Animated Solution for Chemistry - Redox Reactions: Among the properties (A) reducing, (B) oxidising and (C) complexing, the set of properties shown by ${\text{CN}}^{-}$ ion towards metal species is

The cyanide ion (${\text{CN}}^{-}$) is often known for its toxicity, but in the realm of inorganic chemistry, it is an absolute superstar. It possesses a fascinating dual personality when interacting with transition metal species. Let's dive deep into its chemical behavior and understand why it acts the way it does.

First and foremost, the cyanide ion is an exceptional ligand. In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. ${\text{CN}}^{-}$ is what we call a strong field ligand.

Why is it so strong? It's because of its ability to participate in $\pi$-backbonding. Not only does it donate a lone pair of electrons from the carbon atom to the metal's empty orbitals (forming a $\sigma$-bond), but it also accepts electron density back from the metal's filled d-orbitals into its empty ${\pi }^{\ast }$ antibonding orbitals. This synergistic effect creates incredibly stable complexes.

For example, when cyanide ions encounter Nickel(II) ions (${\text{Ni}}^{2+}$), they readily form a stable, square planar complex:

This perfectly illustrates the complexing property (C) of the cyanide ion.

Now, here is where things get even more interesting. Cyanide doesn't just sit around forming complexes; it can also actively participate in redox reactions. Specifically, it acts as a reducing agent.

Imagine cyanide meeting a metal ion that is relatively eager to gain an electron, such as Copper(II) (${\text{Cu}}^{2+}$). You might expect them to simply form a complex like $[\text{Cu}(\text{CN}{)}_4{]}^{2-}$. However, ${\text{Cu}}^{2+}$ is a decent oxidizing agent, and ${\text{CN}}^{-}$ is a willing reducing agent.

When they react, ${\text{CN}}^{-}$ reduces ${\text{Cu}}^{2+}$ to ${\text{Cu}}^{+}$. In the process, the cyanide ion itself gets oxidized to a fascinating gas called cyanogen, $(\text{CN}{)}_2$. The newly formed ${\text{Cu}}^{+}$ then reacts with excess cyanide to form a stable complex. The overall reaction looks like this:

In this reaction, the oxidation state of copper goes from +2 to +1. This clearly demonstrates the reducing property (A) of the cyanide ion.

To sum it all up, the cyanide ion is a versatile chemical actor. It forms strong coordinate bonds to create stable complexes, and it can donate electrons to reduce certain metal ions. It does not, however, act as an oxidizing agent. Therefore, the correct set of properties shown by the ${\text{CN}}^{-}$ ion towards metal species is reducing (A) and complexing (C).