Question

Which of the following shows dep \(^{2}\) hybridisation and a square planar geometry? (a) \(\mathrm{SF}_{6}\) (b) \(\mathrm{Br} \mathrm{F}_{5}\) (c) \(\mathrm{PCl}_{5}\) (d) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\)

Short answer

[Ni(CN)4]2- shows dsp^2 hybridisation and a square planar geometry.

Step-by-step solution

Identify the central atom's valence electrons and coordination number

First, identify the number of valence electrons for the central atom in each molecule or complex ion. The coordination number refers to the number of atoms or ligands directly bonded to the central atom.

Apply the VSEPR theory for predicting geometry

Use the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the molecular geometry based on the number of electron pairs (bonding and non-bonding) surrounding the central atom.

Determine the hybridisation

Determine the hybridisation of the central atom by considering both the electron domains around the atom and the predicted molecular geometry from Step 2.

Match the hybridisation and molecular geometry with options

Match the determined hybridisation and predicted molecular geometry for each option to find which one exhibits dsp^2 hybridisation and square planar geometry.

Key concepts

VSEPR Theory

Understanding the shape of molecules starts with the Valence Shell Electron Pair Repulsion (VSEPR) theory. The fundamental idea is that electron pairs, both bonding and non-bonding, will arrange themselves as far apart as possible around a central atom to minimize repulsion. This spatial arrangement of electron pairs dictates the molecular geometry of the molecule.
For example, a molecule with four electron pairs would typically form a tetrahedral shape to keep the electron pairs as distant from each other as possible. However, if some of these pairs are non-bonding ('lone pairs'), they will push the bonding pairs closer together, leading to variations such as a trigonal pyramidal or bent shape. Understanding VSEPR theory is key for predicting the geometry of molecules, which is necessary for determining hybridisation.

Valence Electrons

Valence electrons are the foundation for bonding in chemistry. They are the electrons in an atom's outermost shell and are involved in forming chemical bonds. The number of valence electrons determines how an atom will interact with others and contributes significantly to the chemical properties of the element.
When determining the molecular geometry or hybridisation, it's crucial to know the valence electrons count for the central atom. This count helps us infer how many electron pairs are available to form bonds or exist as lone pairs. For instance, in the context of hybridisation, an atom like carbon, with four valence electrons, can form four bonds or a combination of bonds and lone pairs, leading to various hybridisations such as sp3, sp2 or sp.

Coordination Number

The coordination number is significant in both chemistry and geometry. It refers to the total number of atoms or ligands directly bonded to a central atom in a complex or molecule. This number is critical for predicting the geometry and understanding the bonding arrangement around the central atom.
In more technical terms, the coordination number correlates with how many 'slots' are available for bonding around a central atom. A coordination number of four suggests four locations for chemical bonds, while six indicates a central atom that can be surrounded by up to six bonds. In the case of square planar geometry, typically seen in transition metal complexes, the coordination number is four.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. The shape of a molecule can affect its physical and chemical properties, such as reactivity, polarity, and phase of matter.
Some common geometries include linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. Each geometry is associated with a specific hybridisation of the central atom's orbitals, such as sp for linear, sp2 for trigonal planar, and sp3 for tetrahedral. In transition metal complexes, you may encounter geometries like square planar, which correlates with dsp^2 hybridisation. Depending on the arrangement of Bonding Groups and Lone Pairs, molecular geometry can vary greatly, even with the same number of regions of electron density, as predicted by the VSEPR theory.