Problem 39 - A Complete Resource Book in Chemistry for JEE Main

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 (c) both tetrahedral (d) square planar and tetrahedral

Step-by-Step Solution

Answer

\(\mathrm{Ni(CO)_4}\) is tetrahedral, \(\mathrm{Ni(PPh_3)_2Cl_2}\) is square planar, so the answer is (b).

Step 1: Determine the Coordination Geometry of Ni(CO)_4

The compound \(\mathrm{Ni(CO)_4}\) is a complex that contains nickel in the 0 oxidation state where \(\text{CO}\) acts as a neutral ligand. Due to having a coordination number of 4 and carbon monoxide's ability as a strong field ligand, this compound adopts a tetrahedral geometry for electronic stability.

Step 2: Determine the Coordination Geometry of Ni(PPh_3)_2Cl_2

\(\mathrm{Ni(PPh}_3\mathrm{)_2Cl}_2\) consists of nickel with ligands that include two phosphine ligands and two chloride ions. In this compound, nickel is in the \(+2\) oxidation state. Typically, \(\text{Ni}^{2+}\) with a coordination number of 4 and involving ligands like \(\text{PPh}_3\) and \(\text{Cl}^-\) tends to form square planar geometry due to the preference of \(\text{d}^8\) metal centers to adopt this arrangement.

Key Concepts

Tetrahedral Geometry

Tetrahedral geometry is a common structural arrangement seen in coordination complexes where four atoms or ligands symmetrically surround a central atom. In this geometry, the bond angles between any two ligands are approximately 109.5 degrees.

This arrangement is prevalent in complexes where the central metal has a d orbital configuration that allows for symmetrical distribution of the ligands. For example, in the compound \(\mathrm{Ni(CO)\_4}\), nickel is in the zero oxidation state, with carbon monoxide acting as a neutral ligand. Because of carbon monoxide's strong field electronic influence, it stabilizes the complex in a tetrahedral shape. This creates a compact and electronically favorable arrangement, minimizing repulsion between the bonding electron pairs.

• It is often seen with central atoms that have a coordination number of 4 and exhibit high symmetry.
• Tetrahedral geometry is less common in \(\mathrm{d^8}\) metal centers due to electronic arrangements that typically favor other types of geometries.
• The tetrahedral shape is versatile and found in non-transition and transition metal complexes alike.

Square Planar Geometry

Square planar geometry is another important coordination geometry characterized by a planar arrangement where four ligands bond at 90-degree angles to the central metal atom.

Typically seen in complexes with a \(\mathrm{d^8}\) electronic configuration like \(\mathrm{Ni^{2+}}\), this geometry provides optimal spatial separation of electronic charges. A prime example is the compound \(\mathrm{Ni(PPh_3)_2Cl_2}\). Here, nickel is in the +2 oxidation state. This oxidation state contributes to the formation of a square planar arrangement, thanks to the favorable overlap of ligand orbitals with the metal's available \(\mathrm{d_{x^2-y^2}}\) orbital.

• Square planar is especially stable for metals with \(\mathrm{d^8}\) electron configurations like platinum or nickel.
• This geometry is usually accompanied by specific ligands that stabilize the \(\mathrm{d^8}\) configuration, leading to planar separation.
• Commonly observed in complexes of transition metals, square planar geometry also impacts the reactivity and electronic properties of the metal center.

Ligand Field Theory

Ligand field theory (LFT) is crucial in understanding the electronic structure of metal complexes. It builds upon crystal field theory by considering covalent as well as ionic bonding aspects, detailing how ligands influence the distribution and energy of d orbitals in the central metal. This theory explains why different geometrical arrangements occur in coordination complexes.

LFT helps determine whether a complex will adopt a tetrahedral or square planar geometry. For example, in \(\mathrm{Ni(CO)_4}\), strong field ligands like carbon monoxide reduce electron repulsion within the orbitals, favoring a tetrahedral shape. Conversely, complexes like \(\mathrm{Ni(PPh_3)_2Cl_2}\) with weaker field ligands often form square planar geometry due to the d orbital splitting pattern, particularly in the \(\mathrm{d^8}\) configuration.

• Ligand field theory explains the magnetic and spectral properties of metal complexes.
• This theoretical framework helps predict the geometry of the complex based on ligand types and metal electron configurations.
• Understanding LFT is crucial when predicting the stability and electronic transitions of metal complexes.