Part 3: Application/Analysis

Question 1

You are given three octahedral complexes of cobalt: hexafluorocobalt(III), hexaaquacobalt(III), and hexaamminecobalt(III). Give the chemical formula and draw the chemical structure for each.

Answer 1

(a) hexafluorocobalt(III): [CoF6]
(b) hexaaquacobalt(III): [Co(OH2)6]
(c) hexaamminecobalt(III): [Co(NH3)6]

Question 2

A pink solid has the formula CoCl3 5NH3 H2O. A solution of this salt is also pink and rapidly precipitates 3mol AgCl(s) on titration with silver nitrate solution.
When the pink solid is heated, it loses 1mol H2O to give a purple solid with the same mole ratio of NH3:Cl:Co as the pink solid.
The purple solid releases two of its chloride ions rapidly; then on dissolution and titration with AgNO3, release one of its chloride ions slowly.
Give the chemical formula and name the two octahedral complexes

Answer 2

Pink: [Co(NH3)5(H2O)]Cl3
pentaammineaquacobalt(III) chloride

Purple: [Co(NH3)5Cl]Cl2
pentaamminechlorocobalt(III) chloride

Question 3

Which coordination complex is likely blue in color? yellow-orange? Green? Use the concept of Crystal field theory to support your answer
(a) hexafluorocobalt(III): [CoF6]
(b) hexaaquacobalt(III): [Co(OH2)6]
(c) hexaamminecobalt(III): [Co(NH3)6]

Answer 3

* When ligands attach to a transition metal to form a coordination complex, electrons in the d orbital split into high energy and low energy orbitals.*

The color of a complex arises from electronic transitions between the t2g and eg orbitals. The energy of the absorbed light corresponds to the Δoct value.

(a) hexafluorocobalt(III): [CoF6] : green
(b) hexaaquacobalt(III): [Co(OH2)6] : blue
(c) hexaamminecobalt(III): [Co(NH3)6]: yellow

Question 4

Explain the principle behind the detection of hydrogen sulfide (H2S) gas using a tin(IV) oxide (SnO2) sensor.

Answer 4

H2S reacts with oxygen ions on the SnO2 surface.

This reaction alters the electron-hole balance in the SnO2.

The resulting change in electrical resistance is used to detect the presence of H2S.

Question 5

Outline the steps involved in the sensing mechanism of a SnO2 sensor towards H2S gas.

Answer 5

Oxygen Adsorption: In clean air, oxygen molecules adsorb onto the SnO2 surface, forming chemisorbed species like O2- and O-. This adsorption process involves the capture of electrons from the conduction band of SnO2, creating electron holes and increasing the sensor’s resistance.

H2S Interaction: When H2S gas encounters the SnO2 surface, it reacts with the adsorbed oxygen ions. This reaction typically involves the reduction of oxygen species (O2-, O-) to water (H2O) or other oxygen-containing compounds.

Electron Release: The reduction reaction releases electrons back into the SnO2 conduction band, neutralizing the electron holes created during oxygen adsorption.

Resistance Change: The decrease in electron holes leads to a decrease in the electrical resistance of the SnO2 sensor.

Signal Detection: The change in resistance is measured by an external circuit, generating an electrical signal that indicates the presence and concentration of H2S gas.

Question 6

Describe the mechanism of action of Wilkinson’s catalyst in the hydrogenation of alkenes. Include the role of each component of the catalyst and the steps involved in the catalytic cycle.

Answer 6

Wilkinson’s catalyst, [RhCl(PPh3)3], is a renowned homogeneous catalyst for the hydrogenation of alkenes

1. * Ligand Dissociation One or two PPh3 ligands dissociate from the rhodium center, creating a vacant coordination site*
2. Oxidative Addition of Hydrogen- Molecular hydrogen (H2) binds to the rhodium center. The Rh(I) center is oxidized to Rh(III)
3. Alkene Coordination-The alkene substrate coordinates to the rhodium center
4. Migratory Insertion-generates an alkyl-rhodium intermediate
5. Reductive Elimination-regenerates the Rh(I) catalyst