2.5: Transition metals

What is a transition metal?

A d-block element with a partially filled d-orbital, that loses e- to form positive ions with s block e- being lost first

What are characteristic properties of transition metals?

• Similar atomic radii

• High densities

• High melting points/boiling points

• Ability to form complexes

• Ability to form coloured ions

• Variable oxidation states

• Good catalysts

What is a complex?

A central metal atom or ion (transition metal) surrounded by ligands

What is a ligand?

A molecule/ion that forms a co-ordinate bond with a transition metal by donating a lone pair of electrons

What is co-ordination number?

The number of co-ordinate bonds to the central metal atom or ion

Label this diagram, give the co-ordinate number and give an alternate way that this complex can be written

• Ligand, complex

• Co-ordination number = 6

• [Co(H₂O)₆]²⁺

What is a monodentate ligand, give examples, and explain how they work in ligand substitution reactions?

• A ligand that forms 1 co-ordinate bond

• H₂O, NH₃

• They are similar in size and uncharged, so they can be exchanged without a change via a ligand substitution reaction without causing a change in co-ordination number

What is the ligand substitution reaction of [Co(H₂O)₆]²⁺ and NH₃?

[Cu(H₂O)₆]²⁺ + 6NH₃ → [Cu(NH₃)₆]²⁺ + 6H₂O

What is the ligand substitution reaction of [Cu(H₂O)₆]²⁺ and NH₃?

[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(H₂O)₂(NH₃)₄)²⁺ + 4H₂O

• Incomplete substitution reactions

• Forms a deep blue solution

What is the ligand substitution reaction of [Cu(H₂O)₆]²⁺ and Cl^-?

[Cu(H₂O)₆]²⁺ + 4Cl^- → [CuCl₄]^2- + 6H₂O

• Cl^- ligand is larger than NH₃ and H₂O, so substitution causes a change in co-ordination number

• Forms a tetrahedral complex

What is a bidentate ligand, and give examples?

• A ligand that is able to form 2 co-ordinate bonds to the central metal atom, as they have 2e- lone pairs

• Ethanedioate and ethane-1,2-diamine

What is a multidentate ligand, and give examples?

• A ligand able to form up to 6 co-ordinate bonds to the central metal ion as they have multiple e^- lone pairs

• EDTA^4- and haem (an iron complex with a multidentate ligand)

What is the ligand substitution reaction of [Fe(H₂O)₆]³⁺ and EDTA^4-?

[Fe(H₂O)₆]³⁺ + EDTA^4- → [FeEDTA]^- + 6H₂O

What is the role of haemoglobin?

• Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, allowing oxygen to be transported in the blood

Why is CO toxic?

• Replaces O₂ coordinately bonded to Fe(II) in haemoglobin

• So oxygen cannot be transported around the body and delivered to tissues

What is the chelate effect?

• In ligand substitution reactions, a positive entropy change is favourable as a more stable complex is being formed

• Meaning it is better to have more moles on the right side of the equation that the left

• So it is favourable so substitute monodentate ligands with bidentate/multidentate ligands

• ∆H for these reactions are very small as bonds being formed is similar to bonds being broken, so enthalpy = 0

What shape and bond angle is formed when transition metals form complexes with small ligands (H₂O/NH₃), and what type of isomerism is shown?

• Octahedral

• 90°

• Cis-trans isomerism

What shape and bond angle is formed when transition metals form complexes with bidentate ligands, and what type of isomerism is shown?

• Octahedral

• 90°

• Optical isomerism

What shape and bond angle is formed when transition metals form complexes with larger ligands, and what type of isomerism is shown?

• Tetrahedral

• 109.5°

• Optical isomerism

What shape and bond angle is formed when Pt/Ni forms complexes with ligands, and what type of isomerism is shown?

• Square planar

• 90°

• Cis-trans isomerism

What is cisplatin and why is only this molecule effective?

• Cancer therapy drug

• Only the cis isomer is effective as natural cells are chiral

What shape and bond angle is formed when Ag⁺ forms complexes with small ligands, and what is the product used for?

• Linear

• 180°

• Tollens’ reagent

How can different transition metals be identified?

Colour

What is the colour of a transition metal dependent on?

• Co-ordination number of the complex

• Type of ligand bonded to the ion

• Oxidation state

How does colour arise in transition metals?

• Some wavelengths of visible light are absorbed

• When light is absorbed, d electrons move from the ground state when light is absorbed

• Remaining wavelengths of light are transmitted and reflected to the human eye

• Reflective wavelengths correspond to a specific colour which is observed by the human eye

How is the energy difference between the ground state and excited state of the d electrons calculated?

∆E = h ν = hc /λ

• ∆E = energy

• h = planck’s constant (6.63×10^-34)

• ν = frequency (Hz)

• c = speed of light (3.00×10^-8m/s)

• λ = wavelength (m)

What are factors that effect energy between ground and excited electrons?

• Co-ordination number of the complex

• Type of ligand bonded to the ion

• Oxidation state

How can the concentration of coloured ions be measured?

• Colorimetry

• Uses absorption of visible light

• Allows a calibration graph of relative absorbance (y) vs % concentration (x)

• Which is then used to calculate the concentration of an unknown substance

What are the different oxidation states of vanadium, what is their ion formula, and what colour is each species?

How are vanadium species in oxidation states IV,III and II formed?

The reduction of vanadate (V) iosn by zinc in acidic solution

What is redox potential, and what is it influenced by?

• How easily an ion can be reduced to a lower oxidation state

• Influenced by pH and the ligand

Why is redox potential influenced by pH?

• For ions to be oxidised, they react with water to produce OH^-, so alkaline conditions are required

• For ions to be reduced, acidic conditions are required

Why is redox potential influenced by ligands?

• Some ligands are stronger than each other (NH₃ is stronger than H₂O)

• So the stronger ligand binds tighter to the central metal ion

• So redox potential becomes more negative

What is Tollens’ reagent used for, and what is the equation?

• Used to differentiate between aldehydes and ketones

• Tollens’ [Ag(NH₃)₂]⁺ is reduced by aldehydes to form a metallic silver mirror

• RCHO + 2[Ag(NH₃)₂]+ + 3OH^- → RCOO^- + 2Ag + 4NH₃ + 2H₂O

What are the equations for the redox titrations of Fe²⁺ and C₂O₂^2- with MnO₄^-

• MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O

• Fe²⁺ → Fe³⁺ + e^-

• C₂O₄^- → 2CO₂ + 2e^-

What are the molar ratios for the redox titrations of Fe²⁺ and C₂O₂^2- with MnO₄^-

• MnO₄^- : C₂O₄^- = 2:5

• MnO₄^- : Fe²⁺ = 1:5

• C₂O₄^{- :} Fe²⁺ = 1:2

What is a catalyst?

• A species that speeds up the rate of reaction by providing an alternative pathway with a lower activation energy without being used up

What is a homogenous catalyst?

A catalyst in the same phase as the reactants

What is a heterogeneous catalyst, and what are some examples?

A catalyst in a different phase to the reactants, where the reaction occurs at active sites on the surface

What are examples of heterogeneous catalysts and what are the equations?

• V₂O₅ acts as a catalyst in the contact process

Overall equation:

• 2SO₂ + O₂ → 2SO₃

Intermediates:

• V₂O₅ + SO₂ → V₂O₄ + SO₃

• V₂O₄ + ½O₂ → V₂O₅

• Fe acts as a catalyst in the Haber process

• N₂ + 3H₂ → 2NH₃

What is the use of catalytic support mediums, and give an example of where it is used?

• Used to maximise surface area of a heterogeneous catalyst and minimise the cost

• Pt is spread out in a honeycomb structure on a catalytic converter

How do solid heterogeneous catalysts work?

• On the surface of a solid catalyst, reactant molecules are absorbed onto active sites

• This increases the proximity of the molecules so they are closer together

• So covalent bonds are weakened, so the reaction takes place more easily

• So rate of reaction is increased

What is strength of adsorption based on, which transition metals have the best adsorption and why?

• Dependent on the type of catalyst

• Fi, Co and Ni are the best, as they increase the rate of reaction the most out of period 4 transition metals and they are cheap

What is catalytic poisoning, what is the consequence of it, and what is an example of a poision in catalysis?

• Heterogeneous catalysts can be poisoned by impurities which block active sites and prevent adsorption

• So bonds between molecules remain strong, so catalyst has no effect on the rate

• Leads to increase in costs as the catalyst has to be replaced and cleaned regularly

• Sulfur poisons Fe catalyst in the Haber process

What is an example of a homogeneous catalyst, and what are the equations?

• Fe²⁺ catalyses S₂O₈^2- and I^-

Overall

• S₂O₈^2- + 2I^- → I₂ + 2SO₄^2-

Intermediates

• S₂O₈^2- + 2Fe²⁺ → 2Fe³⁺ + 2SO₄^2-

• 2Fe³⁺ + 2I^- → 2Fe²⁺ + I₂

Why would S₂O₈^2- and I^- be unable to react if not for a catalyst

They would both repel each other as they are both negative

What is autocatalysis?

• One of the products as a catalyst for the reaction

• So as product increases, the rate of reaction increases as the reaction becomes catalysed^@

What is an example of autocatalysis, and what are the equations and conditions?

• Mn²⁺ ions are used as a catalyst in the redox reaction between MnO₄^- and C₂O₄^2- ions in acidic solutions

• Requires a small amount of heat

Overall

• 2MnO₄^- + 16H⁺ + 5C₂O₄^2- → 2Mn²⁺ + 8H₂O + 10CO₂

Intermediates

• 4Mn²⁺ + MnO₄^- + 8H⁺ → 5Mn³⁺ + 4H₂O

• 2Mn³⁺ + C₂O₄^2- → 2CO₂ + 2Mn²⁺