Question

In which one of the following transition metal complexes, does the metal exhibit zero oxidation state? (a) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) (b) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{X}_{3}\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) (d) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{SO}_{4}\)

Short answer

The complex with zero oxidation state is \( [\mathrm{Ni} (\mathrm{CO})_4] \).

Step-by-step solution

Identify the Metal and Ligands

First, identify the metal and the ligands in each of the given complexes:(a) \( [\mathrm{Ni} (\mathrm{CO})_4] \) where the metal is Ni and the ligand is CO.(b) \( [\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]\mathrm{X}_3 \) where the metal is Fe and the ligand is \( \mathrm{H}_2\mathrm{O} \), and \( \mathrm{X} \) represents the anion.(c) \( [\mathrm{Co}(\mathrm{NH}_3)_6]\mathrm{Cl}_3 \) where the metal is Co and the ligand is \( \mathrm{NH}_3 \).(d) \( [\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]\mathrm{SO}_4 \) where the metal is Fe and the ligand is \( \mathrm{H}_2\mathrm{O} \).

Check Ligand Properties

Next, analyze the properties of the ligands:- CO is a neutral ligand and is known to act as a strong field ligand and can stabilize low oxidation states, even zero.- \( \mathrm{H}_2\mathrm{O} \) and \( \mathrm{NH}_3 \) are neutral ligands that do not stabilize zero oxidation state as effectively as CO.

Determine Oxidation States

Calculate the oxidation state for each metal in the complexes:(a) \( [\mathrm{Ni}(\mathrm{CO})_4] \) has Ni in a zero oxidation state because CO is neutral and contributes no charge.(b) \( [\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]\mathrm{X}_3 \): Fe must be at a +3 oxidation state to balance the charges with the \( \mathrm{X}_3 \) anions.(c) \( [\mathrm{Co}(\mathrm{NH}_3)_6]\mathrm{Cl}_3 \): Co must also be at a +3 oxidation state to balance the charges from the 3 Cl^- ions.(d) \( [\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]\mathrm{SO}_4 \): Fe is in a +2 oxidation state to balance the negative charge of \( \mathrm{SO}_4^{2-} \).

Conclude the Zero Oxidation State Complex

From the calculations, (a) \( [\mathrm{Ni}(\mathrm{CO})_4] \) is the only complex where the metal is in a zero oxidation state due to the neutrality of the CO ligands stabilizing a zero charge state on nickel.

Key concepts

Oxidation States

An oxidation state is a conceptual tool to understand electron transfer in chemical compounds. In transition metal complexes, understanding oxidation states is crucial because it dictates the electronic arrangement, reactivity, and properties of the complex.
The oxidation state of a metal is determined by the overall charge balance between the metal atom and the ligands bound to it. For example, consider the complex \([\mathrm{Ni}(\mathrm{CO})_4]\). Here, carbon monoxide (CO) is a neutral ligand, contributing no charge. Therefore, the oxidation state of nickel in this complex is zero because there's no additional charge to balance.
Commonly, oxidation states vary from negative values up to the maximum oxidation number possible for a metal, but a zero oxidation state is special due to its precise electron balance. It mostly occurs in metal carbonyls like \([\mathrm{Ni}(\mathrm{CO})_4]\) where CO neutral ligands effectively stabilize the metal in this unique electronic arrangement.

Coordination Chemistry

Coordination chemistry focuses on the properties and structures of complexes formed by metals and their surrounding ligands. Transition metal complexes consist of a central metal atom bonded to a set of molecules or ions, termed ligands, which donate electron pairs to the metal.
The structure and reactivity of these complexes significantly depend on the nature and number of ligands in "coordination" with the metal. A pivotal example is \([\mathrm{Ni}(\mathrm{CO})_4]\), where carbon monoxide acts as a ligand. This complex has a tetrahedral geometry with four CO ligands symmetrically surrounding one nickel atom.

• Coordination number: This represents the number of ligand attachments around a central metal. In \([\mathrm{Ni}(\mathrm{CO})_4]\), the coordination number is 4, indicating four CO ligands are bound to nickel.
• Stereochemistry: The spatial arrangement of different ligands influences the complex's steric and electronic properties, leading to various geometrical shapes such as octahedral, square planar, etc.

Coordination chemistry plays a central role in understanding many chemical processes, including catalysis, material synthesis, and biological systems.

Ligand Properties

Ligands are critical components in coordination complexes as they influence the metal's properties, including its oxidation state, magnetic behavior, and overall stability. A ligand's nature—such as its ability to donate or withdraw electron density—determines how it interacts with the metal.
In the complex \([\mathrm{Ni}(\mathrm{CO})_4]\), carbon monoxide (CO) serves as a neutral and strong field ligand. Such ligands are capable of stabilizing the metal in low or zero oxidation states because they engage in effective \pi-backbonding. This backbonding involves the transfer of electron density from filled metal orbitals back to the ligand's CO anti-bonding orbitals, resulting in enhanced stability of the zero oxidation state.
Conversely, ligands like \( \mathrm{H}_2\mathrm{O} \) and \( \mathrm{NH}_3 \) are weaker field and better suited for higher oxidation states, due to less effective \pi-backbonding.

• Neutral ligands: These ligands like CO and \mathrm{H}_2 \mathrm{O} contribute no charge.
• Effect on properties: The type of ligands can significantly influence a metal's coordination environment, affecting stability, solubility, and reactivity.

Understanding ligand properties is crucial in predicting and manipulating the behavior of transition metal complexes in various applications.