Principles of Transition Metal Chemistry (Edexcel A Level Chemistry): Flashcards

The overall charge on a complex ion is determined by summing the oxidation state of the central metal ion and the charges of all the ligands.

If ligands are neutral, the overall charge equals the oxidation state of the metal.

A ligand is a molecule or ion that forms a coordinate (dative covalent) bond with a transition metal by donating a lone pair of electrons to the bond.

When the overall ion is an anion, the metal name is modified with the suffix -ate and sometimes a Latin stem is used.

For example, copper becomes cuprate and nickel becomes nickelate.

In a complex ion, the ligands donate a lone pair of electrons to the central metal ion, forming a coordinate (dative covalent) bond.

The coordination number of the iron ion in [Fe(CN)₆]^3- is 6, because there are six cyanide ligands each forming one coordinate bond with the central Fe³⁺ ion.

The coordination number is the total number of coordinate bonds formed between ligands and the central metal atom or ion in a complex.

False.

Ligands can be neutral molecules or negatively charged ions, as long as they possess a lone pair of electrons available for donation. For example, H₂O and NH₃ are neutral ligands.

The ligand ammonia is named ammine when part of a complex, spelt with two m's, and the prefix for six ligands is hexa.

Transition metal compounds appear coloured because they absorb certain frequencies of visible light, and the colour observed is the complementary colour to the light absorbed.

D-orbital splitting is the separation of the five d-orbitals into two groups of different energy when ligands bond to a central transition metal ion, enabling electrons to absorb visible light and move to the higher energy level.

False.

Zn²⁺ has a completely filled 3d¹⁰ subshell. No d-orbital splitting and electron promotion is possible, so Zn²⁺ compounds are colourless.

A compound appears green because it absorbs all parts of the visible spectrum except green light, which is transmitted.

The four factors that influence ΔE are:

• The size and type of ligands

• The nuclear charge and identity of the metal ion

• The oxidation state of the metal

• The shape of the complex

The two complexes have different colours because NH₃ has a higher charge density than H₂O, causing greater d-orbital splitting.

[Cu(H₂O)₆]²⁺ is light blue; [Cu(NH₃)₄(H₂O)₂]²⁺ is dark blue.

True.

A higher oxidation state creates a stronger interaction between the metal ion and its ligands, increasing the splitting energy ΔE. For example, [Fe(H₂O)₆]³⁺ absorbs in the blue region (appearing orange), while [Fe(H₂O)₆]²⁺ absorbs in the red region (appearing green).

The splitting energy ΔE is directly proportional to the frequency of the light absorbed and inversely proportional to its wavelength.

A larger ΔE means higher-frequency (shorter-wavelength) light is absorbed.

Complementary colours are colours that are directly opposite each other on the colour wheel.

A monodentate ligand is one that forms only one dative bond to the central metal ion, because it has a single lone pair available for donation.

Four examples of monodentate ligands are:

• Water (H₂O)

• Ammonia (NH₃)

• Chloride ions (Cl^-)

• Cyanide ions (CN^-)

A bidentate ligand is one that forms two dative bonds to the central metal ion, because it contains two atoms each with a lone pair available for donation.

The two common bidentate ligands are:

• 1,2-diaminoethane (H₂NCH₂CH₂NH₂), also written as 'en'

• Ethanedioate ion (C₂O₄^2-), also written as 'ox'

A monodentate ligand forms one dative bond; a bidentate ligand forms two; EDTA^4- is hexadentate and forms six dative bonds.

False.

Monodentate ligands can be neutral molecules or anions. H₂O and NH₃ are neutral monodentate ligands; Cl^- and CN^- are anionic monodentate ligands. The requirement is a lone pair, not a charge.

A multidentate ligand is one that forms more than two dative bonds to the central metal ion, because it contains more than two atoms with lone pairs available for donation.

True.

1,2-diaminoethane (en) contains two nitrogen atoms, each with a lone pair. It is therefore a bidentate ligand that chelates the metal ion by forming two coordinate bonds.

EDTA^4- is classified as a hexadentate ligand because it contains six atoms with lone pairs, allowing it to form six dative bonds to a metal ion.

An octahedral complex is a transition metal complex in which the central metal ion forms six coordinate (dative covalent) bonds with surrounding ligands, giving bond angles of 90°.

In an octahedral complex, six coordinate bonds may be formed with six monodentate ligands — for example, water or ammonia molecules — or with three bidentate ligands, each donating two bonds.

Bidentate ligands are ligands that each form two dative covalent bonds to the central metal ion. Three bidentate ligands give six coordinate bonds in total, producing an octahedral complex. Examples include 1,2-diaminoethane and the ethanedioate ion.

An octahedral complex can be formed with six monodentate ligands (such as H₂O, NH₃, OH^-, or SCN^-), three bidentate ligands (such as 1,2-diaminoethane or ethanedioate), or one multidentate ligand such as EDTA^4-, which forms all six coordinate bonds.

The coordination number is the total number of dative covalent bonds formed between the central metal ion and its ligands. In an octahedral complex, the coordination number is 6.

They are described as having six-fold coordination because the central metal ion forms six separate coordinate bonds, one to each of the six monodentate ligands surrounding it.

True.

All six ligands are arranged symmetrically around the central metal ion in three dimensions, giving bond angles of exactly 90° throughout the complex.

EDTA^4- is a multidentate ligand that forms all six coordinate bonds to the central metal ion by itself, satisfying the six-fold coordination requirement of an octahedral complex in a single ligand.

False.

When applying electron pair repulsion theory to transition metal complexes, the 3d electrons on the metal ion and the overall charge on the complex are ignored. Only the electron pairs donated by the ligands are counted.

A tetrahedral complex is a transition metal complex in which the central metal ion forms four coordinate bonds with surrounding ligands arranged at bond angles of 109.5°.

Chloride ligands are large, so only four can fit around the central metal ion without excessive steric repulsion. This gives a tetrahedral arrangement with bond angles of 109.5°.

False.

Tetrahedral complexes have bond angles of 109.5°, not 90°. Bond angles of 90° are characteristic of octahedral and square planar complexes.

The coordination number of a tetrahedral complex is 4, since the central metal ion forms four dative covalent bonds with the surrounding ligands.

Tetrahedral complexes most commonly form with four chloride ligands because these ligands are too large for six to fit around the central metal ion.

A tetrahedral complex has a coordination number of 4 and bond angles of 109.5°. An octahedral complex has a coordination number of 6 and bond angles of 90°.

False.

Complexes with four coordinate bonds can also adopt a square planar geometry. Tetrahedral geometry is most common when the ligands are large (such as Cl^-), while square planar geometry occurs with certain ligands such as CN^-.

The tetrachlorocuprate(II) ion, [CuCl₄]^2-, is an example of a tetrahedral complex. The four large Cl^- ligands adopt a tetrahedral arrangement around the Cu²⁺ central ion.

Cisplatin is a square planar platinum(II) complex, [PtCl₂(NH₃)₂], used as an anticancer drug. It works by binding to nitrogen atoms on DNA bases, distorting the DNA double helix and preventing cancer cells from replicating.

Nitrogen atoms on DNA bases are better ligands than water molecules. Nitrogen forms stronger dative covalent bonds to the platinum centre, so cisplatin displaces the water ligands and binds preferentially to the DNA bases.

True.

Cisplatin, [PtCl₂(NH₃)₂], is a square planar complex. All bond angles between adjacent ligands in a square planar complex are 90°.

Cancer cells replicate much faster than normal cells. Because cisplatin works by preventing DNA replication, it has a greater effect on rapidly dividing cells. However, it does also affect healthy cells that replicate quickly, such as hair follicles.

A cis-isomer is a geometric isomer in which two identical groups are on the same side of a molecule. In cisplatin [PtCl₂(NH₃)₂], the two Cl^- ligands and the two NH₃ ligands are each on the same side of the square planar complex.

Cisplatin works by entering the cell and undergoing ligand exchange, replacing Cl^- with water. It then binds to nitrogen atoms on DNA bases, distorting the helix and preventing DNA from replicating.

Cisplatin binds to DNA in all rapidly dividing cells, not only cancer cells. Hair follicle cells divide quickly, so they are also affected by cisplatin, causing hair loss during treatment.

False.

Only the cis-isomer (cisplatin) is effective as an anticancer drug. Trans-platin, where the Cl^- ligands are on opposite sides of the square planar complex, cannot bind to two adjacent nitrogen atoms on the same DNA strand in the same way.

Cisplatin has been used to treat a range of cancers including testicular, ovarian, cervical, breast, lung and brain cancer. It was discovered in the 1960s and remains an important chemotherapy agent.

Haemoglobin is a protein complex that transports oxygen in the blood. It contains four haem groups, each with an Fe²⁺ ion at its centre, which forms dative covalent bonds with oxygen molecules to carry them around the body.

Oxygen is a poor ligand and forms only weak dative covalent bonds to the Fe²⁺ ion. This is important because the weak bonds break easily, allowing oxygen to be released into cells where it is needed.

True.

CO forms stronger dative covalent bonds to Fe²⁺ than O₂ does. This binding is essentially irreversible, preventing oxygen from being transported to cells and making CO highly toxic.

In haemoglobin, each haem group contains an Fe²⁺ ion at the centre of a square planar complex. Oxygen molecules bond weakly to this ion, allowing them to be transported and released into cells.

CO undergoes ligand exchange, binding strongly and irreversibly to the Fe²⁺ ion in haemoglobin in place of oxygen. This forms carboxyhaemoglobin, preventing the transport of oxygen to cells.

Carboxyhaemoglobin is the complex formed when carbon monoxide replaces oxygen as the ligand bound to the Fe²⁺ ion in haemoglobin. It is darker red than oxyhaemoglobin and is a sign of carbon monoxide poisoning.

Each Fe²⁺ ion forms four dative covalent bonds to nitrogen atoms from the haem group in a square planar arrangement, a fifth dative bond to the protein (globin), and a sixth bond to an oxygen molecule when carrying oxygen.

True.

Anaemia occurs when a person does not have enough haemoglobin in their blood. A deficiency in iron means insufficient Fe²⁺ is available to form the haem complexes needed for oxygen transport. This can be treated by taking iron sulfate tablets.

Carbon monoxide poisoning produces a darker red colour in the blood. This is because carboxyhaemoglobin (CO bound to Fe²⁺) is darker red than oxyhaemoglobin (O₂ bound to Fe²⁺).

A heterogeneous catalyst is in a different physical state (phase) from the reactants.

The reaction occurs at active sites on the surface of the catalyst.

Transition metals can form more than one stable oxidation state, allowing them to accept and lose electrons easily.

This means they can be oxidised and then reduced (or vice versa) during a catalytic cycle, facilitating redox reactions.

False.

Iron is a solid catalyst while the reactants N₂ (g) and H₂ (g) are gases, so iron acts as a heterogeneous catalyst.

A heterogeneous catalyst is in a different physical state from the reactants, while a homogeneous catalyst is in the same physical state as the reactants.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

The catalyst is iron (Fe), acting as a heterogeneous catalyst because it is a solid while the reactants are gases.

True.

The ability to adopt multiple stable oxidation states means transition metals can be readily oxidised and reduced within a catalytic cycle, making them effective redox catalysts.

A homogeneous catalyst is in the same physical state (phase) as the reactants.

Reactants and catalyst are typically all in aqueous solution or all in the gas phase.

The two types are heterogeneous and homogeneous catalysts.

A heterogeneous catalyst is in a different physical state from the reactants; a homogeneous catalyst is in the same physical state as the reactants.

A heterogeneous catalyst is in a different physical state (phase) from the reactants.

The reaction occurs at active sites on the surface of the solid catalyst.

A catalytic converter is a device in a car exhaust system containing finely divided platinum and rhodium on a ceramic support.

It converts toxic CO and NO into less harmful N₂ and CO₂ via surface adsorption: 2NO (g) + 2CO (g) → N₂ (g) + 2CO₂ (g).

False.

Adsorption occurs only at the surface of a substance; absorption involves a substance becoming distributed throughout another (like water in a sponge).

An active site is the location on the surface of a heterogeneous catalyst where adsorption of reactant molecules occurs.

Reactants bind here, bonds are weakened, and reaction takes place before products are desorbed.

In the Contact Process, V₂O₅ is reduced to V₂O₄ when it converts SO₂ to SO₃, then re-oxidised back to V₂O₅ by reaction with oxygen.

This demonstrates the variable oxidation state of vanadium (+5 → +4 → +5).

The Contact Process is the industrial manufacture of sulfuric acid.

The key step converts SO₂ (g) to SO₃ (g) using vanadium(V) oxide (V₂O₅) as the heterogeneous catalyst.

Adsorption is the process by which reactant molecules become attached to the surface of a heterogeneous catalyst at its active sites.

This weakens bonds in the adsorbed reactants, making them more reactive.

If adsorption is too strong (e.g. tungsten, W), products cannot desorb from the surface.

If adsorption is too weak (e.g. silver, Ag), reactants do not bind in sufficient concentration to react. Metals such as Ni and Pt have an intermediate strength that is most effective.

True.

A larger surface area provides more active sites for reactant molecules to adsorb onto, increasing the rate of reaction. This is why catalysts are often finely divided or coated onto a porous support.

A homogeneous catalyst is in the same physical state (phase) as the reactants.

A key feature is the formation of an intermediate species for which a specific formula can be written.

Fe²⁺ ions act as a homogeneous catalyst via a two-step redox cycle:

1. Fe²⁺ reduces S₂O₈^2- to SO₄^2-, forming Fe³⁺

2. Fe³⁺ oxidises I^- to I₂, regenerating Fe²⁺

Fe²⁺ is oxidised to Fe³⁺ in the first step of the catalytic cycle, then reduced back to Fe²⁺ in the second step, fulfilling its role as a catalyst.

Both S₂O₈^2- and I^- are negatively charged, so electrostatic repulsion between the ions reduces the frequency of successful collisions.

Adding Fe²⁺ ions provides an alternative pathway that avoids direct collision between the two negative ions.

True.

A key feature of homogeneous catalysis is the formation of an identifiable intermediate species, unlike heterogeneous catalysis where the reaction occurs at surface active sites.

Fe²⁺ catalyses: S₂O₈^2- (aq) + 2I^- (aq) → I₂ (aq) + 2SO₄^2- (aq)

It is homogeneous catalysis because the catalyst (Fe²⁺) and all reactants and products are in the same phase — aqueous solution.

True.

A homogeneous catalyst provides a reaction pathway of lower activation energy by forming an intermediate species. This is shown on an energy profile as a two-step pathway with lower energy barriers than the uncatalysed route.

Adding starch to the reaction mixture produces a blue-black colour when iodine (I₂) is formed.

This acts as a visual indicator of the product and can be used to monitor the reaction rate.

Autocatalysis describes a reaction that is speeded up by one of its own products, which acts as a catalyst for the reaction.

The rate increases over time as more catalyst is produced.

The gradient becomes steeper as the reaction proceeds, meaning the rate increases over time.

This is the opposite of a normal reaction where the rate decreases as reactants are consumed, and occurs because more autocatalyst is produced as the reaction progresses.

In the reaction between manganate(VII) ions and oxalate ions, the autocatalyst is Mn²⁺ ions, which are formed as a product of the reaction.

Mn²⁺ ions undergo a redox cycle between two oxidation states:

1. Mn²⁺ is oxidised to Mn³⁺ by MnO₄^-

2. Mn³⁺ is reduced back to Mn²⁺ by C₂O₄^2-

As more Mn²⁺ product accumulates, the rate of reaction increases.

False.

Mn²⁺ ions are a product of the reaction, so they are not present at the beginning. The rate is initially slow and then increases as Mn²⁺ accumulates.

The reaction can be followed using a colorimeter, which measures how the absorption of light changes as the purple MnO₄^- ions are consumed.

The accelerating rate is shown by an increasingly rapid change in colour.

True.

Mn²⁺ (aq) is oxidised to Mn³⁺ (aq) by MnO₄^-, then Mn³⁺ is reduced back to Mn²⁺ by C₂O₄^2-. This redox cycle allows Mn²⁺ to be regenerated and act as an autocatalyst.

Mn²⁺ ions appear as a product in the overall equation for the reaction, and the rate increases as more Mn²⁺ accumulates.

Adding Mn²⁺ ions at the start speeds up the reaction immediately, confirming Mn²⁺ is the autocatalyst.