Square Planar Shape of Molecules

The Square Planar molecular geometry tells us about the stereochemistry of various molecular compounds. When there are four bonds and two lone pairs on the molecule’s central atom, this shape is Square Planar. Xenon tetrafluoride (XeF4) is a molecular compound with a square planar structure. The XeF4 molecule comprises six equally spaced sp3d2 (or d2sp3) hybrid orbitals arranged at 90° angles.

VSEPR Theory

• VSEPR theory is a model that helps us predict the geometrical structure of various molecular compounds with the help of the no. of lone pairs surrounding the central atom.
• The theory assumes that the molecule tries to achieve a geometry in which the repulsion between the electrons in the atom’s valence shell is minimum.
• According to VSEPR Theory, a square planar molecular geometry corresponds to an AB4C2 molecule.

Bond angle

• The bond angle in square planar is 90 degrees.
• In a square planar-shaped molecule, the lone pairs are on opposite sides of the central atom of the molecule, with the four bonded neighbouring atoms all lying in one plane. The bonding pairs experience equal repulsion from both lone pairs, so the four bonds are split on a 2-D plane with 90 degrees of separation between each bonded pair. 

Hybridisation

• The hybridisation of atomic orbitals forms a new set of orbitals called hybridised orbitals to attain better stability.
• The hybrid orbitals have some characteristic geometry from the time of formation. But some factors affect the geometry and therefore alter the overall shape of the molecules. 
• Some aspects are the number and position of lone pairs, type of substituent attached to the central atom, etc.
• Square planar is one of the geometries characterising a specific hybridisation of atoms. It is a planar structure having one central atom bonded to 4 other atoms. The hybridisation that is achieved by this shape is dsp2.
• The energy of 3d-orbitals of an atom is either similar to 3s and 3p or 4s and 4p. Thus it makes the d-orbitals hybridise with both groups. If they hybridise with the former group, they form inner orbital complexes, as the principal quantum number for all the orbitals is the same.
• The hybridisation of 3d orbitals with 4s and 4p orbitals is termed outer orbital complexes, as the principal quantum number of the orbitals except the d-orbitals is one unit higher.

Electron Geometry

• The shape of the orbitals in a square planar is octahedral. 
• Two orbitals contain lone pairs of electrons on opposite sides of the central atom. The four bonded atoms lie on the 2-D plane giving the compound a square planar shape.

Examples 

XeF4  : Molecular Geometry

• It becomes very easy to predict the geometry of a molecule if we know the Lewis structure of that given molecule.
• As we know, Xenon has got two lone pairs of electrons. To minimise the electron repulsion, it will keep its lone pair in a perpendicular plane to help it avoid the repulsion forces.
• Now, as there are four Fluorine atoms bonded to Xenon, it will achieve a geometry in which all these four atoms are in one plane, thus forming a square planar structure for the molecule.
• The electronic geometry of XeF4 is Octahedral, while the molecule geometry is square planar.

Other Chemical compounds which have these types of molecular geometry are- 

[Ni(CN)4]2−  and  [Pt(Cl)4]2−

The square planar shape is also possible when the central atom exhibits sp3d2 hybridisation with two lone pairs at the axial positions.

For example, the molecule XeF4 has a square planar shape and has two lone pairs placed at axial positions to minimise the repulsive forces.

Conclusion: 

According to VSEPR Theory, a square planar molecular geometry corresponds to an AB4C2 molecule. The bond angle in square planar is 90 degrees. The lone pairs are on opposite sides of the central atom in a square planar, keeping the four bonding electron pairs on one plane. The remaining four atoms connected to the central atom give the molecule a square planar shape—the shape of the orbitals in an octahedral.