Question

Carbon monoxide, CO, is an important ligand in coordination chemistry. When CO is reacted with nickel metal, the product is \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right],\) which is a toxic, pale yellow liquid. (a) What is the oxidation number for nickel in this compound? (b) Given that \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) is a diamagnetic molecule with a tetrahedral geometry, what is the electron configuration of nickel in this compound? (c) Write the name for \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) using the nomenclature rules for coordination compounds.

Short answer

The oxidation number of nickel in \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) is 0. The electron configuration of nickel in this compound is \([\mathrm{Ar}]\,3d^{10}\). The name for the coordination compound is tetracarbonylnickel.

Step-by-step solution

Part (a): Determining the oxidation number of nickel

To determine the oxidation number of nickel in the molecule, we need to consider the charge of its ligands, CO. In this case, CO is a neutral ligand, as it doesn't have any charge. Since the molecule itself is neutral (not charged), the oxidation number of nickel is 0, as it is surrounded by neutral ligands.

Part (b): Determining the electron configuration of nickel

Nickel is a transition metal with an atomic number of 28. The electron configuration for neutral nickel is \([\mathrm{Ar}]\,3d^8\,4s^2\). To explain the diamagnetic property of \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\), we need to analyze the d-orbitals. Since the molecule is diamagnetic, all the d-orbitals must be paired to ensure no unpaired electrons are present. The ligands occupy four orbitals in \((sp^3)-d\) hybridization. The neutral nickel has two electrons in the 4s orbital and eight electrons in the 3d orbital. The four CO ligands donate four electrons to the nickel atom, filling the 3d orbital and leaving the 4s orbital empty. The electron configuration of nickel in this compound is thus \([\mathrm{Ar}]\,3d^{10}\).

Part (c): Naming the coordination compound

According to the nomenclature rules for coordination compounds, we name the ligands first, followed by the metal name. When naming ligands, neutral ligands are named as they are, while anionic ligands use the suffix "-o." In this case, the ligand is CO, which is neutral, so its name remains unchanged. The metal is nickel, and since the oxidation number is 0, we don't need to add a Roman numeral after the metal name, as we normally would if it had a nonzero oxidation number. Thus, the name for \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) is tetracarbonylnickel.

Key concepts

Oxidation Number

In coordination chemistry, determining the oxidation number of a central metal is crucial. It helps us understand the electron distribution in the compound. Let's delve into how we determine the oxidation number for nickel in the compound \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\).
When analyzing coordination complexes, we need to consider the nature of the ligands. Ligands such as carbon monoxide (CO) are quite special as they are neutral and do not carry any charge. Therefore, when they bind to metals like nickel, they do not alter the oxidation state of the metal.
In \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\), each CO contributing no charge means that the entire molecule remains neutral. Consequently, the oxidation number of nickel is 0. This conclusion aids in understanding how electrons are organized in the compound and influences the property analysis of the entire structure.

Electron Configuration

The electron configuration is key to understanding the magnetic properties and shapes of coordination compounds. Nickel, a transition metal with an atomic number of 28, has an electron configuration of \([\mathrm{Ar}]3d^8 4s^2\) when in its neutral state, outside of a complex.
When nickel forms the \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) complex, it undergoes a reorganization to accommodate the four CO ligands. These ligands contribute electrons that occupy the nickel's orbitals. This results in the electron configuration changing to \([\mathrm{Ar}]3d^{10}\), where all d-orbitals are filled.
The filling of these d-orbitals leads to the molecule being diamagnetic. This means that all electrons are paired in the orbitals, ensuring there are no unpaired ones that would contribute to magnetic properties. This also aligns with the observed tetrahedral geometry of the complex, a common configuration for complexes with complete d subshells.

Nomenclature of Coordination Compounds

Naming coordination compounds involves a systematic approach based on the components of the compound. The name is constructed by listing the ligands first, followed by the metal center. For the complex \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\), we must apply these rules.
Ligands are named before the metal, and in the case of neutral ligands like CO, their names remain unchanged. Hence, we call it 'carbonyl'. Nickel is the central metal, and its name remains as is unless it requires a Roman numeral to indicate a positive oxidation state; however, a numeral is not needed here since the oxidation number of nickel is 0.
Therefore, the formal name for \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) is tetracarbonylnickel. Prefixes like 'tetra-' indicate the number of such ligands present, completing the systematic name for clear communication in chemical contexts.