Animated Solution for Chemistry - s and p-Block Elements: Ammonia forms the complex ion $[Cu(N{H}_3{)}_4{]}^{2+}$ with copper ions in the alkaline solutions but not in acidic solutions. What is the reason for it?

A Tale of Two Suitors

Imagine you are observing a chemical dance inside a beaker. On one side, we have the elegant Ammonia ($N{H}_3$) molecule, and on the other, the transition metal Copper ion ($C{u}^{2+}$). In a normal, alkaline, or neutral environment, these two are destined to pair up. Ammonia acts as a Lewis base, gracefully donating its lone pair of electrons to the electron-deficient Copper ion, forming the beautiful deep-blue coordination complex $[Cu(N{H}_3{)}_4{]}^{2+}$.

But what happens when we change the environment? What if we introduce an acidic medium into this beaker? Suddenly, a new, highly aggressive suitor enters the fray: the Proton ($H^{+}$).

To understand the outcome of this chemical rivalry, we must look closely at the Nitrogen atom in Ammonia. Nitrogen possesses a lone pair of electrons—a dense cloud of negative charge that is highly attractive to anything lacking electrons. This lone pair is the sole reason Ammonia can act as a Lewis base (an electron-pair donor).

In an alkaline medium, there is no competition. The Copper ion ($C{u}^{2+}$) happily accepts these lone pairs from four different Ammonia molecules to form its stable complex.

However, an acidic medium is defined by an abundance of $H^{+}$ ions. An $H^{+}$ ion is essentially a bare proton. It has no electrons, a tiny size, and an incredibly high charge density. This makes it a tremendously strong Lewis acid.

When Ammonia is placed in this acidic environment, a kinetic and thermodynamic race begins. The bare proton ($H^{+}$) is much smaller and more mobile than the bulky Copper ion. Before the Ammonia molecule can even approach the Copper ion, the proton swoops in and attacks the lone pair on the Nitrogen atom.

The reaction is instantaneous:

Ammonia is converted into the Ammonium ion ($N{H}_4^{+}$). In this new molecule, the lone pair that was once available for bonding is now completely shared with the proton. The Nitrogen atom no longer has any free electrons to offer.

Consequently, the Copper ion is left stranded. Because the protons have coordinated with all the available Ammonia molecules to form $N{H}_4^{+}$ ions, there are simply no $N{H}_3$ molecules left to form the $[Cu(N{H}_3{)}_4{]}^{2+}$ complex. The acidic medium effectively neutralizes Ammonia's ability to act as a ligand for the metal ion.