A complex is a molecule or ion formed by a central metal atom or ion surrounded by one or more ligands that donate lone pairs to form dative covalent bonds.

What shape and coordination number does [Cu(H 2 O) 6 ] 2+ have?

[Cu(H 2 O) 6 ] 2+ is octahedral with a coordination number of 6 . Six water ligands each donate a lone pair to form six dative covalent bonds with the central Cu 2+ ion.

True or False? Chloride ion ligands form octahedral complexes with Cu 2+ ions.

False. Chloride ions are large ligands, so only four fit around the central metal ion, forming a tetrahedral complex [CuCl 4 ] 2- with a coordination number of 4.

The overall charge of [CuCl 4 ] 2- is calculated as ( .......... ) + (4 × .......... ) = .......... .

The overall charge of [CuCl 4 ] 2- is calculated as ( +2 ) + (4 × −1 ) = −2 .

What is the overall charge of [Co(H 2 O) 6 ] 2+ and how is it determined?

The overall charge is 2+ . It is calculated as the charge on Co 2+ (+2) plus the charge from six neutral water ligands (6 × 0), giving a total of +2.

True or False? The coordination number of a complex is equal to the number of ligands present.

False. The coordination number is the number of dative covalent bonds formed, not the number of ligands. A bidentate ligand contributes 2 to the coordination number despite being one ligand.

Define coordination number .

A coordination number is the total number of dative covalent bonds formed between the ligands and the central metal atom or ion.

What is the formula and charge of the cobalt(II) complex formed with four chloride ligands?

The complex is [CoCl 4 ] 2- . The overall charge is (2+) + (4 × −1) = −2 .

Water and ammonia are .......... ligands that form .......... complexes with Co 2+ and Cu 2+ .

Water and ammonia are neutral ligands that form octahedral complexes with Co 2+ and Cu 2+ .

A ligand is a molecule or ion that has one or more lone pairs of electrons, which it donates to a central metal atom or ion to form dative covalent bonds.

What is a monodentate ligand? Give two examples.

A monodentate ligand forms only one dative bond to the central metal ion. Examples include water (H 2 O) and ammonia (NH 3 ).

True or False? 1,2-diaminoethane (en) is a bidentate ligand.

True. 1,2-diaminoethane has two nitrogen atoms, each with a lone pair, allowing it to form two dative bonds to the central metal ion.

EDTA 4- is a .......... ligand because it forms .......... dative bonds to the central metal ion.

EDTA 4- is a hexadentate ligand because it forms six dative bonds to the central metal ion.

Define bidentate ligand .

A bidentate ligand is a ligand that contains two atoms with lone pairs of electrons, allowing it to form two dative covalent bonds to the central metal ion.

Through which atoms does the ethanedioate ion (ox) form dative bonds?

The ethanedioate ion (C 2 O 4 2- ) forms dative bonds through the lone pairs on two oxygen atoms , one from each carboxylate group.

True or False? Chloride ions are monodentate ligands.

True. Chloride ions (Cl - ) have lone pairs but can only donate one lone pair to form a single dative bond with the central metal ion.

In the complex [Fe(H 2 O) 6 ] 2+ , the water ligands each donate a .......... from the .......... atom to form a dative bond.

In the complex [Fe(H 2 O) 6 ] 2+ , the water ligands each donate a lone pair from the oxygen atom to form a dative bond.

Why do larger monodentate ligands tend to form tetrahedral rather than octahedral complexes?

Larger ligands, such as Cl - , cause more steric repulsion. Only four of them can fit around the central metal ion, resulting in a tetrahedral geometry rather than six ligands in an octahedral arrangement.

Define coordinate bond .

A coordinate bond (also called a dative covalent bond) is a bond formed when one species donates both electrons in the shared pair, as occurs when a ligand bonds to a central metal ion in a complex.

What bond angle is found in an octahedral complex?

Octahedral complexes have bond angles of 90° .

True or False? A tetrahedral complex has a bond angle of 90 o .

False. A tetrahedral complex has a bond angle of 109.5 o . A bond angle of 90 o is found in both octahedral and square planar complexes.

A linear complex has .......... coordinate bonds and a bond angle of .......... o .

A linear complex has two coordinate bonds and a bond angle of 180 o .

What geometry does cisplatin adopt and what is its bond angle?

Cisplatin adopts a square planar geometry with bond angles of 90° .

True or False? Four coordinate bonds always give a tetrahedral complex.

False. Four coordinate bonds can give either a tetrahedral (bond angle 109.5 o ) or a square planar (bond angle 90 o ) complex, depending on the ligands involved.

An octahedral complex can form with six .......... ligands or three .......... ligands.

An octahedral complex can form with six monodentate ligands or three bidentate ligands.

Define square planar complex .

A square planar complex is a transition metal complex with four coordinate bonds arranged in a flat square around the central metal ion, with bond angles of 90 o .

Why do chloride ligands commonly form tetrahedral rather than octahedral complexes?

Chloride ions are large ligands, so steric repulsion limits the number that can fit around the central metal ion to four , resulting in a tetrahedral geometry.

Define ligand exchange .

Ligand exchange (or ligand substitution) is a reaction in which one or more ligands in a complex are replaced by different ligands, forming a more stable complex.

What colour change occurs when excess concentrated ammonia is added to [Cu(H 2 O) 6 ] 2+ ?

The solution changes from blue (the hexaaqua complex) through a pale blue precipitate to a deep blue solution of [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ .

True or False? When water ligands in [Cu(H 2 O) 6 ] 2+ are replaced by Cl - , the coordination number stays the same.

False. The coordination number changes from 6 to 4 because chloride ions are larger than water molecules, so only four chloride ligands fit around the central metal ion.

Adding concentrated HCl to [Cu(H 2 O) 6 ] 2+ produces a .......... solution of .......... as water ligands are replaced by chloride ions.

Adding concentrated HCl to [Cu(H 2 O) 6 ] 2+ produces a yellow solution of [CuCl 4 ] 2- as water ligands are replaced by chloride ions.

What is the product and colour when excess concentrated ammonia is added to [Co(H 2 O) 6 ] 2+ ?

Complete ligand substitution forms [Co(NH 3 ) 6 ] 2+ , which is a brown solution.

True or False? Ligand exchange always changes the coordination number of a complex.

False. If the incoming and outgoing ligands are of similar size , there is no change in coordination number or geometry. Changes occur only when ligand sizes differ significantly.

What precipitate forms when NaOH is added dropwise to [Co(H 2 O) 6 ] 2+ ?

A blue precipitate of [Co(H 2 O) 4 (OH) 2 ] forms as two water ligands are partially substituted by two hydroxide ligands.

When water is added to [CuCl 4 ] 2- , the .......... ligands displace the chloride ions and the .......... solution returns.

When water is added to [CuCl 4 ] 2- , the water ligands displace the chloride ions and the blue solution returns.

Define hexaaqua complex .

A hexaaqua complex is a transition metal complex in which six water molecules act as ligands, each donating a lone pair to the central metal ion to form an octahedral complex such as [Cu(H 2 O) 6 ] 2+ .

Define standard electrode potential .

A standard electrode potential ( E θ ) is the potential of a half-cell measured relative to the standard hydrogen electrode under standard conditions, indicating how readily a species is reduced.

How do you calculate the standard cell potential for a redox reaction?

E θ cell = E θ reduction − E θ oxidation . A positive value indicates the reaction is feasible.

True or False? A more positive standard electrode potential means a species is more readily oxidised.

False. A more positive E θ means the species is more readily reduced (gains electrons more readily). The more negative the E θ , the more readily the species is oxidised.

Half equations are always written as .......... reactions. The species with the more .......... E θ acts as the oxidising agent.

Half equations are always written as reduction reactions. The species with the more positive E θ acts as the oxidising agent.

Given E θ for Fe 3+ /Fe 2+ = +0.77 V and Cu 2+ /Cu + = +0.15 V, calculate E θ cell for the reaction Fe 3+ + Cu + → Fe 2+ + Cu 2+ .

E θ cell = (+0.77) − (+0.15) = +0.62 V . The positive value confirms the reaction is feasible .

True or False? A positive standard cell potential guarantees that a reaction will occur in practice.

False. A positive E θ cell indicates feasibility but does not guarantee the reaction will occur. A large activation energy barrier may prevent the reaction from proceeding.

In the half equation Fe 3+ + e - ⇌ Fe 2+ , the .......... ion is the oxidising agent and iron undergoes .......... .

In the half equation Fe 3+ + e - ⇌ Fe 2+ , the Fe 3+ ion is the oxidising agent and iron undergoes reduction .

Define feasibility of a redox reaction .

Feasibility of a redox reaction is whether it is thermodynamically likely to occur, predicted by comparing standard electrode potentials: a positive E θ cell value indicates a feasible reaction.

In a redox reaction, which half equation is reversed to represent oxidation?

The half equation with the less positive (or more negative) standard electrode potential is reversed, so the species is oxidised rather than reduced.

What is the overall equation for the redox reaction between MnO 4 - and C 2 O 4 2- in acid solution?

5C 2 O 4 2- (aq) + 2MnO 4 - (aq) + 16H + (aq) → 2Mn 2+ (aq) + 8H 2 O (l) + 10CO 2 (g)

Define autocatalyst .

An autocatalyst is a product of a reaction that catalyses the same reaction, causing it to accelerate as more product is formed. In the MnO 4 - /C 2 O 4 2- reaction, Mn 2+ acts as the autocatalyst.

True or False? In the reaction of Fe 2+ with MnO 4 - in acid, the endpoint is when a permanent pink colour appears.

True. The purple MnO 4 - colour disappears as it reacts with Fe 2+ . The endpoint is when all Fe 2+ has reacted and a trace of permanent pink appears, indicating a slight excess of MnO 4 - .

Write the overall equation for the reaction of Fe 2+ with MnO 4 - in acid.

5Fe 2+ (aq) + MnO 4 - (aq) + 8H + (aq) → Mn 2+ (aq) + 4H 2 O (l) + 5Fe 3+ (aq)

In the Cu 2+ /I - redox system, Cu 2+ is .......... to Cu + while I - is .......... to I 2 .

In the Cu 2+ /I - redox system, Cu 2+ is reduced to Cu + while I - is oxidised to I 2 .

True or False? In the Cu 2+ /I - titration, starch is added at the start to indicate the endpoint.

False. Starch is added when most of the iodine colour has faded , not at the start. It turns deep blue/black in the presence of the remaining I 2 and the endpoint is when this colour disappears.

A LevelChemistryCambridge (CIE)Flashcards28. Chemistry of Transition ElementsGeneral Characteristic Chemical Properties of the First Set of Transition Elements, Titanium to Copper

General Characteristic Chemical Properties of the First Set of Transition Elements, Titanium to Copper (Cambridge (CIE) A Level Chemistry): Flashcards

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FrontTransition Metal Complexes
Define complex.

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Define complex.
A complex is a molecule or ion formed by a central metal atom or ion surrounded by one or more ligands that donate lone pairs to form dative covalent bonds.
What shape and coordination number does [Cu(H₂O)₆]²⁺ have?
[Cu(H₂O)₆]²⁺ is octahedral with a coordination number of 6. Six water ligands each donate a lone pair to form six dative covalent bonds with the central Cu²⁺ ion.
True or False?
Chloride ion ligands form octahedral complexes with Cu²⁺ ions.
False. Chloride ions are large ligands, so only four fit around the central metal ion, forming a tetrahedral complex [CuCl₄]^2- with a coordination number of 4.
The overall charge of [CuCl₄]^2- is calculated as (..........) + (4 × .......... ) = .......... .
The overall charge of [CuCl₄]^2- is calculated as (+2) + (4 × −1) = −2.
What is the overall charge of [Co(H₂O)₆]²⁺ and how is it determined?
The overall charge is 2+. It is calculated as the charge on Co²⁺ (+2) plus the charge from six neutral water ligands (6 × 0), giving a total of +2.
True or False?
The coordination number of a complex is equal to the number of ligands present.
False. The coordination number is the number of dative covalent bonds formed, not the number of ligands. A bidentate ligand contributes 2 to the coordination number despite being one ligand.
Define coordination number.
A coordination number is the total number of dative covalent bonds formed between the ligands and the central metal atom or ion.
What is the formula and charge of the cobalt(II) complex formed with four chloride ligands?
The complex is [CoCl₄]^2-. The overall charge is (2+) + (4 × −1) = −2.
Water and ammonia are .......... ligands that form .......... complexes with Co²⁺ and Cu²⁺.
Water and ammonia are neutral ligands that form octahedral complexes with Co²⁺ and Cu²⁺.
Define ligand.
A ligand is a molecule or ion that has one or more lone pairs of electrons, which it donates to a central metal atom or ion to form dative covalent bonds.
What is a monodentate ligand? Give two examples.
A monodentate ligand forms only one dative bond to the central metal ion. Examples include water (H₂O) and ammonia (NH₃).
True or False?
1,2-diaminoethane (en) is a bidentate ligand.
True. 1,2-diaminoethane has two nitrogen atoms, each with a lone pair, allowing it to form two dative bonds to the central metal ion.
EDTA^4- is a .......... ligand because it forms .......... dative bonds to the central metal ion.
EDTA^4- is a hexadentate ligand because it forms six dative bonds to the central metal ion.
Define bidentate ligand.
A bidentate ligand is a ligand that contains two atoms with lone pairs of electrons, allowing it to form two dative covalent bonds to the central metal ion.
Through which atoms does the ethanedioate ion (ox) form dative bonds?
The ethanedioate ion (C₂O₄^2-) forms dative bonds through the lone pairs on two oxygen atoms, one from each carboxylate group.
True or False?
Chloride ions are monodentate ligands.
True. Chloride ions (Cl^-) have lone pairs but can only donate one lone pair to form a single dative bond with the central metal ion.
In the complex [Fe(H₂O)₆]²⁺, the water ligands each donate a .......... from the .......... atom to form a dative bond.
In the complex [Fe(H₂O)₆]²⁺, the water ligands each donate a lone pair from the oxygen atom to form a dative bond.
Why do larger monodentate ligands tend to form tetrahedral rather than octahedral complexes?
Larger ligands, such as Cl^-, cause more steric repulsion. Only four of them can fit around the central metal ion, resulting in a tetrahedral geometry rather than six ligands in an octahedral arrangement.
Define coordinate bond.
A coordinate bond (also called a dative covalent bond) is a bond formed when one species donates both electrons in the shared pair, as occurs when a ligand bonds to a central metal ion in a complex.
What bond angle is found in an octahedral complex?
Octahedral complexes have bond angles of 90°.
True or False?
A tetrahedral complex has a bond angle of 90^o.
False. A tetrahedral complex has a bond angle of 109.5^o. A bond angle of 90^o is found in both octahedral and square planar complexes.
A linear complex has .......... coordinate bonds and a bond angle of .......... ^o.
A linear complex has two coordinate bonds and a bond angle of 180^o.
What geometry does cisplatin adopt and what is its bond angle?
Cisplatin adopts a square planar geometry with bond angles of 90°.
True or False?
Four coordinate bonds always give a tetrahedral complex.
False. Four coordinate bonds can give either a tetrahedral (bond angle 109.5^o) or a square planar (bond angle 90^o) complex, depending on the ligands involved.
An octahedral complex can form with six .......... ligands or three .......... ligands.
An octahedral complex can form with six monodentate ligands or three bidentate ligands.
Define square planar complex.
A square planar complex is a transition metal complex with four coordinate bonds arranged in a flat square around the central metal ion, with bond angles of 90^o.
Why do chloride ligands commonly form tetrahedral rather than octahedral complexes?
Chloride ions are large ligands, so steric repulsion limits the number that can fit around the central metal ion to four, resulting in a tetrahedral geometry.
Define ligand exchange.
Ligand exchange (or ligand substitution) is a reaction in which one or more ligands in a complex are replaced by different ligands, forming a more stable complex.
What colour change occurs when excess concentrated ammonia is added to [Cu(H₂O)₆]²⁺?
The solution changes from blue (the hexaaqua complex) through a pale blue precipitate to a deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺.
True or False?
When water ligands in [Cu(H₂O)₆]²⁺ are replaced by Cl^-, the coordination number stays the same.
False. The coordination number changes from 6 to 4 because chloride ions are larger than water molecules, so only four chloride ligands fit around the central metal ion.
Adding concentrated HCl to [Cu(H₂O)₆]²⁺ produces a .......... solution of .......... as water ligands are replaced by chloride ions.
Adding concentrated HCl to [Cu(H₂O)₆]²⁺ produces a yellow solution of [CuCl₄]^2- as water ligands are replaced by chloride ions.
What is the product and colour when excess concentrated ammonia is added to [Co(H₂O)₆]²⁺?
Complete ligand substitution forms [Co(NH₃)₆]²⁺, which is a brown solution.
True or False?
Ligand exchange always changes the coordination number of a complex.
False. If the incoming and outgoing ligands are of similar size, there is no change in coordination number or geometry. Changes occur only when ligand sizes differ significantly.
What precipitate forms when NaOH is added dropwise to [Co(H₂O)₆]²⁺?
A blue precipitate of [Co(H₂O)₄(OH)₂] forms as two water ligands are partially substituted by two hydroxide ligands.
When water is added to [CuCl₄]^2-, the .......... ligands displace the chloride ions and the .......... solution returns.
When water is added to [CuCl₄]^2-, the water ligands displace the chloride ions and the blue solution returns.
Define hexaaqua complex.
A hexaaqua complex is a transition metal complex in which six water molecules act as ligands, each donating a lone pair to the central metal ion to form an octahedral complex such as [Cu(H₂O)₆]²⁺.
Define standard electrode potential.
A standard electrode potential (E^θ) is the potential of a half-cell measured relative to the standard hydrogen electrode under standard conditions, indicating how readily a species is reduced.
How do you calculate the standard cell potential for a redox reaction?
E^θ_cell = E^θ_reduction − E^θ_oxidation. A positive value indicates the reaction is feasible.
True or False?
A more positive standard electrode potential means a species is more readily oxidised.
False. A more positive E^θ means the species is more readily reduced (gains electrons more readily). The more negative the E^θ, the more readily the species is oxidised.
Half equations are always written as .......... reactions. The species with the more .......... E^θ acts as the oxidising agent.
Half equations are always written as reduction reactions. The species with the more positive E^θ acts as the oxidising agent.
Given E^θ for Fe³⁺/Fe²⁺ = +0.77 V and Cu²⁺/Cu⁺ = +0.15 V, calculate E^θ_cell for the reaction Fe³⁺ + Cu⁺ → Fe²⁺ + Cu²⁺.
E^θ_cell = (+0.77) − (+0.15) = +0.62 V. The positive value confirms the reaction is feasible.
True or False?
A positive standard cell potential guarantees that a reaction will occur in practice.
False. A positive E^θ_cell indicates feasibility but does not guarantee the reaction will occur. A large activation energy barrier may prevent the reaction from proceeding.
In the half equation Fe³⁺ + e^- ⇌ Fe²⁺, the .......... ion is the oxidising agent and iron undergoes .......... .
In the half equation Fe³⁺ + e^- ⇌ Fe²⁺, the Fe³⁺ ion is the oxidising agent and iron undergoes reduction.
Define feasibility of a redox reaction.
Feasibility of a redox reaction is whether it is thermodynamically likely to occur, predicted by comparing standard electrode potentials: a positive E^θ_cell value indicates a feasible reaction.
In a redox reaction, which half equation is reversed to represent oxidation?
The half equation with the less positive (or more negative) standard electrode potential is reversed, so the species is oxidised rather than reduced.
What is the overall equation for the redox reaction between MnO₄^- and C₂O₄^2- in acid solution?
5C₂O₄^2- (aq) + 2MnO₄^- (aq) + 16H⁺ (aq) → 2Mn²⁺ (aq) + 8H₂O (l) + 10CO₂ (g)
Define autocatalyst.
An autocatalyst is a product of a reaction that catalyses the same reaction, causing it to accelerate as more product is formed. In the MnO₄^-/C₂O₄^2- reaction, Mn²⁺ acts as the autocatalyst.
True or False?
In the reaction of Fe²⁺ with MnO₄^- in acid, the endpoint is when a permanent pink colour appears.
True. The purple MnO₄^- colour disappears as it reacts with Fe²⁺. The endpoint is when all Fe²⁺ has reacted and a trace of permanent pink appears, indicating a slight excess of MnO₄^-.
Write the overall equation for the reaction of Fe²⁺ with MnO₄^- in acid.
5Fe²⁺ (aq) + MnO₄^- (aq) + 8H⁺ (aq) → Mn²⁺ (aq) + 4H₂O (l) + 5Fe³⁺ (aq)
In the Cu²⁺/I^- redox system, Cu²⁺ is .......... to Cu⁺ while I^- is .......... to I₂.
In the Cu²⁺/I^- redox system, Cu²⁺ is reduced to Cu⁺ while I^- is oxidised to I₂.
True or False?
In the Cu²⁺/I^- titration, starch is added at the start to indicate the endpoint.
False. Starch is added when most of the iodine colour has faded, not at the start. It turns deep blue/black in the presence of the remaining I₂ and the endpoint is when this colour disappears.
In the Cu²⁺/I^- system, what is the role of sodium thiosulfate in the second reaction?
Sodium thiosulfate reacts with the iodine produced: I₂ (aq) + 2S₂O₃^2- (aq) → 2I^- (aq) + S₄O₆^2- (aq). The moles of thiosulfate are used to calculate the moles of Cu²⁺.
In the MnO₄^-/C₂O₄^2- reaction, the oxidation number of manganese changes from .......... to .......... , so MnO₄^- is .......... .
In the MnO₄^-/C₂O₄^2- reaction, the oxidation number of manganese changes from +7 to +2, so MnO₄^- is reduced.
Define redox titration.
A redox titration is a titration in which one species is oxidised and another is reduced, used to find the concentration of an unknown species by measuring the volume of a standard solution required to reach the endpoint.

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