Question

Consider the complex ion \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Cl}_{2}(\mathrm{OH})_{2}\right]^{2-}\). (a) Identify the ligands and their charges. (b) What is the oxidation number of nickel? (c) What is the formula for the sodium salt of this ion?

Short answer

Answer: The ligands in the complex ion are Water (H2O) with a charge of 0, Chloride (Cl-) with a charge of -1, and Hydroxide (OH-) with a charge of -1. The oxidation number of nickel is +2. The formula for the sodium salt of this complex ion is Na₂[Ni(H₂O)₂Cl₂(OH)₂].

Step-by-step solution

(a) Identifying Ligands and Charges

Ligands are ions or molecules that bind to a central metal atom or ion. They are often Lewis bases which donate electron pairs to the metal ion. We have Water (H2O), Chloride (Cl-), and Hydroxide (OH-) as potential ligands in this complex ion. The charges of these ligands are as follows: 1. Water (H2O) is a neutral molecule with 0 charge. 2. Chloride (Cl-) is an anion with a -1 charge. 3. Hydroxide (OH-) is an anion with a -1 charge.

(b) Oxidation Number of Nickel

To find the oxidation number of nickel, we must consider the charge balance of the entire complex ion. Since it has a net charge of 2-, we know the charge of the nickel ion plus the total charge of the ligands must equal -2. Let's denote the oxidation number of nickel as "x". The equation for charge balance in this complex ion is: x + (2 * 0) + (2 * -1) + (2 * -1) = -2 Solving for x: x - 2 - 2 = -2 x = +2 Thus, the oxidation number of nickel is +2.

(c) Sodium Salt Formula

To find the formula of the sodium salt of this complex ion, we need to determine how many sodium ions are needed to counterbalance its charge. Since the complex ion has a charge of 2-, we need two sodium ions with charges of +1 each to neutralize the overall charge. Therefore, the formula for the sodium salt of this complex ion is: \(\mathrm{Na}_{2}\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}_{2}(\mathrm{OH})_{2}\right]\)

Key concepts

Ligands and Charges

Ligands are critical components in the formation of complex ions, which are central to many reactions and compounds in chemistry. They are atoms, ions, or molecules that donate electron pairs to a central metal atom or ion, forming a coordinate covalent bond. The nature of ligands plays a significant role in the properties of the resulting complex ion, such as its color, reactivity, and magnetic properties.

In our example, the complex ion \(\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}_{2}(\mathrm{OH})_{2}\right]^{2-}\), contains water (H2O), chloride (Cl-), and hydroxide (OH-) ligands. Understanding their charges is essential:

• Water, in this context, is a neutral molecule with no charge.
• Chloride has a negative charge, written as Cl-.
• Hydroxide also carries a negative charge, noted as OH-.

It's important to remember that the charge of a ligand contributes to the overall charge of the complex ion. Neutral ligands do not affect the overall charge, whereas negatively charged ligands add to the negative charge of the complex. In our example, both the chloride and hydroxide ligands contribute to the complex ion's net charge.

Oxidation Number Determination

Determining the oxidation number of a metal in a complex ion is crucial for understanding its chemistry, as it influences properties like the complex's reactivity and potential reactions. The oxidation number is essentially the charge that the metal would have if all ligands were removed along with their electron pairs.

To determine the oxidation number, we balance the charges from the metal and ligands to match the complex ion's overall charge. In our example, the complex ion \(\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}_{2}(\mathrm{OH})_{2}\right]^{2-}\) has an overall charge of 2-. Considering the contributions from the ligands, we can set up an equation where the unknown oxidation number of nickel (Ni) is 'x'.

The charge balance equation then looks like this:
\[x + (2 \times 0) + (2 \times -1) + (2 \times -1) = -2\]
Solving for 'x' gives us:\[x - 4 = -2\]
\[x = +2\]
So, the nickel in the complex ion has an oxidation number of +2, which is typical for many nickel complexes.

Formulas of Ionic Compounds

The formula of an ionic compound reflects the balance of charges between the constituent ions. Each formula unit must have a net charge of zero, meaning the total positive charge equals the total negative charge. To find this balance, cations (positively charged ions) and anions (negatively charged ions) combine in ratios that result in neutrality.

For complex ions, the process involves pairing the complex with counterions that balance out its charge. In the case of \(\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}_{2}(\mathrm{OH})_{2}\right]^{2-}\), the negative charge is 2-, requiring counterbalancing cations. Sodium (Na+) is often used as a counterion due to its +1 charge and prevalence. To neutralize our complex ion's 2- charge, we need two sodium ions, leading to the salt's formula:
\[\mathrm{Na}_{2}\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}_{2}(\mathrm{OH})_{2}\right]\]
This tells us that for each complex ion, two sodium ions are needed to maintain the required balance in the ionic compound.