Due to the presence of ligands it is difficult to predict the actual number of unpaired electrons in a complex

Impact of d-d splitting on Paramagnetism

• When bonded to ligands the we have learned that the 3d shell splits into 2 different energy levels.
• When this happens electrons will fill the lowest energy level first following the aufbau principle.
• For Example:

• Here we can see that the effect of ligands means that the the ion goes from 5 unpaired electrons down to one unpaired electron as it is more energetically stable to fill the lower energy shell first.
• So Fe3+ in the ground state exhibits more magnetic force than [Fe(H₂O)₆]⁺³

Catalytic Activity

• All reactions are accompanied by changes in enthalpy.
• The enthalpy rises as the reaction starts because energy is being put in to break bonds
• It reaches a maximum then starts to fall as bonds are formed and energy is released.

Activation Energy

• Reactants will only be able to proceed to products if they have enough energy
• The energy is required to overcome an energy barrier
• Only those reactants with enough energy will get over
• The minimum energy required is known as the ACTIVATION ENERGY

Collision Theory

According to COLLISON THEORY a reaction will only take place if…

• PARTICLES COLLIDE
• PARTICLES HAVE AT LEAST A MINIMUM AMOUNT OF ENERGY
• PARTICLES ARE LINED UP CORRECTLY

To increase the chances of a successful reaction you need to...

• HAVE MORE FREQUENT COLLISONS
• GIVE PARTICLES MORE ENERGY or
• DECREASE THE MINIMUM ENERGY REQUIRED

Maxwell-Boltzmann Distribution

• The area under the curve beyond E_a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react.
• If a catalyst is added, the Activation Energy is lowered - E_a will move to the left.

Catalysts Lower Ea

Catalysts work by providing…

“AN ALTERNATIVE REACTION PATHWAY WHICH HAS A LOWER ACTIVATION ENERGY”

A GREATER PROPORTION OF PARTICLES WILL HAVE ENERGIES

IN EXCESS OF THE MINIMUM REQUIRED SO MORE WILL REACT

Heterogeneous Catalysts

Format Catalysts are in a different phase to the reactants

e.g. a solid catalyst in a gaseous reaction

Action takes place at active sites on the surface of a solid

gases are adsorbed onto the surface they form weak bonds with metal atoms

Catalysis is thought to work in three stages...

Adsorption

Reaction

Desorption

Adsorption (STEP 1)

• Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier

Reaction (STEPS 2 and 3)

• Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur. This increases the chances of favourable collisions taking place.

Desorption (STEP 4)

• There is a re-arrangement of electrons and the products are then released from the active sites

Catalysis of gaseous reactions can lead to an increase in rate in several ways

• one species is adsorbed onto the surface and is more likely to undergo a collision

• one species is held in a favourable position for reaction to occur

• adsorption onto the surface allows bonds to break and fragments react quicker

• two reactants are adsorbed alongside each other give a greater concentration

Transition Metal Catalysts - Specific Examples

These work because of their ability to change oxidation state

Metal Complexes acting as lewis acids

• A Lewis acid is a Lone Pair Acceptor.
• Transition metals therefore behave as Lewis acids when bonding with ligands
• When these ions are placed into water they all form hexacqua ions as water is a ligand.
• When water behaves as a ligand the charge density of the cation weakens the O-H bonds.
• This means the waters can lose their protons when acting as a ligand:

[Fe(H₂O)₆]⁺³ + H₂O ⇋ [Fe(H₂O)₅(OH)]⁺² + H₃O⁺

• This process is called Deprotonation and increases the [H⁺] concentration of the solution and thus decreases pH.
• All of these hydrated metal ions exhibit this acidic behaviour and act as weak acids.
• +3 ions have a greater charge density and their hexaqua ions lose protons more readily as the O-H bond is weaker than in +2 ions.
• +3 ions therefore have a higher Ka and a lower pH.