Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

In the coordination compound \(\mathrm{K}_{4}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]\), the oxidation state of nickel is (a) \(-1\) (b) 0 (c) \(+1\) (d) \(+2\)

Step-by-Step Solution

Answer

The oxidation state of nickel is 0.

Step 1: Identify components of the complex

The coordination compound is \( \mathrm{K}_{4}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \). It contains potassium (K), nickel (\(Ni\)), and the cyanide ligand (\(\mathrm{CN}^{-}\)).

Step 2: Determine potassium's contribution

Potassium (\(\mathrm{K}\)) is a metal that commonly forms a \(+1\) charge. Since there are 4 potassium ions, the total charge contribution from potassium is \(+4\).

Step 3: Define the known charge of cyanide

The cyanide ion (\(\mathrm{CN}^{-}\)) has a charge of \(-1\). Since there are four cyanide ions in the complex \(\left(\mathrm{Ni}(\mathrm{CN})_{4}\right)\), their total charge is \(-4\).

Step 4: Calculate the charge balance

The overall compound is neutral, so the sum of the charges from the ions must equal zero: \[ x + 4(+1) + 4(-1) = 0 \] where \(x\) is the oxidation state of nickel. Simplifying, we have \[ x + 4 - 4 = 0 \] which reduces to \(x = 0\).

Step 5: Conclude with nickel's oxidation state

The oxidation state of nickel (\(Ni\)) in the compound is 0, following the calculation from charge balances in the complex.

Key Concepts

Oxidation State

The oxidation state of an element in a compound indicates the hypothetical charge that an atom would have if all bonds to atoms of different elements were fully ionic. For the compound \( \mathrm{K}_{4}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \), the oxidation state of nickel (\(Ni\)) is - our primary focus.

When we look at the potassium part, it has an oxidation state of "+1" which is standard in its compounds. Since there are four potassium ions, the total contribution is "+4".

The cyanide ion (\(\mathrm{CN}^{-}\)) contributes a "-1" charge. With four cyanide ions, their total contribution is "-4".

To figure out the oxidation state of nickel, the compound is known to be neutral overall (\(0\)) so: \[ x + 4 - 4 = 0 \] where \(x\) is the oxidation state of nickel. Solving this gives \(x = 0\). Thus, we determine the oxidation state of nickel is zero.

Charge Balance

Charge balance in coordination compounds is crucial for their stability and neutrality. In the case of \( \mathrm{K}_{4}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \), the overall charge must be zero to maintain neutrality.

• Cations such as potassium (\( \mathrm{K}^{+} \)) have positive charges, here it contributes a "+4" total charge from four potassium ions.
• An ions like cyanide (\(\mathrm{CN}^{-}\)) contribute negative charges; four of these contribute a "-4" total.

The sum of all charges should equal zero, compelling the oxidation state of nickel (\(Ni\)) to balance it, making its oxidation state zero. This careful balancing act ensures that the compound doesn't have any net charge, maintaining its stable configuration.

Ligands

Ligands play a fundamental role in coordination compounds. They are ions or molecules that can donate a pair of electrons to the central metal atom, forming a coordination bond. In the compound \( \mathrm{K}_{4}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \), the ligand involved is cyanide (\(\mathrm{CN}^{-}\)).

• Cyanide is known as a strong field ligand, meaning it exerts a significant crystal field splitting, affecting the electronic properties of the central metal, nickel in this case.
• Cyanide donates an electron pair, forming a coordination bond with nickel, thereby helping to determine the geometry and properties of the complex.

Recognizing ligands and their effects is crucial for understanding structural formations and electronic characteristics of coordination compounds.

Transition Metals

Transition metals, such as nickel in \( \mathrm{K}_{4}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \), are characterized by their ability to form various oxidation states and complex ions. They lie in the d-block of the periodic table and have partially filled d orbitals.

• The rich chemistry of transition metals allows them to form stable coordination compounds with different ligands due to their flexible oxidation states.
• These metals also participate in numerous physical and chemical processes due to their unique ability to engage in electron sharing through d orbitals.

The prominence of transition metals in chemistry is due to their versatile nature, enabling them to act as catalysts and participate in redox reactions essential to both industrial applications and biological systems.