Problem 17 - Chemistry The Science in Context

Question

Which ion is the counterion in the coordination compound \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{CN})_{4}\right] ?\)

Step-by-Step Solution

Answer

Answer: The counterion in the given coordination compound is the sodium (Na) ion.

Step 1: Identify the central atom and ligands in the coordination compound

In the given compound, \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]\), the central atom within the square bracket is \(\mathrm{Zn}\) (zinc) and the ligands surrounding it are four \(\mathrm{CN}^-\) (cyanide) ions.

Step 2: Calculate the charge on the central atom and the ligands

Zinc (Zn) typically has a charge of +2, while the cyanide ligands each have a charge of -1. This means that the overall charge on the complex within the square bracket is +2 - (4 × -1) = +2 + 4 = +6.

Step 3: Balance the charge with counterions

To balance the +6 charge on the complex, we need to have counterions with a total charge of -6. In this compound, we have two sodium (Na) ions, each with a charge of +1. Therefore, the total charge on the sodium ions is +2, and they are the counterions in the given coordination compound. So, the counterion in the coordination compound \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]\) is the sodium (Na) ion.

Key Concepts

Ligands in Coordination Compound

In coordination chemistry, ligands play a crucial role as they are the molecules or ions that form bonds with a central metal atom, usually through donation of electron pairs. The nature of the ligands can significantly affect the properties and reactivity of the coordination compound.
For the coordination compound \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]\), the ligands involved are \(\mathrm{CN}^-\), also known as cyanide ions. Each cyanide ion has a charge of -1, and in this compound, there are four cyanide ions attached to the central zinc (Zn) atom.

• The bonding between the cyanide ligands and the zinc is achieved through the lone electron pair on the carbon atom of each \(\mathrm{CN}^-\), which coordinates to the zinc atom.
• The presence of these ligands provides negative charges that must be balanced by the overall coordination complex, influencing the geometry and electronic structure.

Coordination compounds can exhibit varied geometries such as tetrahedral, square planar, and octahedral, typically dictated by the ligands and central atom combination.

Central Atom in Complexes

The central atom in a coordination complex is the metallic atom around which ligands are coordinated. It is typically a transition metal because these metals have vacant d-orbitals that can accommodate pairs of electrons from the ligands.
In the compound \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]\), zinc (Zn) serves as the central atom:

• Zinc, a transition metal, has inherent properties that allow it to stabilize multiple bonding partners – in this case, the four cyanide ions.
• The geometry of this complex is likely tetrahedral due to the coordination number of 4, which allows the cyanide ions to effectively surround the zinc atom.

The central atom's oxidation state (often determined from the charge balance with ligands) is +2 for zinc in this complex. Understanding the role of the central atom helps predict the complex’s stability, magnetic properties, and reaction tendencies.

Counterion Identification

Counterions are species present in the compound that balance the charge, but are not part of the inner coordination sphere. They do not bond directly with the central atom but are crucial for maintaining charge neutrality, as they are typically located outside the coordination complex.
In the case of \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]\), sodium ions \((\mathrm{Na}^+)\) serve as the counterions:

• The overall charge of the inner complex \(\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]\) is determined by the charges of the zinc and the cyanide ligands, resulting in an overall charge of -2.
• To balance this negative charge, two sodium ions (each with a +1 charge) are present, counterbalancing the -2 charge from the inner complex.

Understanding counterions helps in predicting compound solubility, electrical conductivity, and overall charge distribution in coordination chemistry.