Transition Metals & Complexes (Edexcel International A Level (IAL) Chemistry): Flashcards

Coordination number is the total number of dative covalent bonds formed between the central metal ion and all the ligands in a complex.

The overall charge is the sum of the oxidation states of the central metal ion and all the ligands. Neutral ligands (e.g. H₂O, NH₃) contribute zero to the charge.

The complex is hexaaquacopper(II). Its coordination number is 6 (six water ligands each donate one lone pair).

In complex notation, the metal ion and ligands are enclosed in square brackets to show the complex unit.

For anionic complexes, the metal name is modified to end in -ate and Latin stems are used where applicable (e.g. copper becomes cuprate, iron becomes ferrate).

True.

Ligands donate a lone pair of electrons to the metal ion to form a dative bond, which is the definition of a Lewis base (electron pair donor).

The complex is hexaamminecobalt(II), with formula [Co(NH₃)₆]²⁺.

Each NH₃ donates one lone pair to Co²⁺.

The ligand ammonia is named ammine when used in complex nomenclature (note the double 'm').

Ligands cause the 3d orbitals to split into two energy levels. Electrons absorb visible light to move to the higher level, and the complementary colour to the absorbed colour is transmitted.

Zn²⁺ has a completely filled 3d¹⁰ subshell and Sc³⁺ has an empty 3d⁰ subshell, so neither can undergo d-electron promotion to absorb visible light.

Complementary colours are two colours directly opposite each other on the colour wheel — the colour absorbed by a complex and the colour observed are complementary to each other.

1. The size and type of ligands

2. The nuclear charge and identity of the metal ion

3. The oxidation state of the metal

4. The shape of the complex

NH₃ ligands have greater charge density than H₂O, causing stronger interaction with Cu²⁺ and larger d-orbital splitting. This shifts the absorbed wavelength, changing the complementary colour observed.

[Fe(H₂O)₆]²⁺ absorbs in the red region and appears green, while [Fe(H₂O)₆]³⁺ absorbs in the blue region and appears orange.

True.

A higher oxidation state creates a stronger interaction with ligands, causing greater splitting of the d-orbitals and absorbing light of higher energy.

[CuCl₄]^2– is yellow, while [Cu(H₂O)₆]²⁺ is light blue. The colour change is due to a change in both ligand type and coordination number (6 to 4).

False.

The colour observed is the complementary colour to the light absorbed. The absorbed wavelength is the one that matches the d-orbital splitting energy.

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

1. Water (H₂O)

2. Ammonia (NH₃)

3. Chloride ion (Cl^–)

4. Cyanide ion (CN^–)

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

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

2. Ethanedioate ion (C₂O₄^2–, written as 'ox')

A multidentate ligand is a ligand that forms more than two dative covalent bonds to the central metal ion, as it contains multiple atoms with lone pairs.

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

False.

In a dative covalent bond, the ligand donates both electrons to the bond. The transition metal ion acts as the electron pair acceptor (Lewis acid).

'en' forms two dative bonds because it contains two NH₂ groups, each with a lone pair on the nitrogen that can be donated to the metal ion.

A ligand must have a lone pair of electrons to donate, so it must be a Lewis base. It may be negatively charged (e.g. Cl^–) or neutral (e.g. H₂O, NH₃).

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

[Cu(H₂O)₆]²⁺ (aq) + 4Cl^– (aq) ⇌ [CuCl₄]^2– (aq) + 6H₂O (l)

The coordination number changes from 6 to 4.

The solution changes from blue ([Cu(H₂O)₆]²⁺) to green (a mixture of both complexes) to yellow ([CuCl₄]^2–).

Chloride ions are larger than water molecules, so fewer can fit around the central metal ion. This causes the coordination number to change from 6 to 4.

The chelate effect is the greater stability of complexes containing bidentate or multidentate ligands compared to those with monodentate ligands, driven by a large positive entropy change.

Chelation produces a net increase in the number of particles in solution, giving a large positive ΔS. The small enthalpy change and large positive entropy change combine to give a negative ΔG^ꝋ.

When EDTA^4– chelates with [Co(H₂O)₆]²⁺, two reactants become seven products, producing a large increase in entropy.

False.

If incoming ligands are of similar size to the departing ligands, the coordination number and geometry of the complex remain unchanged.

False.

The oxidation number of copper remains +2 throughout the ligand exchange reaction — no redox occurs, only a substitution of ligands.

An octahedral complex is a transition metal complex with a coordination number of 6, in which six dative bonds point towards the vertices of an octahedron. The bond angles are 90°.

A tetrahedral complex is a transition metal complex with a coordination number of 4, in which four dative bonds point towards the vertices of a tetrahedron. The bond angles are 109.5°.

Chloride ions are large, so only four can fit around the central metal ion. This gives a coordination number of 4 and a tetrahedral geometry.

Three bidentate ligands each form two dative bonds, giving six bonds in total. For example, three 1,2-diaminoethane ('en') molecules coordinate to a metal ion to form an octahedral complex.

The bond angles in an octahedral complex are 90° and in a tetrahedral complex are 109.5°.

These complexes show cis-trans (geometrical) isomerism, where the 'cis' form has the two different ligands adjacent and the 'trans' form has them opposite each other.

True.

Octahedral complexes with three bidentate ligands (e.g. [Ni(en)₃]²⁺) have no plane of symmetry, giving non-superimposable mirror images that are optical isomers.

It is slowly oxidised to [Fe(H₂O)₆]³⁺, changing colour from pale green to yellow as Fe²⁺ is oxidised to Fe³⁺.

EDTA^4– is a hexadentate (multidentate) ligand that forms six dative bonds and can form an octahedral complex on its own with a central metal ion.

A square planar complex is a complex with four coordinate bonds arranged in a flat, square geometry with bond angles of 90°.

The bond angles in a square planar complex are 90°.

Cisplatin exhibits cis-trans (geometric) isomerism. It arises because the square planar geometry restricts rotation, so ligands can be arranged on the same side (cis) or opposite sides (trans) of the complex.

Cisplatin passes through the cell membrane and undergoes ligand exchange, where the chlorine ligands are replaced by water molecules.

False.

Only cis-platin is effective in cancer treatment. Trans-platin cannot bind to DNA in the same way and does not have the same therapeutic effect.

Cisplatin undergoes ligand exchange inside the cell, replacing its chloride ligands with water. Nitrogen atoms on DNA bases then displace the water ligands, forming dative covalent bonds with the platinum centre, distorting the DNA and preventing replication.

False.

Cisplatin binds to healthy cells as well as cancerous ones. Hair follicles are affected because they replicate rapidly, making them vulnerable to the drug's mechanism.

Cisplatin is a square planar platinum(II) complex used as an anticancer drug, with two chloride and two ammonia ligands arranged on the same side of the complex.

Haemoglobin is a protein complex containing iron(II) at the centre of haem groups, which transports oxygen around the body in the blood.

The iron ion at the centre of the haem group is iron(II), with an oxidation state of +2.

Oxygen forms a dative covalent bond with the iron(II) centre. The bond is weak, allowing oxygen to detach easily and be transported into cells.

True.

CO forms stronger dative covalent bonds with the iron(II) centre than O₂, preventing oxygen from binding and being carried to cells.

When carbon monoxide binds to haemoglobin, the complex is called carboxyhaemoglobin, which has a darker red colour compared to oxyhaemoglobin.

Anaemia is a condition in which a person does not have enough haemoglobin in their blood. Iron deficiency anaemia can be treated by taking iron sulfate tablets in the diet.

There are five dative covalent bonds to the Fe²⁺ ion: four from nitrogen atoms in the haem group (in a square planar arrangement) and one from the protein (globin) chain.

False.

Carbon monoxide is a better ligand than oxygen. It binds more strongly to iron(II), blocking oxygen transport and making it toxic.