Transition Elements

Question 1

What are Transition Elements?

Answer 1

A transition element is a d-block element that forms one or more ions with PARTIALLY FILLED D SUBSHELLS.

(Thus NOT Sc3+: vacant d subshell or Zn2+: Completely full d subshell)

Question 2

Why does atomic radii and ionisation energy remain relatively invariant across transition elements?

Answer 2

1. Nuclear charge increases due to the increasing number of protons.
2. Electrons are added to the PENULTIMATE 3d orbitals, increasing shielding effect, which cancels out the increase in nuclear charge, thus invariant.

Question 3

Why are Transition elements much denser than s-block elements?

Answer 3

They have higher atomic mass, and smaller atomic radius, and so have a more closely packed metallic structure that results in more atoms per unit volume.

Question 4

Why do transition elements have higher melting and boiling point than their s-block elements?

Answer 4

For s-block elements, only valence electrons from 4s orbitals are delocalized -> weaker metallic bonding.

Whereas for transition elements, both valence electrons from 4s and 3d are delocalized due to their similar energy levels, resulting in more energy needing to overcome its stronger metallic bonding.

Question 5

Why can transition elements exhibit variable O.S.? And how do we know the maximum oxidation state?

Answer 5

Their 4s and 3d electrons are similar in energy. Once the 4s electrons are removed, some or all of the 3d electrons may also be removed or shared without requiring much energy.

Max O.S. = Number of 4s electrons and number of UNPAIRED 3d electrons (Cu is an exception as it can have an O.S. of +3)

Question 6

What is a complex?

Answer 6

A complex is a chemical compound consisting of a CENTRAL ATOM OR ION, surrounded by ligands bonded to it by DATIVE bonds.

Question 7

What is a ligand?

Answer 7

A ligand is a neutral molecule or an 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.

Question 8

Why do transition elements form complexes readily?

Answer 8

1) They have HIGH CHARGE DENSITY and hence strong polarising power. They can attract ligands which are rich in electrons.
2) They have vacant, low-lying orbitals which can accept lone pairs of electrons from ligands to form dative bonds with them.

Note: During the formation of a complex, the transition metal ion acts as Lewis Acid and the ligands act as Lewis Base

Question 9

What is the denticity of a ligand?

Answer 9

The number of coordinate bonds formed between a central metal ion and the ligand

Question 10

What is the colour of [Co(H2O)6]2+?

Answer 10

Pink

Question 11

What is the colour of [CuCl4]2-?

Answer 11

Yellow

Question 12

What is the colour of [Cu(H2O)6]2+?

Answer 12

Light blue

Question 13

What is the colour of [Cu(NH3)4]2+ or [Cu(H2O)2(NH3)4]2+?

Answer 13

Dark blue

Question 14

What is the colour of [Fe(H2O)5(SCN)]2+?

Answer 14

BLOOD red note that CN- has -1 charge

Question 15

What is the Crystal Field Theory?

Answer 15

1) When a transition metal atom/ion interacts with its surrounding ligands, its d orbitals are split into 2 groups of 2 energy levels.
2) According to whether it has an octahedral or tetrahedral coordination geometry, the d orbitals will have different energy levels. Octahedral: x^2-y^2 z^2 on top vice versa for tetrahedral

Question 16

Why do all 5 d orbitals have higher energy levels than when the ion was isolated?

Answer 16

All 5 d orbitals experience repulsion as the ligands approach the metal ion.

Question 17

Why are the dz^2 and dx^2-y^2 orbitals are destabilized to a greater extent and have higher energy levels than dxy, dyz and dxz in an octahedral complex? 2-3

Answer 17

dz^2 and dx^2-y^2 orbitals have lobes that are in the region of the ligands along the x, y and z-axes. Hence, there is greater repulsion between the dz^2 and dx^2-y^2 orbitals and the ligands. As a result, these orbitals are more destabilised and higher in energy.

Whereas for the dxy, dyz and dxz orbitals, they have lobes that are in between the region of the ligands along the x, y and z-axes. Hence, there is less repulsion between the dxy, dyz and dxz orbitals and ligands. As a result, these orbitals are less destabilized and lower in energy.

This is the splitting of the d orbitals into two different energy levels.

Question 18

Why are transition elements complexes usually coloured?

Answer 18

1) In the presence of ligands, the d orbitals of the central ion/atom are split into two groups of two different energy levels.
2) When white light shines on the complex, a d electron from the lower energy level undergoes d-d transition and is promoted to a higher energy vacant or partially filled d orbital. During the transition, the d electron absorbs a certain wavelength of light from the visible spectrum.
3) The colour observed is complementary to the colour absorbed.

Question 19

What affects the colour of complexes?

Answer 19

Depends on the ENERGY GAP between the two groups of d orbitals, which is dependent on:
1) IDENTITY OF METAL and its OXIDATION STATE (difference in electronic configuration)– higher OS leads to higher energy gap because higher OS reflects higher charge density
2) SHAPE of complex ion
3) Nature of LIGAND (stronger-> larger energy gap)

Question 20

What reaction is it when one or more ligands in a complex ion is replaced by a different one? (Either because it is stronger, or having a much higher conc.)

Answer 20

A ligand exchange reaction.

Question 21

How do transition elements and their compounds act as heterogenous catalysts?

Answer 21

1. Reactant molecules diffuse towards the solid catalyst surface are adsorbed onto the active sites on the surface.
2. Adsorption increases the concentration of the reactant molecules on the catalyst surface, orientates the reactant molecules in the right orientation for reaction to occur, and the formation of weak bonds during adsorption weakens the intramolecular bonds within reactant molecules, thus lowering the activation energy.
3. Product molecule desorbs and diffuses away from the catalyst surfaces, freeing up active sites on the catalyst surfaces for adsorption of new reactant molecules.

Question 22

Why are transition metal ions good homogenous catalysts for Redox reactions?

Answer 22

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

Question 23

Possible complexes of Sc3+

Answer 23

[Sc(H2O)6)]3+ colourless

Question 24

Possible complexes of Co2+

Answer 24

[Co(H2O)6]2+ pink
[Co(NH3)6]2+ yellow/ brown

Question 25

Possible complexes of Cr3+

Answer 25

[Cr(H2O)6]3+ green
[Cr(OH)6]3- dark green

Question 26

Possible complexes of Cu2+

Answer 26

[CuCl4]2- yellow
[Cu(H2O)6]2+ light blue
[Cu(NH3)4]2+ or Cu(NH3)4(H2O)2]2+ dark blue

Question 27

Possible complexes of Fe2+

Answer 27

[Fe(H2O)6]2+ pale green
[Fe(CN)6]4- yellow

Question 28

Possible complexes of Fe3+

Answer 28

[Fe(H2O)6]3+ pale yellow
[Fe(H2O)5(SCN)]2+ blood red
[Fe(CN)6]3- red

Question 29

Possible complexes of Mn2+

Answer 29

[Mn(H2O)6]2+ pink

Question 30

Possible complexes of Ni2+

Answer 30

[Ni(H2O)6]2+ green
[Ni(NH3)6]2+ blue

Question 31

Possible complexes of Zn2+

Answer 31

[Zn(NH3)4]2+ colourless

Question 32

Why are transition metals heterogenous catalysts?

Answer 32

They can accept electron pairs from reactant molecules due to the presence of low lying vacant orbitals, and use the electrons in its d orbitals to form the temporary bonds with the reactants at the solid surface