Draw geometrical isomers of each of the following complex ions. a. \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{-}\) b. \(\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{I}_{2}{ }^{2+}\) c. \(\mathrm{Ir}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\) d. \(\mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}^{+}\)

Coordination compounds, also known as coordination complexes, play a pivotal role in the realm of chemistry. They consist of a central metal atom or ion bonded to a surrounding array of molecules or anions, known as ligands. These ligands possess 'donor atoms' with lone pairs of electrons, and they form coordinate bonds with the metal atom or ion by donating these pairs.

The formation of these compounds is fascinating because it includes the interaction between the orbitals of the metal and the ligands, creating a unique arrangement that can greatly affect the properties of the compound. One interesting application is the use of coordination compounds in various industries, including pharmaceuticals and materials science.

The coordination number is a term used to describe the number of points at which ligands are attached to the central atom or ion in a coordination compound. It is a critical factor in determining the geometry of the complex. For example, with a coordination number of six, an octahedral geometry is often observed. However, the coordination number doesn't just influence the shape; it can also impact the color, magnetism, and reactivity of the coordination compound. Understanding a complex's coordination number helps in predicting its physical and chemical properties, making it a key concept in inorganic chemistry.

Cis-trans isomerism is a form of stereoisomerism that's particularly intriguing within coordination chemistry. This type of isomerism occurs when two identical or similar ligands are oriented differently around the central atom in an octahedral or square planar complex. In the 'cis' form, the like ligands are adjacent to each other, while in the 'trans' form, they are on opposite sides.

This seemingly small difference in spatial arrangement can lead to vastly varied properties, like solubility and reactivity. The classic example of cis-trans isomerism's importance is seen in the drug cisplatin, which is used in cancer treatment, whereas its trans isomer is ineffective in treating cancer.

Geometry and Ligand Arrangement

Octahedral complexes are six-coordinated structures forming an octahedron shape. The six ligands are symmetrically placed around the metal center at 90-degree angles from each other. This arrangement is one of the most common geometries in coordination chemistry and offers a perfect stage for the study of isomerism.

Geometrical Isomers

Given their symmetric property, octahedral complexes can give rise to geometrical isomers. The diverse spatial layouts of the ligands can result in compounds with distinct chemical properties despite having the same formula. These isomers include the cis and trans forms, as well as the less commonly discussed facial (fac) and meridional (mer) isomers, which refer to the arrangement of three identical ligands in these frameworks. Such isomerism plays a crucial role in the biological activity and chemical application of these compounds.