Transition Metals

How do transition metal characteristics arise?

The characteristics of elements titanium to copper arise from an incomplete d sub level in atoms or ions

What are the four characteristics of a transition metal?

• Complex formation

• Formation of coloured ions

• Variable oxidation state

• Catalytic activity

Why are scandium and zinc not a transition metal?

They don’t have variable oxidation states, and have a complete d orbital and so doesn’t meat the criteria of incomplete d orbital

What is a complex?

A central metal ion surrounded by ligands

What is a ligand?

An atom, ion or molecule which can donate an electron pair

What is co-ordinate bonding?

When the shared pair of electrons in the covalent bond comes from only one of the bonding atoms

What is co-ordination number?

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

What are the different types of ligands and give examples?

Monodentate- form one coordinate bond per ligand e.g. water, NH3 and Cl^-

Bidentate- two atoms with lone pairs which can form two coordinate bonds per ligand e.g. 1,2-diaminoethane and ethanedivoate ion

Multindentate e.g. EDTA^4-

What is a ligand substitution reaction?

One in which one ligand is coordinately bonded to a metal atom/ion and is replaced by another ligand

What is similar about the ligand NH₃ and H₂O and their reactions?

• Similar in size and uncharged

• So exchange of these ligands occurs without change of the co-ordination number e.g. with copper and cobalt

• [Co(H₂O)₆]²⁺(aq) + 6NH₃ (aq)  [Co(NH₃)₆]²⁺(aq) + 6H₂O (l)

• In the case of copper the substitution may be incomplete:

• e.g. [Cu(H₂O)6]²⁺(aq) + 4NH₃ (aq) to [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 4H₂O (l)

How does the reaction with chloride ions differ?

• The chloride ion ligand is larger than the uncharged water and ammonia, so therefore ligand exchange can involve a change of coordination number

• e.g. addition of concentrates HCl to aqueous ions of copper and cobalt leads to a change in coordination number from 6 to 4

• [Cu(H₂O)₆]²⁺ + 4Cl^- to [CuCl₄]^2- + 6H₂O [Co(H₂O)₆]²⁺ + 4Cl^- to [CoCl₄]^2- + 6H₂O

How does this differ when the solid metal chloride is dissolved in water?

If dissolved in water, it forms the aqueous hexaaqua (metal) complex, not the chloride complex

e.g. [Cu(H₂O)6]²⁺ not [CuCl₄]^2-

Draw the structure of the bidentate ligand 1,2- diaminoethane

There are 3 bidentate ligands in this complex, each bonding in twice to the metal ion. It has a coordination number of 6

Draw the structure of the bidentate ligand ethandioate ion

C₂O₄^2-

• Octahedral shape with 90 degree angles

What are the equations to show formation of bidentate ligands?

• [Cu(H₂O)₆]²⁺ + 3NH₂CH₂CH₂NH₂ to [Cu(NH₂CH₂CH₂NH₂)₃]²⁺ + 6H₂O

• [Cu(H₂O)₆]²⁺ + 3C₂O₄^2-to [Cu(C₂O₄)₃]^4- + 6H₂O

Draw the structure of the multidentate ligand EDTA^4-

• Forms six coordinate bonds per ligand

• [Cu(H₂O)₆]²⁺ + EDTA^4-to [Cu(EDTA)]^2- + 6H₂O

What is an example of an iron complex with a multi dentate ligand?

• Haem

• Each Fe²⁺ containing harm complex can accept a pair of electrons and form a coordinate bond from an oxygen molecule. This is how oxygen is transported around the blood

Why does this relate to how carbon monoxide is toxic?

• Carbon monoxide also contains a lone pair of electrons on the oxygen atom, and this can form a stable complex with haemoglobin

• When incomplete combustion of fuels occur and CO is produced, as when it replaces the oxygen it prevents oxygen being carried in the blood

How can a complex be made more stable?

• This substitution of a monodentate ligand with a bi/multidentate ligand leads to a more stable complex. This is called the chelate effect

How can we explain the chelate effect?

• Positive entropy change, as there are more moles of products than reactants

• When there are more moles of products than reactants, there is an increase in entropy which creates more disorder

• If there are a similar number bonds in both complexes, the enthalpy change will be small and change in entropy is positive, so Gibbs will be negative, so a stable complex is formed

What is the shape of a complex ion with six coordinate bonds?

Octahedral

• TM ions commonly form octahedral complexes with small ligands like water and ammonia

What is the shape of a complex ion with four coordinate bonds?

Tetrahedral

• forms with larger ligands like chloride ions

What shape do platinum and nickel end up forming?

• Square planar e.g. cisplatin

• No overall charge

What is the shape of a complex ion with two coordinate bonds?

Linear

[Ag(NH₃)₂]⁺ as used as Tollen’s reagent

How is stererisomerism shown in these shapes?

• cis-trans isomerism in square planar complexes

  • only the cis isomer of the cisplatin drug shows any anti-cancer activity

  • cis= z trans= e

• Cis-trans isomerism in octahedral complexes

• Optical isomerism in octahedral complexes

What three things do colour changes arise from?

• Oxidation state

• Co-ordination number

• Ligand

How do colour arise?

• In these coloured compounds and complexes, the d sub shell must be partially filled

• The d sub shell is split into two distinct sets of orbitals that have a difference in energy between them

• Energy in the visible region causes d electron transition and electrons become excited, so are promoted to the higher energy orbitals (excited state) from the ground state

• A portion of visible light is absorbed to promote the d electrons to higher energy levels; the light that is not absorbed in transmitted to give the substance colour

What is the equation linking colour, wavelength and frequency of light absorbed?

• energy difference between orbitals= frequency of light absorbed x Planck’s constant

  or

• energy difference between orbitals= (planks constant x speed of light) / wavelength of light absorbed

How do we change colours?

• Changing a ligand or changing coordination number will alter the energy split between the d orbitals, change the energy difference and hence the frequency of light absorbed and transmitted

Why is absorption of visible light used in spectrometry?

• To determine the concentration of coloured ions

• If visible light of increasing frequency is passed through a sample of coloured complex ion, some of the light is absorbed

• The amount of light absorbed is proportional to the concentration of the absorbing species

• Complexes which have only a pale colour and do not absorb light strongly have a suitable ligand added to intensify the colour

• Spectrometers contain a coloured filter, which is chosen to allow the wavelengths of light through that would most strongly be absorbed by the coloured solution

How is a colorimeter used to determine conc of an coloured ions in a solution?

• Add an appropriate ligand to intensify colour

• Make up solutions of known concentration

• Measure absorption or transmission using colorimeter

• Plot graph of absorption vs concentration

• Measure absorption of unknown and compare

How is vanadium reduced into its successive oxidation states?

Addition of zinc in acidic solution- strong reducing agent

What are vanadium’s 4 main oxidation states and their colour?

• VO₂⁺ Oxidation state +5 ( a yellow solution)

• VO²⁺ Oxidation state +4 (a blue solution)

• V³⁺ Oxidation state +3 (a green solution)

• V²⁺ Oxidation state +2 (a violet solution)

What is the redox potential influenced by?

The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH and by the ligand.

[Ag(NH₃)₂]⁺ is used in Tollens’ reagent to distinguish between aldehydes and ketones. Aldehydes reduce the silver in the Tollens’ reagent to silver.

Red 1⁄2 eq: [Ag(NH₃)₂]⁺ + e-  Ag +2NH₃
Ox 1⁄2 eq: CH₃CHO + H₂OCH₃CO₂H + 2H⁺ + 2e^-

What is a catalyst?

A substance that increases rate of reaction but is not used up itself, by providing an alternative pathway with a lower activation energy. It speeds up rate at which an equilibrium is reached by speeding up the forwards and backwards reaction by the same amount

What are the two types of catalyst?

Homogeneous and heterogeneous

What is a homogeneous catalyst?

In the same phase as the reactants

How do homogeneous catalysts work?

They proceed via an intermediate species formed from a reactant and the catalyst, which reacts further and regenerates the catalyst

What are examples of homogeneous catalysts?

• Fe²⁺ and Fe³⁺ can catalyse the reaction between iodide ions and peroxodisulfate ions. The reaction will go by a two-step alternative route involving an intermediate species

  • S₂O₈^2- + 2Fe²⁺ to SO₄^2- + 2Fe³⁺ and then:

  • 2Fe³⁺ + 2I^- to I₂ + 2Fe²⁺

    Overall equation is:

  • S₂O₈^2–_(aq) + 2I^– _(aq) ==> 2SO₄^2–_(aq) + I_2(aq)

  • The collision of an Fe³⁺ ion and an I^– ion involves a positive ion–negative ion attraction, reducing repulsion, so this interaction which has a much lower activation energy.

  • Iron (II) is the catalyst because the oxidation state has changed

• Conc hydrogen sulphate can be used to catalyse esterification between carboxylic acid and alcohol

• Depletion of ozone: chlorine gas is the catalyst and is in the same phase as ozone gas when ut increases the destruction of the ozone layer

What is a heterogeneous catalyst?

Different phase from reactant (e.g. solid and gas). Catalyst is usually solid and reaction takes place on surface with gases and solutions

What are examples of heterogeneous catalysts?

• Catalytic cracking: catalyst is zeolite/ aluminium

• Catalytic converters: made of platinum, palladium, and rhodium

• The Haber process: catalyst is iron, used to increased RoR between nitrogen and hydrogen to produce ammonia

• Contact process: catalyst is vanadium oxide (V₂O₅), used to increase RoR between SO₂ and O₂

Why are heterogeneous catalysts often coated on an inert support medium?

Thin coating can be used to maximise surface area, as metals are expensive so minimises costs

How do heterogeneous catalysts works?

• Reactant molecules are adsorbed onto active sites on the surface of the catalyst

• The bonds in the reactant molecules are weakened

• The reactant molecule are held in a more favourable conformation for reaction

• The product molecules are desorbed from the surface of the catalyst

How can heterogeneous catalysts become poisoned?

• May be poisoned by impurities in the reaction mixture

• Catalyst is poisoned as the active sites on the surface of catalyst become blocked and therefore inactive, and consequently have a reduced efficiency, which has cost implications

What two things would make a heterogeneous catalyst most efficient?

• Adsorb the reactant for so long the reactants stay in the active sites to react

• Desorb the reactants quickly enough that the active sites free up and can adsorb more reactants

What is autocatalysis?

Creates the catalyst itself in the reaction

What are the equations and explanation for how V₂O₅ acts as catalyst in the contact process?

• SO₂ + V₂O₅ to SO₃ + V₂O₄ (reduction)

• V₂O₄ + 0.5O₂ to V₂O₅ (oxidation)

• Overall equation: 2 SO₂(g) + O₂(g) → 2 SO₃(g)

• Variable oxidation states throughout these reactions

How does Mn²⁺ autocatalyse the reaction between C₂O₄^2- and MnO₄^-?

Oxidation of ethanedioic acid:
C₂O₄^2- to 2CO₂ + 2e^-

Reduction of MnO₄^- to Mn²⁺
MnO₄^- + 8H⁺ + 5e^- to Mn²⁺ + 4H₂O

• Overall equation
  5C₂O₄^2- + MnO₄^- + 16H+ to Mn²⁺ + 8H₂O + 10CO₂

• Mn²⁺ : autocatalyst

• Catalyses reaction by reacting with manganate to form manganese:

  • 2MnO₄^- +4Mn²⁺ + 8H⁺ to 5Mn³⁺ + 4H₂O

• And then the Mn³⁺ reacts with ethanedioic:

  • 2Mn³⁺ + C₂O₄^2- to 2CO₂ + 2Mn²⁺

• Overall equation:

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

Outcome is the same but has been sped up by manganese, as it is a positive catalyst in the mix with negative ions so no repulsion to overcome