Stereoisomerism

Cis and trans isomers are geometric isomers that differ in ligand positioning around the metal center. In cis isomers, identical ligands are adjacent to each other on the same side of the molecule. In trans isomers, identical ligands are positioned directly opposite each other. These arrangements produce completely different physical and chemical properties, such as color, polarity, and solubility, even though the complexes have identical chemical formulas.

Optical isomers, or enantiomers, are non-superposable mirror images of each other, like left and right hands. Unlike geometric isomers, optical isomers have nearly identical physical and chemical properties. They differ only in how they interact with plane-polarized light and how they react with other enantiomers. This makes optical isomers particularly important in biological systems, where living organisms often incorporate only one specific enantiomer.

Dextrorotatory (d) and levorotatory (l) optical isomers are enantiomers distinguished by their interaction with plane-polarized light. Dextrorotatory isomers rotate the plane of polarized light to the right, while levorotatory isomers rotate it to the left. For example, one enantiomer of tris(ethylenediamine)cobalt(III) is dextrorotatory and the other is levorotatory, despite having identical molecular formulas and nearly identical chemical properties.

Fac-mer isomerism occurs in octahedral complexes with two sets of three identical ligands, such as MA3B3 complexes. In the fac isomer, the three identical ligands occupy opposite faces of the octahedron. In the mer isomer, three identical ligands form an arc around the central metal atom. These geometric arrangements create distinct isomers with different properties.

The spatial arrangement of ligands determines the distribution of electron density and bond dipoles around the metal center. In cis isomers, unequal electron distribution creates a polar molecule. In trans isomers, opposing ligands cause bond dipoles to cancel, resulting in a nonpolar molecule. This spatial difference explains why geometric isomers exhibit different solubilities, reactivities, and other chemical properties despite having identical formulas.

Cisplatin and transplatin are cis and trans geometric isomers of diamminedichloroplatinum(II), a square-planar complex. In cisplatin, the two chloride ligands are adjacent on the same side of the molecule. In transplatin, the chloride ligands are on opposite sides. These structural differences result in dramatically different biological activities, making cisplatin an effective anticancer drug while transplatin is biologically inactive.

Many coordination complexes, such as [M(en)3]n+ where en is ethylenediamine, form enantiomers because their three-dimensional structures are non-superposable mirror images. The cis isomer of [Co(en)2Cl2]+ exists as a pair of optical isomers. These enantiomers have identical properties except when interacting with other chiral molecules, such as DNA helices, where only one enantiomer binds selectively.