\(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) is found to exist in two geometric isomers designated I and II, which react with oxalic acid as follows: $$\begin{aligned}\mathrm{I}+\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4} & \longrightarrow\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4}\right] \\\\\mathrm{II}+\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4} & \longrightarrow\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{HC}_{2} \mathrm{O}_{4}\right)_{2}\right]\end{aligned}$$ Comment on the structures of I and II.

Understanding the square planar geometry is vital to grasp why certain isomers in coordination chemistry behave the way they do. Let's tackle this concept which is often associated with transition metal complexes, such as those formed by platinum(II). This geometry involves a central metal atom surrounded by ligands positioned at the corners of a square. Picture a flat square table with the metal atom as the tabletop and four legs as the ligands; this is the essence of square planar geometry.

In terms of bonding, the metal atom in this configuration has eight electrons in its d-orbital which create a coordination number of four. This underpins the four ligands it can bond with. A key characteristic of this geometry is its symmetrical nature, which paves the way for geometric isomerism—compounds with the same formula but different spatial arrangements of the atoms.

These differences in arrangement lead to the fascinating property of having distinct isomers, each with unique physical and chemical properties. Isomers with the square planar geometry may exhibit what's called 'cis' and 'trans' configurations. In the 'cis' form, similar ligands are adjacent to each other, while in the 'trans' form, they are opposite each other. This distinct positioning crucially influences how the complex will react with other molecules, such as in ligand substitution reactions.

When it comes to coordination chemistry, platinum(II) complexes are some of the most well-studied and intriguing systems. The metal platinum, when in its +2 oxidation state, forms bonds readily with a variety of ligands, creating diverse coordination compounds. Among them, the square planar complexes take center stage due to their unique chemical behaviors and uptake in research and industrial applications.

These platinum(II) compounds have a notable tendency to undergo geometrical changes leading to the generation of geometric isomers. The nature of these platinum complexes is such that they exhibit a high degree of stability, which is why they are commonly found in catalysis and as active centers in certain pharmaceutical compounds, most famously anti-cancer drugs.

Understanding the platinum(II) complexes' structure-function relationship is essential. It underpins not only the compound's reactivity but also its biological interactions. The two classic examples of geometric isomers in these compounds, 'cis' and 'trans', provide fundamental insights into how similar atoms or groups of atoms can confer vastly different properties based on their arrangement around the central platinum atom.

Ligand substitution reactions are a cornerstone of coordination chemistry, where one ligand in a coordination complex is replaced by another. Such reactions are particularly relevant in platinum(II) chemistry due to the metal’s ability to form stable, yet reactive, complexes. But how do these ligand substitution reactions unfold?

In the simplest sense, these reactions can follow two general pathways: associative or dissociative. In an associative mechanism, a new ligand comes in and attaches to the metal center before an old ligand leaves, temporarily increasing the coordination number. Conversely, in a dissociative mechanism, a ligand departs first, creating a site of reactivity before a new ligand attaches.

The type of pathway such a reaction follows can be influenced by several factors, including the geometry of the complex. For instance, a square planar complex may be more inclined toward a dissociative mechanism because the symmetry and stability of the planar arrangement must first be disturbed to create an open site. The distinct behavior shown by the two isomers in the given exercise is a clear reflection of how their structural differences affect their reactivity in ligand substitution reactions, with one allowing a bidentate ligand to attach at a single site, while the other permits two monodentate ligands to attach.