Consider the following isomers [Cr(NH3)2Cl4]-.
(a) Label the isomers as cis or trans.
(b) Which isomers are identical, and which are different?
(c) Do any of these isomers exist as enantiomers? Explain.

Verified step by step guidance

1. <b>Step 1:</b> Understand the structure of the complex [Cr(NH3)2Cl4]-. This is an octahedral complex with chromium as the central metal ion, two ammonia (NH3) ligands, and four chloride (Cl-) ligands.
2. <b>Step 2:</b> Identify the possible geometric isomers. In an octahedral complex, the two NH3 ligands can either be adjacent to each other (cis) or opposite each other (trans).
3. <b>Step 3:</b> Label the isomers: If the two NH3 ligands are adjacent, the isomer is 'cis'. If they are opposite, the isomer is 'trans'.
4. <b>Step 4:</b> Determine which isomers are identical or different. In this case, the cis and trans isomers are different due to their distinct spatial arrangements.
5. <b>Step 5:</b> Consider the possibility of enantiomers. Enantiomers are non-superimposable mirror images. In an octahedral complex with two identical ligands, enantiomers are not possible unless there is a chiral center, which is not present in this case. Therefore, these isomers do not exist as enantiomers.

Key Concepts

Isomerism

Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements or spatial orientations. In coordination chemistry, isomers can be classified into structural isomers, which differ in the connectivity of atoms, and stereoisomers, which have the same connectivity but differ in the arrangement of atoms in space. Understanding isomerism is crucial for identifying and labeling the isomers of coordination complexes.

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism where the relative positions of substituents around a central atom differ. In coordination complexes, 'cis' refers to isomers where similar ligands are adjacent to each other, while 'trans' indicates that similar ligands are opposite each other. This concept is essential for determining the spatial arrangement of ligands in the given isomers of [Cr(NH3)2Cl4]-.

Enantiomers

Enantiomers are a specific type of stereoisomer that are non-superimposable mirror images of each other, often containing a chiral center. For a compound to have enantiomers, it must lack an internal plane of symmetry. In the context of the isomers of [Cr(NH3)2Cl4]-, identifying whether any of the isomers are chiral will determine if they can exist as enantiomers, which is crucial for answering part (c) of the question.