Among the properties (a) reducing, (b) oxidizing, (c) complexing, the set of properties shown by ${\text{C}}{{\text{N}}^ - }$ ion towards metal species is:
A. a, c, b
B. b, c
C. c, a
D. a, b

Hint: To determine the answer we should know the meaning of oxidizing, reducing and complexing. We will check how the ${\text{C}}{{\text{N}}^ - }$ reacts with metal to decide whether it is oxidizing or reducing. If the ${\text{C}}{{\text{N}}^ - }$ forms complex we can say that it is a complexing agent. if ${\text{C}}{{\text{N}}^ - }$ lose electrons we will sat it as a reducing agent. if ${\text{C}}{{\text{N}}^ - }$ gain electrons we will say it is an oxidizing agent.


Complete solution:
The species which causes the reduction of other species is known as the reducing agent and the species which causes the oxidation of other species is known as the oxidizing agent.
The species which form complexes are known as complexing agents.
Some examples are shown as follows:
 \[{\left[ {{\text{Fe(CN}}{{\text{)}}_{\text{6}}}} \right]^{4 - }}\], \[{\left[ {{\text{Cu(CN}}{{\text{)}}_4}} \right]^{3 - }}\], \[{\left[ {{\text{Fe(CN}}{{\text{)}}_{\text{6}}}} \right]^{3 - }}\]
In the presence of ${\text{C}}{{\text{N}}^ - }$, the metals form complex with ${\text{C}}{{\text{N}}^ - }$. Some so, ${\text{C}}{{\text{N}}^ - }$ is a complexing agent.
Cyanide${\text{C}}{{\text{N}}^ - }$ has negative charge so it cannot accept electrons and cannot undergo reduction. So, it donates the electrons to the metal vacant d-orbital and forms a complex. As cyanide ${\text{C}}{{\text{N}}^ - }$ donate electrons which are accepted by metal, so metal undergoes reduction. The reduction is caused by cyanide ${\text{C}}{{\text{N}}^ - }$ so, cyanide ${\text{C}}{{\text{N}}^ - }$ is a reducing agent.
So, the set of properties shown by ${\text{C}}{{\text{N}}^ - }$ ion towards metal species is (a) reducing, and (c) complexing.

Therefore, option (C) is correct.


Note: An atom loss or gain electrons to form a bond with other atoms. The atom which loses electrons goes under the oxidation process and the atom which gains electrons goes under the reduction process. When both the processes take place simultaneously the reaction is known as redox reaction that is reduction plus oxidation. The ‘red’ stands for reduction and ‘ox’ stands for oxidation. The reaction in which electrons are transferred between two reactants is known as the redox reaction.