There are isomeric coordination compounds that have isomerism only within the ligand groups as the overall isomerism. These are Ligand Isomers. Bis(1,3-diaminopropane)platinum(2+) and bis(1,2-diaminopropane) platinum(2+) are two ions that exhibit isomerism. The development of coordination chemistry required the presence of coordination molecules with the same formula but various ligand configurations. Isomers are two or more compounds that have the same formula but distinct atom configurations. Because isomers have various physical and chemical properties, it’s crucial to identify which isomer we’re dealing with if there are multiple isomers.

Isomers are compounds that have the same molecular formula but various structural formulae, and they don’t always have the same properties. Stereoisomers, enantiomers, and geometrical isomers are among the various types of isomers. Structural isomerism and stereoisomerism are the two basic types of isomerism (spatial isomerism). 

Ligands

A ligand is an ion or molecule (functional group) that forms a coordination complex by binding to a central metal atom. Metal–ligand bonding can be either covalent or ionic in nature. Formal donation of one or more of the ligand’s electron pairs, frequently through Lewis Bases, is required for bonding with the metal. Lewis bases are considered ligands. However, Lewis acidic “ligands” have been observed in rare situations. Ligand substitution rates, ligand reactivity, and redox are all factors that influence the reactivity of the central atom in a complex. In many practical domains, such as bioinorganic and environmental chemistry, homogeneous catalysis, and pharmaceutical chemistry, ligand selection is crucial. Charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons provided to the metal are all used to classify ligands. The ligands are considered as electron donors, whereas the metals are viewed as electron acceptors, resulting in Lewis bases and Lewis acids, respectively. The neutral molecules water (H2O), ammonia (NH3), and carbon monoxide (CO), as well as the anions cyanide (CN–), chloride (Cl–), and hydroxide, are examples of frequent ligands (OH–). Cations (e.g., NO+, N2H5+) and electron-pair acceptors are occasionally used as ligands.

Structural Isomers

• The isomers in solvate or hydrate isomerism have the same composition but differ in the number of solvent ligand molecules and the counter ion in the crystal lattice.
• Ionisation isomerism occurs when two isomers have the same composition but produce distinct ions in solution. This isomerism happens when the complex’s centre ion is also a potential ligand.
• Ambidentate ligands that can bind in many locations cause linkage isomerism. NO2 can bond to a metal at either the N or O atoms, for example.
• Both positive and negative ions of a salt are complex ions in coordination isomerism, and the two isomers differ in the distribution of ligands between the cation and the anion.

Explanation

Ligand isomerism in coordination complexes is a type of structural isomerism caused by the presence of ligands that can take on distinct isomeric forms. Ligand isomerism is determined by the position isomer of the ligand. There are isomeric coordination compounds that have isomerism only within the ligand groups as the overall isomerism. This isomerism is a sort of isomerism that is unique. Diamino propane, for example, can have amine groups in either the terminal (1,3-) or 1,2-positions. The ions bis(1,3-diaminopropane)platinum(2+) and bis(1,2-diaminopropane)platinum(2+) are another example of isomerism. Diaminopropane, for example, has two isomers that differ in the connection of the amine groups, 1,2-diaminopropane and 1,3-diaminopropane, resulting in two ligand isomers: 1,2-diaminopropane and 1,3-diaminopropane. 

Key points

• When the ligands have the same link, but the bonds are in different orientations relative to one another, stereoisomers form. 
• The two ligands are on the same side of the complex in cis molecules. Similar ligands are on the opposite sides of the molecules in trans molecules. Stereoisomerism does not exist in tetrahedral compounds.
• The isomer is called facial or fac, when three identical ligands fill one face. The isomer is said to be meridional or mer, if all three ligands and the metal ion are in the same plane.
• When a molecule is not superimposable with its mirror image, it is called optical isomerism.
• The chemical composition of structural isomers is the same, but the linkages are different.

Conclusion

The ligand or ligands are neutral molecules or ions (or atoms or groups of atoms) that are directly connected to the core metal ion or atom in the complex ion by coordinate bonds. Ligand isomerism is a sort of structural isomerism that occurs when there are multiple isomeric forms of the same ligand. Ligands are typically thought of as electron donors that are attracted to the metal in the complex’s core. A ligand is any substance capable of donating a pair of electrons to a metal. A ligand might be a neutral molecule or a negatively or positively charged ion. Ligands can be neutral or negatively charged entities that have accessible electron pairs.