Question

The nickel(II) ion exists as \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) in aqueous solution. Why is such a solution acidic? As part of your answer include a balanced equation depicting what happens when \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) interacts with water.

Short answer

The solution is acidic due to ionization of water ligands, forming \(\text{H}_3\text{O}^+\).

Step-by-step solution

Understand the Complex Ion

The complex ion \(\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+}\) consists of a central nickel ion surrounded by six water molecules acting as ligands. In this structure, nickel is in the +2 oxidation state.

Consider the Nature of Water Ligands

Water molecules are neutral but can act as weak acids, where the hydrogen atoms can be donated as protons (\(\text{H}^+\)). In a coordination complex like \(\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+}\), the central metal ion can polarize the O-H bonds in water.

Predict the Interaction with Water

The positive charge of \(\text{Ni}^{2+}\) can increase the bond polarization in the water molecules, making it easier for an \(\text{H}^+\) ion to be released from a water ligand, thus forming hydronium ions \(\text{H}_3\text{O}^+\).

Write the Balanced Chemical Equation

When \(\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+}\) interacts with water, it can be represented by the following reaction: \[\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+} + \text{H}_2\text{O} \rightleftharpoons \left[\text{Ni}\left(\text{H}_2\text{O}\right)_5\left(\text{OH}\right)\right]^{+} + \text{H}_3\text{O}^{+}\] This shows that the release of \(\text{H}^+\) leads to the formation of hydronium, rendering the solution acidic.

Key concepts

Coordination Complex

A coordination complex involves a central metal ion that is bonded to surrounding molecules or ions, known as ligands. These ligands can be anything with lone pairs of electrons that can form coordinate bonds with the metal ion.
In the case of the nickel(II) ion, the complex is represented by \(\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+}\). This complex consists of a nickel ion surrounded by six water molecules functioning as ligands.
These water molecules coordinate through their oxygen atom's lone pairs, holding the structure intact via coordinate covalent bonds. The central nickel is in the +2 oxidation state, giving the overall complex a 2+ charge.

• This means that all six water molecules are acting together to stabilize the charged nickel ion by donating electron density.

This process of forming such stable complexes involves various interactions, including electrostatic attractions and orbital overlaps. This makes coordination complexes a fundamental concept in understanding how metal ions behave in aqueous solutions.

Nickel(II) Ion

The nickel(II) ion plays an integral role in forming the coordination complex by acting as the central metal ion. In this state, nickel has a positive charge of +2, denoted as \(\text{Ni}^{2+}\).
Nickel's oxidation state is a crucial aspect because it affects how the nickel interacts with water molecules. When nickel is in its +2 state, it becomes a powerful attractor for the electron-rich oxygen of water molecules.

• The strength of this attraction helps in forming strong coordinate covalent bonds.
• This results in the coordination of nickel by water molecules to form \(\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+}\).

Such interactions highlight the ability of metal ions like nickel to form stable and intricate coordination complexes.

Hydronium Ion Formation

When the complex \(\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+}\) interacts with water, the polarization of the water molecule bonds becomes significant. The nickel ion, with its positive charge, can enhance this polarization, making the oxygen and hydrogen atoms in water more distinct in their charges.
This polarity facilitates the release of a hydrogen ion, forming a hydronium ion \(\text{H}_3\text{O}^+\), while the original water ligand transforms into a hydroxide ligand bound to nickel:

• This transformation is expressed by the chemical equation:

\[\left[\text{Ni}\left(\text{H}_2\text{O}\right)_6\right]^{2+} + \text{H}_2\text{O} \rightleftharpoons \left[\text{Ni}\left(\text{H}_2\text{O}\right)_5\left(\text{OH}\right)\right]^{+} + \text{H}_3\text{O}^{+}\]
The formation of hydronium ions in solution means that it becomes acidic since hydronium ions define acidity in aqueous solutions.
This process illustrates a common way metal-ligand interactions can influence the acidity of metal ion solutions.