[Inorganic] Transition Elements

[DEFINITION]

What is a transition element?

[DEFINITION]

What is a transition element?

a d-block element that forms one or more stable ions with partially filled d subshells

KEY ORBITAL CHARACTERISTICS OF TRANSITION ELEMENTS

1. 4s and 3d orbitals have similar energy levels

2. Across the period, electrons are added to the penultimate 3d orbital

[ATOMIC STRUCTURE]

State the electronic configuration of chromium and explain why it is different from expected.

[Ar]3d54s1. The half-filled 3d subshell is more stable than the [Ar]3d44s2 configuration.

[PHYSICAL PROPERTIES]

State the electronic configuration of copper and explain why it is different from expected.

[Ar]3d104s1. The fully filled 3d subshell is more stable than the [Ar]3d94s2 configuration.

[PHYSICAL PROPERTIES]

Explain why atomic radii remains relatively invariant across transition elements.

1. Nuclear charge increases due to increasing number of protons

2. Electrons are added to the penultimate 3d orbitals

3. Shielding effect increases, almost cancelling out the increase in nuclear charge

4. Effective nuclear charge remains relatively invariant

5. Atomic radius remains relatively invariant

[PHYSICAL PROPERTIES]

Account for the differences in trend of atomic radii between transition elements and the rest of period 4 main group elements.

Atomic radius decreases significantly across the period 4 main group elements, but remains relatively invariant across transition elements.

Across the period for the main group elements, electrons are added to the same valence shell, causing shielding effect to remain relatively constant. However, for transition elements, electrons are added to the penultimate 3d orbital, causing an increase in shielding effect which almost cancels out the increase in nuclear charge.

[PHYSICAL PROPERTIES]

Why do transition metals have smaller atomic radii than s-block metals from the same period?

• Transition metals have higher nuclear charge as they have more protons

• Each additional electron is added to the penultimate 3d orbital, which provide a poor shield between valence 4s electrons and the nucleus

• Valence 4s electrons are strongly attracted by the nucleus

• Higher effective nuclear charge = smaller atomic radii

[PHYSICAL PROPERTIES]

By comparing the electronic configurations of Cu and Zn, explain why Zn has a significantly higher first ionisation energy than copper.

Cu: [Ar]3d104s1, Zn: [Ar]3d104s2

• Zn has one more proton than Cu, and has greater nuclear charge

• The additional electron in Zn is added to the valence 4s orbital, which experiences the same shielding effect from inner electrons as Cu.

• Effective nuclear charge increases

[PHYSICAL PROPERTIES]

Explain why first ionisation energy remains relatively invariant across transition elements.

1. Nuclear charge increases due to increasing number of protons

2. Electrons are added to the penultimate 3d orbitals

3. Shielding effect increases, almost cancelling out the increase in nuclear charge

4. Effective nuclear charge remains relatively invariant

5. Energy required to remove valence electron remains relatively invariant

[PHYSICAL PROPERTIES]

Account for the differences in trend of first ionisation energy between transition elements and the rest of period 4 main group elements.

First ionisation energy increases significantly across the period 4 main group elements, but remains relatively invariant across transition elements.

Across the period for the main group elements, electrons are added to the same valence shell, causing shielding effect to remain relatively constant. However, for transition elements, electrons are added to the penultimate 3d orbital, causing an increase in shielding effect which almost cancels out the increase in nuclear charge.

[PHYSICAL PROPERTIES]

Explain the difference in density between transition elements and s-block elements like calcium.

*density is mass/volume

• Transition elements have higher atomic masses and smaller atomic radii.

• Close-packed metallic structure results in more atoms per unit volume.

[PHYSICAL PROPERTIES]

Explain the difference in MP/BP between transition elements and s-block elements like calcium.

• In s-block elements, valence electrons from 4s orbitals are delocalised in the metallic lattice structure.

• In transition elements, electrons from both 4s and 3d orbitals are delocalised due to similar energy levels.

• More energy is required to overcome stronger metallic bonding in transition elements than s-block elements.

[CHEMICAL PROPERTIES]

Explain why transition elements are able to exhibit variable oxidation states, while s-block elements exhibit limited oxidation states.

• 4s and 3d electrons are similar in energy. Once 4s electrons are removed, 3d electrons may also be removed or shared without requiring much more energy.

• For s-block elements, the subsequent removal of electrons from inner shell p orbitals is not energetically favourable.

Maximum oxidation state of first row transition element =

no. of 4s electrons + no. of unpaired 3d electrons

[DEFINITION]

What is a complex?

a chemical compound consisting of a central atom or ions, surrounded by ligands, bonded to it by dative bonds

[DEFINITION]

What is a ligand?

a neutral molecule or anion containing at least one atom with a lone pair of electrons that can be donated to form a dative bond with a metal atom/ion

[COMPLEX FORMATON]

Explain why transition elements are able to form complexes readily.

• high charge densities = high polarising power = able to attract electron-rich ligands

• vacant low-lying orbitals = accept lone electron pairs from ligands to form dative bonds

[COMPLEX FORMATION]

Why do Group 1 metal ions not form aqua complexes in water?

Group 1 metal ions have lower charge densities. They form ion-dipole interactions with water molecules, giving hydrated cations.

[COMPLEX IONS]

Label the parts of the chemical formula of a complex ion.

1. Coordination sphere: central atom/ion + ligands (whatever is in the square brackets)

2. Counterion: ion accompanying the complex ion to maintain electrical neutrality

3. Net charge: sum of oxidation state

4. Oxidation number of metal atom/ion: compare net charge of complex and total charge of ligands

5. Coordination number: no. of dative bonds the central atom/ion forms with ligands

[COMPLEX IONS]

What is the denticity of a ligand?

The number of dative bonds formed between ligand and central metal ion.

[COMPLEX IONS - SHAPE]

Shape of complex ion with coordination number 2

linear

[COMPLEX IONS - SHAPE]

Possible shapes of complex ions with coordination number 4

1. tetrahedral

2. square planar

[COMPLEX IONS - SHAPE]

Shape of complex with coordination number 6

octahedral

[COMPLEX IONS - COLOUR]

Colour of [Co(H₂O)₆]2+

pink

[COMPLEX IONS - COLOUR]

Colour of [CuCl₄]2-

yellow

[COMPLEX IONS - COLOUR]

Colour of [Cu(H₂O)₆]2+

light blue

[COMPLEX IONS - COLOUR]

Colour of [Cu(NH₃)₄]2+/[Cu(H₂O)₂(NH₃)₄]2+

dark blue

[COMPLEX IONS - COLOUR]

Colour of [Fe(H₂O)₅(SCN)]2+

blood red

[COMPLEX IONS - COLOUR]

What are the two phenomena that influence the colour of a transition metal complex?

1. d orbital splitting

2. d-d transition

[COMPLEX IONS - COLOUR]

Outline the process of d orbital splitting in the formation of an octahedral complex. (Crystal Field Theory)

1. Ligands approach the metal ion along the x, y and z-axes. IN the presence of ligands, all 5 d-orbitals are destabilised and hence have higher energy levels than when the ion was isolated.

2. In the octahedral complex, the dz2 and dx2-y2 orbitals have lobes in the region of the ligands along the x, y and z-axes, while dxy, dyz and dxz orbitals have lobes that are in between the region of the ligands. There is greater repulsion between the dz2 and dx2-y2 orbitals and the ligands. These orbitals are destabilised to a greater extent and have higher energy levels.

3. Hence, in the presence of ligands, there is splitting of the d orbitals into two energy levels.

[COMPLEX IONS - COLOUR]

Outline the process of d-d transition in the formation of an octahedral complex.

1. When a complex comes under white light, an electron in a d orbital of lower energy level absorbs a photon of light.

2. The electron undergoes an electronic transition, where it is promoted to a vacant or partially-filled d orbital of higher energy.

[COMPLEX IONS - COLOUR]

Explain why transition element complexes are usually coloured.

1. (d orbital splitting) In the presence of ligands, the d orbitals of the transition metal ion are split into two groups of different energy levels.

2. (d-d transition) When white light shines on the complex, a d electron is promoted to a higher energy vacant or partially-filled d orbital.

3. (absorbed wavelength) During this transition, the d electron absorbs a wavelength of light from the visible spectrum.

4. (transmitted light) The transmitted light, which is complementary to the wavelength of light absorbed, gives the transition metal complex its colour.

[COMPLEX IONS - COLOUR]

State the three pairs of complementary absorbed light-transmitted light colours.

1. Red-green

2. Blue-orange

3. Yellow-violet

[COMPLEX IONS - COLOUR]

Explain why an aqueous solution of Cu2+ ions appears blue.

1. (details of complex - ligands, shape and formula) In aqueous solutions, each Cu2+ ion is surrounded by six H₂O ligands in an octahedral [Cu(H₂O)₆]2+ complex.

2. (d orbital splitting) In the presence of H₂O ligands, the d orbitals of Cu2+ are split into two groups of different energy levels.

3. (d-d transition) When white light shines on the complex, a d electron undergoes d-d transition and is promoted to a higher energy vacant or partially-filed d orbital.

4. (absorbed wavelength) During the transition, the d electron absorbs light from the orange region of the visible spectrum.

5. (transmitted colour) The blue colour of the transmitted light is observed, the complementary colour of the wavelength absorbed.

[COMPLEX IONS - COLOUR]

The colour of a complex depends on (1) in turn depends on (2)

(1) the wavelength of light absorbed during d-d transition (2) the energy gap between the two groups of d orbitals, delta E

[COMPLEX IONS - COLOUR]

3 factors that affects ∆E

1. identity of metal and its oxidation state

2. shape of complex ion

3. nature of ligand

[COMPLEX IONS - COLOUR]

Why are Sc(III), Zn(II) and Cu(I) compounds white in colour? Explain using your understanding of d orbital splitting or d-d transition.

They contain an empty or filly-filled d subshell. No d-d transition is possible, hence they do not absorb any light.

[COMPLEX IONS - COLOUR]

For the same metal in different oxidation states, does the ion with a higher oxidation state have a larger or smaller energy gap? Explain using your understanding of d orbital splitting or d-d transition.

1. Higher oxidation state = greater charge + smaller ion = higher charge density

2. Since ligands are pulled closer to the central metal ion, shorter metal-ligand bond = greater extent of repulsion

3. This results in more destabilisation during d orbital splitting, leading to a larger energy gap

[COMPLEX IONS - COLOUR]

How do the shapes of complex ions affect the energy gap between the two groups of d orbitals?

Generally, due to the greater number of ligands approaching an octahedral complex than a tetrahedral complex, there is greater repulsion and hence destabilisation, resulting in a higher energy gap between high and low energy groups of d orbitals.

[COMPLEX IONS - COLOUR]

How do the nature of ligands affect the energy gap?

Stronger field ligands result in larger energy gaps.

[LIGAND EXCHANGE]

When can a weaker ligand replace a stronger ligand from a complex?

When the weaker ligand is present in higher concentration. (applying Le Chatelier’s Principle)

[LIGAND EXCHANGE]

Explain the dissolving of pale blue Cu(OH)₂ precipitate upon addition of excess NH₃ (aq), to give a dark blue solution.

*Formation of pale blue precipitate: [Cu(H₂O)₆]2+ (aq) + 2OH- (aq) ⇌ Cu(OH)₂ (S) + 6H₂O (l)

Ligand exchange occurs.

1. Stronger NH₃ ligands replace weaker H₂O ligands in the [Cu(H₂O)₆]2+ complex to form a dark blue [Cu(NH₃)₄]2+ complex. [Cu(H₂O)₆]2+ + 4NH₃ ⇌ [Cu(NH₃)₄]2+ + 6H₂O

2. The concentration of [Cu(H₂O)₆]2+ decreases

3. By Le Chatelier’s Principle, the position of equilibrium (of the formation of pale blue precipitate) shifts left to increase the concentration of [Cu(H₂O)₆]2+.

4. The ionic product of Cu(OH)₂ falls below its Ksp. Hence, the pale blue precipitate of Cu(OH)₂ dissolves.

[LIGAND EXCHANGE]

Explain the colour changes that occur then concentrated hydrochloric acid is added to blue Cu2+ solution.

*[Cu(H₂O)₆]2+ is blue, [CuCl4]2- is yellow.

Blue to green to yellow.

Ligand exchange occurs.

1. Due to high [Cl-], Cl- ions replace H₂O ligands in the blue [Cu(H₂O)₆]2+ complex to form yellow [CuCl₄]2- complex.

2. The intermediate green colour is due to the presence of both the blue [Cu(H₂O)₆]2+ and yellow [CuCl₄]2- complex ions.

3. As more concentrated HCl is added, [Cl-] increases further. The position of equilibrium shifts right to decrease [Cl-], resulting in the formation of more [CuCl₄]2- ions. The solution turns yellow.

[LIGAND EXCHANGE]

Explain how haemoglobin transports oxygen around the body. Hence, explain why carbon monoxide and cyanide are toxic to the human body.

• An oxygen ligand replaces the water ligand in haemoglobin to form oxyhaemoglobin in a reversible reaction.

• In the presence of a stronger CO/CN- ligand, the water ligand is replaced irreversibly, preventing oxygen molecules form binding to the central Fe2+ ion. Body cells are deprived of oxygen.

[CATALYSIS]

What makes transition elements and their compounds good heterogenous catalysts?

Temporary weak bonds can be formed with reactants at the solid surface.

1. Presence of low lying vacant d orbitals: transition elements can accept electron pairs from reactant molecules

2. Presence of electrons in d orbitals: temporary bonds can be formed with reactant molecules

[CATALYSIS]

What makes transition elements and their compounds good homogenous catalysts?

They exhibit variable oxidation states: allows for the formation of intermediates via alternative reaction pathways that have lower activation energies.