Why is PtC{l_4}^{2 - } square planar?

The PtCl42− ion is square planar because of its electron configuration and the geometry that minimizes the repulsion between its electrons.

First, let's look at the electron configuration of a platinum (Pt) atom. Platinum is in the 5th period of the periodic table and has an atomic number of 78. The electron configuration of a neutral platinum atom is [Xe] 4f145d96s1, but when it forms an ion with a 2- charge (Pt2+), two electrons are removed. So, the electron configuration of Pt2+ is [Xe] 4f145d8.

Now, let's consider the PtCl42− ion. This ion has a 2- charge, which means it has gained two extra electrons compared to Pt2+. When these two extra electrons are added to the electron configuration of Pt2+, they fill the 5d subshell. The electron configuration of PtCl42− is [Xe] 4f145d10.

In a square planar complex, the central metal atom/ion is surrounded by four ligands in the same plane. The d orbitals of the central metal are involved in bonding with the ligands. In the case of PtCl42−, the 5d orbitals are used for bonding. When you have 10 electrons in the 5d orbitals, they fill all available d orbitals. This results in a very stable electronic configuration, known as a filled d subshell.

In a square planar geometry, the ligands occupy the four equatorial positions, and the d orbitals with their electrons are arranged in a square plane perpendicular to the ligand plane. This arrangement minimizes electron repulsion because the electrons are as far apart from each other as possible within the same energy level, leading to a stable and energetically favorable square planar geometry.

In summary, the square planar geometry of PtCl42− is a result of the electron configuration of the platinum ion (Pt2+) and the need to minimize electron repulsion by filling the 5d orbitals with 10 electrons in a square plane.