Question

Which of these octahedral complexes would you expect to exhibit geometric isomerism? Explain. (a) \(\left[\mathrm{CrOH}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}\) (b) \(\left[\mathrm{CrCl}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)\left(\mathrm{NH}_{3}\right)_{3}\right]^{+}\) (c) \(\left[\mathrm{CrCl}_{2}(\mathrm{en})_{2}\right]^{+}\) (d) \(\left[\mathrm{CrCl}_{4}(\mathrm{en})\right]^{-}\) (e) \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]^{3+}\)

Short answer

Of the given complexes, [CrCl2(H2O)(NH3)3]+ and [CrCl4(en)]- are expected to exhibit geometric isomerism.

Step-by-step solution

Analyzing the complex [CrOH(NH3)5]2+

This complex has two ligands: OH- and NH3, in a 5:1 ratio respectively. However, due to the unique position of OH-, there is no possible different spatial arrangement of these ligands around Chromium. Thus, it does not exhibit geometric isomerism.

Analyzing the complex [CrCl2(H2O)(NH3)3]+

The complex has three different types of ligands: Cl-, H2O, and NH3. There are different possible spatial arrangements for these ligands around Chromium (i.e., the Cl- ions could be adjacent (cis) or opposite (trans) each other). Hence, it exhibits geometric isomerism.

Analyzing the complex [CrCl2(en)2]+

This complex has two types of ligands: Cl- and en (which is a bidentate ligand and occupies two sites). However, due to the nature of the ethylenediamine (en) ligand, there is no alternative placement for the Cl- ions, so no geometric isomerism can occur.

Analyzing the complex [CrCl4(en)]-

In this complex, there are two types of ligands: Cl- and en, in a 4:1 ratio respectively. There can be different spatial arrangements of these ligands around the Chromium, so this complex can exhibit geometric isomerism.

Analyzing the complex [Cr(en)3]3+

In this case, there is only one type of ligand, en, which makes alternate arrangement impossible. Thus, it does not exhibit geometric isomerism.

Key concepts

Coordination Chemistry

Coordination chemistry delves into the study of compounds that feature central metal atoms or ions bonded to a surrounding array of molecules or ions, called ligands. These complexes can be categorized based on their properties, shapes, and bonding arrangements.

The metal-ligand bond is a crucial aspect of coordination compounds, often displaying a coordinate covalent nature where both electrons in the bond originate from the ligand. This intricacy leads to a vast variety of coordination complexes with diverse geometries and chemical behaviors.

Central to understanding coordination chemistry is recognizing that the specific properties of these complexes—color, magnetic behavior, and reactivity—are highly dependent on the metal ion, the ligands attached to it, and the overall three-dimensional structure of the complex. For students grappling with coordination chemistry, visualizing the three-dimensional arrangements and the impact of different ligands can significantly aid comprehension.

Ligands Arrangement

The arrangement of ligands around a metal ion in a coordination complex is a concept that beautifully showcases the interplay between structure and function in chemistry. Ligands can be neutral molecules like water or ammonia, negatively charged ions like chloride or hydroxide, or even larger organic molecules that can form multiple bonds with the metal center, known as multidentate ligands.

The spatial distribution of these ligands is influenced by several factors, including their size, electronic demand, and the preferences of the metal ion. For example, in a coordination complex, certain ligands may prefer to be opposite to each other (trans configuration) or next to each other (cis configuration), leading to different isomers of the same compound.

Understanding ligands arrangement is vital for students as it directly relates to the potential reactivity and interactions of a coordination compound in a chemical system. Visualizing models and engaging with practical examples can clarify these concepts.

Cis-Trans Isomerism

Cis-trans isomerism is one of several forms of geometric isomerism—variations in spatial arrangement—that can occur in coordination complexes, particularly in octahedral and square planar compounds.

The term 'cis' refers to isomers where two identical or similar ligands are adjacent to each other on the same side of a central atom, while 'trans' isomers have these ligands opposite from one another. This difference might seem small, but it can lead to dramatically different chemical and physical properties. For instance, cisplatin is an effective anticancer drug, whereas its trans isomer is not.

Students should note that for cis-trans isomerism to exist, there must be at least two different types of ligands, and each type must have more than one instance within the complex. A thorough understanding of these concepts and being aware of the conditions for isomerism can deepen the students' knowledge in the fascinating field of coordination chemistry.