Problem 47 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The geometry of \(\mathrm{Ni}(\mathrm{CO})_{4}\) and \(\mathrm{Ni}\left(\mathrm{PPh}_{3}\right)_{2} \mathrm{Cl}_{2}\) are (a) both square planar (b) tetrahedral and square planar, respectively (c) both tetrahedral (d) square planar and tetrahedral, respectively

Step-by-Step Solution

Answer

(b) Tetrahedral and square planar, respectively.

Step 1: Determine Geometry of Ni(CO)_4

Ni(CO)_4 is a nickel carbonyl complex, where carbon monoxide (CO) is a strong field ligand. The nickel in this complex is in the zerovalent oxidation state, meaning it has an electronic configuration of \([Ar]3d^{10}4s^0\). CO ligands facilitate the pairing of electrons. Due to the configuration and nature of CO, Ni(CO)_4 forms a tetrahedral geometry.

Step 2: Determine Geometry of Ni(PPh_3)_2Cl_2

In the complex \(\mathrm{Ni}\left(\mathrm{PPh}_{3}\right)_{2}\mathrm{Cl}_{2}\), nickel is typically in the +2 oxidation state, leading to an electronic configuration of \([Ar]3d^8\). Phosphines (PPh_3) and chloride ions are ligands here. This configuration typically favors a square planar geometry for \(\mathrm{Ni}(II)\) complexes in particular environments, like with the presence of soft phosphine ligands.

Step 3: Compare the Geometries

From the previous steps, the geometry of \(\mathrm{Ni}\left(\mathrm{CO}\right)_{4}\) is tetrahedral and \(\mathrm{Ni}\left(\mathrm{PPh}_{3}\right)_{2}\mathrm{Cl}_{2}\) is square planar. Therefore, the correct option reflecting these configurations is (b) tetrahedral and square planar, respectively.

Key Concepts

Molecular Geometry

In coordination chemistry, understanding molecular geometry is essential because it determines many physical and chemical properties of the complex. The molecular geometry of a coordination complex depends on the number and arrangement of the ligands around the central metal atom.
Ligands are the atoms, ions, or molecules attached to the central metal atom. Depending on their orientation, they influence the 3D shape of the complex. For instance, in the case of Ni(CO)_4, the ligand CO leads to a tetrahedral geometry, while for Ni(PPh_3)_2Cl_2, soft phosphine ligands and chloride ions result in a square planar configuration. The shape of these complexes is often determined by the number of electron pairs around the metal and the types of ligands. Factors like steric effects, electronic effects, and ligand size can influence these geometries.

Ligand Field Theory

Ligand field theory is a powerful model used to describe the bonding, orbital arrangement, and properties of transition metal complexes. It is an extension of crystal field theory and helps predict the splitting of d-orbitals in different geometrical settings. This prediction is crucial for determining the geometry of coordination compounds.

• Strong field ligands, such as CO, lead to a greater splitting of d-orbitals.
• Weak field ligands, such as Cl, result in smaller splitting.

In a tetrahedral complex like Ni(CO)_4, strong field ligands pair with electrons, affecting the electronic distribution and stabilizing the tetrahedral geometry. On the other hand, a square planar complex like Ni(PPh_3)_2Cl_2 features specific electron pairing and repulsion, stabilizing the planar geometry.
Ligand field theory helps us understand how ligands' nature influences the electronic arrangement and geometry of the complex.

Electronic Configuration

The electronic configuration of the central metal atom plays a crucial role in determining the geometry and stability of coordination compounds. It provides insight into the oxidation state and distribution of electrons in the metal's d-orbitals.
For instance, nickel in Ni(CO)_4 is in the zero oxidation state, giving a configuration of [Ar]3d^{10}4s^0. In this state, all d-orbitals are completely filled, facilitating the adoption of a tetrahedral shape as the CO ligands interact with the metal. In contrast, nickel in Ni(PPh_3)_2Cl_2 typically exhibits a +2 oxidation state, resulting in an electronic configuration of [Ar]3d^8. Here, only some d-orbitals are filled, creating a scenario favorable for square planar geometry due to the presence of phosphine and chloride ligands. Thus, the electronic configuration helps predict both the geometry and the reactivity of coordination complexes by showing how electrons are distributed within the central metal.