Topic 15 Transition Metals Flashcards
What causes the colour of a transition metal complex?
d orbitals are split by the energy in the ligands. Light is needed for electron promotion. The colour not absorbed is the colour seen.
Transition metal
A d-block element that forms one or more stable ions with incompletely-filled d-orbitals.
6 properties of transition metals
- Hard solids.
- Have high melting & boiling temperatures.
- Act as catalysts.
- Form coloured ions & compounds.
- Form ions with different oxidation numbers.
- Form ions with incompletely-filled d-orbitals.
Scandium & Zinc
They are d-block elements, but not transition metals as they only form one ion and they do not form coloured compounds. Zn2+ & Sc3+.
What do transition metal ions involving higher oxidation numbers usually contain?
An electronegative element, e.g., oxygen.
How do transition metals form ions with different oxidation numbers?
They lose their 4s electrons before their 3d electrons. They can lose a variable number of electrons.
Ti
OS: +3, +4
V
OS: +2, +3, +4; +5
Cr
OS: +3, +6
Mn
OS: +2, +4; +7
Fe
OS: +2, +3
Co
OS: +2, +3
Ni
OS: +2
Cu
OS: +1, +2
Why does highest common oxidation number increase from Ti to Mn?
All the 4s & 3d electrons become involved in bonding.
Why are ions with higher oxidation numbers less common from Fe to Cu?
The increasing nuclear charge means electrons are more strongly attracted to the nucleus and less likely to be involved in bonding.
Equation for a displacement reaction involving a transition metal ion
Mg(s) + Fe2+ (aq) –> Mg2+ (aq) + Fe(s)
Why do transition metal ions attract electron-rich species, incl. water molecules, more strongly?
Transition metal ions tend to have smaller ionic radii than non-transition metals in the same period.
Ligand
Species that uses a lone pair of electrons to form a dative bond with a metal ion.
Complex
A species containing a metal ion joined to ligands.
Complex ion
A complex with an overall positive or negative charge.
Coordination number
Number of dative bonds in a complex.
What letter do ligands with a negative charge end in?
o, e.g., Cl- = chloro.
Common ligands
- Water H2O, aqua.
- Hydroxide OH-, hydroxo.
- Ammonia NH3, ammine.
What happens when white light is passed through a solution containing a transition metal complex?
Some wavelengths of light are absorbed by the complex, so the light emerging will contain proportionately more of the complementary colour. E.g., red light will emerge green/blue.
What ions do not form coloured compounds?
Ions with completely filled 3d energy levels & ions with no electrons in their 3d orbitals.
The amount of energy gained by an electron when it is promoted in a transition metal
Proportional to the frequency of light absorbed.
Inversely proportional to the wavelength of light.
How do changes in its electron configuration give a transition metal ion its colour?
When ligands attach to the metal ion, the d-orbital splits into 2 levels with slightly different energies. When one of the electrons in the lower level absorbs energy, it is promoted/excited to a higher energy level. The bigger the energy difference between the two levels, the more energy the electron absorbs.
Six-fold coordination
Complexes, in which there are six ligands forming coordinate bonds with the transition metal ion.
Example of a linear transition metal complex
[H3N–> Ag <–NH3]+ This is formed in Tollen’s test.
How can we predict the shape of a transition metal complex?
Halve the number of electrons donated. E.g., 12 electrons donated = octahedral, usually with 6 ligands. Other common shapes are tetrahedral, square planar & linear.
Square planar
Shape containing a central atom or ion surrounded by 4 atoms or ligands in the same plane with bond angles of 90 degrees.
Cis-platin
Square planar molecule, with a Pt(II) ion, two ammonia ligands & two chloride ion ligands, used in cancer treatment because it forms a bond between the two strands of DNA, so that it cannot separate and cells cannot divide.
Trans-platin
More toxic & less effective in cancer treatment.
Monodentate ligand
One that forms a dative bond with a metal ion.
Bidentate ligand
One that forms 2 dative bonds with a metal ion.
Multidentate ligand
One that forms several dative bonds with a metal ligand.
NH2CH2CH2NH2
Bidentate ligand ethylenediamine/ 1,2-diaminoethane/ en.
It uses the lone pairs on each of the nitrogen atoms to attach to the metal ion.
EthyleneDiamineTetraAcetic Acid
EDTA4-, a multidentate ligand. It is the same as en, except each of the hydrogens attached to the nitrogen atoms is replaced by -CH2COOH. Then each of the H+ ions in the COOH groups is lost. This means EDTA4- can form 6 dative bonds as it has 6 lone pairs.
Ligand exchange reaction: monodentate ligands are replaced by a bidentate or multidentate ligand.
The total number of species increases meaning the systems is more disordered, so the entropy of the system increases. This increases the stability of the products instead of the reactants, so product formation is favoured.
The largest part of haemoglobin is protein. Within which, there are 4 haem groups. What is inside each haem group?
There are 4 nitrogen atoms that hold an Fe2+ ion by forming dative bonds with it in a square planar structure. There is a fifth dative bond from the protein to the Fe2+ ion.
What allows carbon monoxide to act as a ligand?
It has a lone pair of electrons on its carbon atom.
What happens when carbon monoxide is breathed in?
Carbon monoxide forms a stronger dative bond with haemoglobin, so the carbon monoxide replaces the oxygen already bound to the haemoglobin in a ligand-substitution reaction.
Haemoglobin + oxygen <—> oxyhaemoglobin
Haemoglobin + carbon monoxide —> carboxyhaemoglobin.
4 main types of transition metal reaction
Redox: the oxidation number of the transition metal ion changes.
Acid-base reaction: at least one of the ligands gains or loses a hydrogen ion.
Ligand exchange:one or more of the ligands around the transition metal ion is replaced by a different ligand.
Coordination number change: the number of ligands changes.
What colour is a solution containing Fe2+ (aq) ions?
Pale green.
What happens when a solution containing Fe2+ ions is exposed to air?
Pale green —> yellow/brown
The oxidation number increases from +2 to +3.
Amphoteric behaviour
The ability of a species to react with both acids & bases.
Ligand-exchange reaction:
[Cu(H2O)4(OH)2] + 4NH3 —> [Cu(NH3)4(H2O)2]2+ + 2H2O + 2OH-
Aqueous ammonia is added to the dark blue precipitate to form a deep blue solution. 4 ammonia molecules replace 2 water molecules & 2 hydroxide ions.
Acid-base reaction: when aqueous sodium hydroxide is added to copper(II) sulfate solution.
A pale blue solution forms a blue precipitate.
[Cu(H2O)6]2+ + 2OH- —> [Cu(H2O)4(OH)2] + 2H2O
2 hydroxide ions have removed hydrogen ions from two of the water ligands & converted them into water molecules. The 2 molecules that have lost hydrogen ions are now hydroxide ligands.
This is reversible, as seen when an acid is added. The same is observed, if aqueous ammonia instead of aqueous NaOH is added.
Change in coordination number: copper (II) sulfate solution & concentrated hydrochloric acid.
When the acid is added slowly & continuously, the colour changes gradually from blue to green to yellow.
[Cu(H2O)6]2+ + 4Cl- <—> [CuCl4]2- + 6H2O
All 6 water ligands have been substituted by 4 chloride ions. The green colour is because the reaction is reversible, so there is a mixture at this point.
NaOH + hexaaqua cobalt (II)
Pink solution forms a blue precipitate. The precipitate gradually changes to pink upon standing. Acid-base reaction: [Co(H2O)6]2+ + 2OH- —> [Co(H2O)4(OH)2] + 2H2O
Two hydroxide ions have removed two of the H+ ions from the water ligands to make water molecules. The same occurs for NH3 instead of NaOH, except 2NH4+ forms instead of 2H2O.
What happens when EXCESS aqueous NH3 is added to hexaaquacobalt (II)?
The precipitate dissolves to form a brown solution by a ligand-exchange reaction.
[Co(H2O)4(OH)2] + 6NH3 —> [Co(NH3)6]2+ 4H2O + 2OH-
What is observed when the brown solution formed by hexaaquacobalt (II) & EXCESS NH3(aq) is left standing, and why?
Due to atmospheric oxygen, the oxidation number of cobalt increases from +2 to +3, and the yellow [Co(NH3)6]3+ ion forms.
When does atmospheric oxidation occur?
With complexes containing a transition metal ion with a +2 oxidation number, but not for those with a +3 oxidation number.
What happens when concentrated hydrochloric acid is slowly added to a solution containing the hexaaquacobalt (II) ion?
The pink solution gradually changes to blue. [Co(H2O)6]2+ + 4Cl- —> [CoCl4]2- + 6H2O.
All 6 water ligands are replaced by 4 chloride ions.
What is observed when NaOH(aq) is added to a solution containing the hexaaquairon (II) ion, and why?
[Fe(H2O)6]2+ + 2OH- —> [Fe(H2O)4(OH)2] + 2H2O
Acid-base reaction. A pale green solution forms a green precipitate.
The same is observed when NH3 (aq) is added, but 2NH4 + is formed instead of 2H2O.
What happens when [Fe(H2O)4(OH)2] is left to stand?
The green precipitate gradually changes to brown due to atmospheric oxidation, forming [Fe(H2O)3(OH)3]. This is the triaquatrihydroxoiron (III) complex.
What is observed when NaOH (aq) or NH3 (aq) is added to a solution containing the hexaaquairon (III) ion?
A yellow-brown solution forms a brown precipitate. Acid-base reaction. No further changes upon standing. No further changes when excess of either reactant is added.
[Fe(H2O)6]3+ + 3OH- —> [Fe(H2O)3(OH)3] + 3H2O
3NH4+ is produced instead of 3H2O, when NH3 is used.
What complicates the colour of chromium compounds?
The presence of a dissolved gas in an aqueous solution affects the colour. The colour depends on concentration.
What happens when chromium (III) complexes react with alkalis?
Acid-base reaction. A green or violet solution forms a green precipitate.
[Cr(H2O)6]3+ + 3OH- —> [Cr(H2O)3(OH)3] + 3H2O
If the alkali is NH3, 3NH3 is a reactant instead of OH- & 3NH4 + is formed instead of H2O.
What happens if an excess of alkali is added to a chromium intermediate?
The green precipitate dissolves to form a green solution. Acid-base reaction. [Cr(H2O)3(OH)3] + OH- –> [Cr(H2O)2(OH)4]- + H2O
If the alkali is more concentrated…
[Cr(H2O)2(OH)4] + 2OH- –> [Cr(OH)6]4- + 2H2O
How can the amphoteric nature of a neutral chromium complex be illustrated?
Adding acid reverses the reactions involving OH- ions.
What happens when an excess of aqueous NH3 is added to the green precipitate [Cr(H2O)3(OH)3]?
[Cr(H2O)3(OH)3] + 6NH3 –> [Cr(NH3)6]3+ + 3H2O + 3OH-
The precipitate dissolves slowly. A violet or purple solution forms.
How is an alkaline solution of [Cr(OH)6]3- oxidised?
2 [Cr(OH)6]3- + 3H2O2 –> 2CrO4 2- + 2OH- + 8H2O.
Oxidizing agent: hydrogen peroxide, H2O2.
Green to yellow.
Chromate (VI) ion/complex
CrO4^2-
What happens when acid is added to chromate(VI) ions?
2CrO4^2- + 2H+ <–> Cr2O7^2- + H2O
Yellow to orange. In alkaline solution: chromate (VI) is more stable. In acidic: dichromate(VI) is more stable. Add alkali to reverse.
What happens when Zn metal is added to an acidic solution containing dichromate (VI) ions?
Reduction.
Orange +6 –> +3 green.
Cr2O7^2- + 14H+ +3Zn –> 2Cr^3+ + 7H2O + 3Zn^2+
Green +3 –> blue +2.
2Cr^3+ + Zn –> 2Cr^2+ + Zn^2+
Summary: add NaOH (aq) to [Cr(H2O)6]3+ green solution
Intermediate: [Cr(H2O)3(OH)3] green precipitate.
Summary: add NaOH (aq) to the intermediate: [Cr(H2O)3(OH)3] green precipitate.
Final: [Cr(OH)6]3- green solution.
Summary: add NH3 (aq) to [Cr(H2O)6]3+ green solution
Intermediate: [Cr(H2O)3(OH)3] green precipitate.
Summary: add NH3 (aq) to intermediate: [Cr(H2O)3(OH)3] green precipitate.
Final: [Cr(NH3)6]3+ violet solution.
Summary: add H2O2/OH- to [Cr(OH)6]3- green solution.
Intermediate: CrO4^2-. Yellow solution.
Summary: add H+ to intermediate CrO4^2-. Yellow solution.
Cr2O7^2- orange solution.
Summary: add Zn/H+ to a yellow solution of CrO4^2-.
Intermediate: [Cr(H2O)6]3+ green solution.
Summary: add Zn/H+ to intermediate: [Cr(H2O)6]3+ green solution.
[Cr(H2O)6]2+ blue solution.
Vanadium
Transition metal with one distinct colour for each of its oxidation numbers, so it’s used in redox reactions.
Vanadium: oxidation number +2
V^2+
Vanadium (II)
Purple (aq)
Vanadium: oxidation number +3
V^3+
Vanadium(III)
Green (aq)
Vanadium: oxidation number +4
VO^2+
Oxovanadium (IV)
Blue (aq)
Vanadium: oxidation number + 5
VO2^+
Dioxovanadium(V)
Yellow (aq)
Source of dioxovanadium(V)
Ammonium vanadate(V) NH4VO3 in acidic solution as it contains the VO2^+ ion.
Reduction of vanadium +5 to +2
Add zinc. There is a gradual colour change from yellow to blue to green to purple.
Heterogeneous catalyst
A catalyst in a different phase to the reactants.
Adsorption
The process that occurs when reactants form weak bonds with the surface of the catalyst.
Desorption
The process that occurs when products leave the surface of the solid catalyst.
What does the fact that catalysis occurs on the surface of the heterogeneous catalyst mean for particle size?
Catalysts are used in finely divided form: small particles not large lumps. Sometimes as a thin coating on a inert support material.
Surface adsorption theory
- Adsorption: one or more reactants attaches to the surface of the catalyst.
- Reaction, following the weakening of bonds in the adsorbed reactants.
- Desorption: the reaction product becomes detached from the surface of the catalyst.
Key reaction of the contact process
2SO2 + O2 <–> 2SO3 all are in the gas phase.
Heterogeneous catalysis: the contact process
- Sulfur dioxide adsorbs onto the vanadium(V) oxide and a redox reaction occurs. the oxidation number of vanadium decreases from +5 to +4. The sulfur trioxide then desorbs.
V2O5 + SO2 –> V2O4 + SO3 - Oxygen reacts with the V2O4 on the surface of the catalyst and another redox reaction occurs.
V2O4 + 1/2O2 –> V2O5
The catalyst is regenerated as oxidation number increases from +4 to +5.
Nitrogen monoxide
Oxidised in the atmosphere to NO2, where it can form acid rain and act as a respiratory irritant.
How does CO form?
Incomplete combustion of hydrocarbon fuels
How does NO form?
Nitrogen + oxygen react at high temperatures in the internal combustion engine.
Transition metals used by catalytic converters
Platinum, rhodium & sometimes palladium.
Summarise catalysis in catalytic converters.
Molecules of CO & NO adsorb onto the metal surface. Their bonds are weakened, so they react to form CO2 + N2 which desorb from the surface of the catalyst.
2CO + 2NO –> 2CO2 + N2
Homogeneous catalyst
A catalyst in the same phase as the reactants.
Autocatalysis
Occurs when a reaction product acts as a catalyst for the reaction.
Key feature of homogeneous catalysis
The formation of an intermediate species for which a specific formula can be written.
Persulfate/Peroxodisulfate ion
S2O8^2-
Why is the reaction S2O8^2- + 2I- –> 2SO4^2- + I2, in which the persulfate ion acts as an oxidizing agent, so slow at room temperature?
The 2 reactant anions repel each other. It can be catalysed by Fe2+ or Fe3+. Everything is in the aqueous phase.
Steps in the homogeneous catalysis of S2O8^2- + 2I- –> 2SO4^2- + I2 by Fe2+
- The Fe2+ ions are positively-charged, so are not repelled by the persulfate ion. They react as follows:
S2O8^2- + 2Fe2+ –> 2SO4^2- + 2Fe3+ - The Fe3+ ions now react with the I- ions:
2Fe3+ + 2I- –> 2Fe2+ + I2
Steps in the homogeneous catalysis of S2O8^2- + 2I- –> 2SO4^2- + I2 by Fe3+.
2Fe3+ + 2I- –> 2Fe2+ + I2
S2O8^2- + 2Fe2+ –> 2SO4^2- + 2Fe3+
The same two reactions as when catalysed by Fe2+, but with their order swapped.
Autocatalysis in the redox titration reaction 2MnO4^- + 5C2O4^2- + 16H^+ –> 2Mn^2+ + 5CO2 + 8H2O
Potassium manganate(VII) acts as an oxidizing agent. Both the reacting species (ethanedioate & potassium manganate(VII)) are negatively-charged, so the reaction is slow initially, but the Mn^2+ ion catalyses the reaction, hence the rate speeds up as the titration continues!
How could impurities in the reactants affect the effectivity of the heterogeneous catalyst?
Impurities adsorb onto the surface/occupy active sites.
Impurities prevent bond weakening in reactants, and there are fewer active sites. Less surface area available for reaction.
Impurities form strong bonds to the surface, so are less likely to desorb.
Ionic equation for the oxidation of [Cr(OH)6]^3-to CrO4^2- by hydrogen peroxide in alkaline conditions:
[Cr(OH)6]^3- + 2OH- –> CrO4^2- + 4H2O + 3e-
How would you prepare dry crystals of a transition metal solid involving water of crystallisation?
- Heat the mixture gently in evaporating basin then leave to crystallise.
- Gravity filtration to separate the crystals.
- Rinse in a small volume of ice-cold water to remove excess reagent.
- Dry in a warm oven for 30 minutes. Warm not hot so as not to remove the water of crystallisation as this would decrease the yield.
What allows iron to act as a catalyst?
Its variable oxidation states.
EDTA^4-
Hexadentate.
Cl- ligand
Larger than the O in the water ligand or the N in the NH3 ligand.
Why can’t NH4^+ ions act as ligands?
They don’t have a lone pair available for bonding.
Why can most transition metals show variable oxidation numbers?
There is only a gradual increase in successive ionisation energies.
Why do solutions containing Cr2+ & Cr3+ ions differ in colour?
Different splitting of d-orbitals, so electrons undergo different d-d transitions/require different amounts of energy to be promoted to a higher d-orbital.
How do impurities affect catalysts?
Impurities adsorb onto the catalyst’s surface, so there’s less surface area of the catalyst available for reaction. Impurities form strong bonds to the surface, so are less likely to desorb.
