Question

Which of the following ligands are capable of linkage isomerism? Explain your answer. $$ \mathrm{SCN}^{-}, \mathrm{N}_{3}^{-}, \mathrm{NO}_{2}^{-}, \mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}, \mathrm{OCN}^{-}, \mathrm{I}^{-} $$

Short answer

The ligands capable of linkage isomerism are \(\mathrm{SCN}^-\) and \(\mathrm{OCN}^-\), as they both have multiple coordinating atoms (S and N for \(\mathrm{SCN}^-\); O and N for \(\mathrm{OCN}^-\)) that can coordinate to a central metal atom/ion, leading to different coordinate bonds. The other ligands either have only one coordinating atom or identical coordinating atoms, which cannot form different coordinate bonds.

Step-by-step solution

Identify atoms capable of coordinating to a metal atom/ion

In order to exhibit linkage isomerism, a ligand must have multiple coordinating atoms (atoms that can bond to a metal center such as N, O, S, etc.). For each given ligand, identify all coordinating atoms. 1. \(\mathrm{SCN}^-\): Two coordinating atoms (S and N) 2. \(\mathrm{N}_{3}^-\): One coordinating atom (N) 3. \(\mathrm{NO}_{2}^-\): One coordinating atom (N) 4. \(\mathrm{NH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{NH}_{2}\): Two coordinating atoms (N) 5. \(\mathrm{OCN}^-\): Two coordinating atoms (O and N) 6. \(\mathrm{I}^-\): One coordinating atom (I)

Determine if the ligands are capable of linkage isomerism

Next, determine which ligands have multiple coordinating atoms that can coordinate to a central metal atom/ion. Linkage isomerism can only occur in ligands with multiple coordinating atoms as these ligands can form different coordinate bonds with the central metal atom/ion. 1. \(\mathrm{SCN}^-\): Capable of linkage isomerism (S and N coordinating atoms) 2. \(\mathrm{N}_{3}^-\): Not capable of linkage isomerism (only one N coordinating atom) 3. \(\mathrm{NO}_{2}^-\): Not capable of linkage isomerism (only one N coordinating atom) 4. \(\mathrm{NH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{NH}_{2}\): Not capable of linkage isomerism (both N atoms are identical and would form the same coordinate bond with the central metal atom/ion) 5. \(\mathrm{OCN}^-\): Capable of linkage isomerism (O and N coordinating atoms) 6. \(\mathrm{I}^-\): Not capable of linkage isomerism (only one I coordinating atom)

List the ligands capable of linkage isomerism

Based on the previous steps, the ligands capable of linkage isomerism are: - \(\mathrm{SCN}^-\) - \(\mathrm{OCN}^-\)

Key concepts

Coordination Compounds

Coordination compounds are chemical compounds that consist of a central metal atom or ion (usually transition metals) surrounded by molecules or anions known as ligands. The ligands donate electron pairs to the metal to form coordinate covalent bonds. These compounds are characterized by their complex structure and the nature of their bonding. Coordination compounds are of great importance in various fields including biochemistry, metallurgy, and industrial chemistry.

For example, haemoglobin, the oxygen-carrying molecule in our blood, is a coordination compound of iron. The central iron atom is bonded to several ligands, including one that can reversibly bind to oxygen. In industry, coordination compounds are used for catalysis, as they can speed up chemical reactions without being consumed in the process. The rich and varied properties of coordination compounds arise from the flexibility in the types and numbers of ligands that can bond to the central metal, as well as the geometry of the resulting structures.

Isomerism in Coordination Chemistry

Isomerism in coordination chemistry refers to the phenomena where two or more coordination compounds possess the same formula but differ in the arrangement of their atoms or bonds. One form of isomerism is linkage isomerism which occurs when a ligand that can coordinate through more than one atom forms different compounds, depending on which atom is directly bonded to the metal center.

Linkage isomerism is exhibited by ambidentate ligands—those with multiple potential bonding sites. For example, a thiocyanate ion (\(\mathrm{SCN}^-\)) can bind to the metal through the sulfur atom to form a thiocyanato complex or through the nitrogen atom to form an isothiocyanato complex. This type of isomerism can affect the properties of the compound, including its reactivity, color, and magnetic properties. Understanding linkage isomerism is crucial for chemists as it can influence the outcomes of chemical reactions and the functionality of coordination compounds in biological systems or industrial applications.

Bidentate Ligands

Bidentate ligands are a type of chelating ligands that have two donor atoms which allow them to attach to a central metal atom or ion at two points. The term 'bidentate' comes from Latin, meaning 'two teeth', which reflects how these ligands 'bite' onto the metal. Bidentate ligands increase the stability of coordination compounds by forming more complex and robust structures through chelation.

One well-known example of a bidentate ligand is ethylenediamine (\(\mathrm{NH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{NH}_{2}\)), which has two nitrogen donor atoms that can bond to the same metal ion. This leads to the formation of a ring that includes the metal ion, known as a chelate ring. Multiple chelate rings can increase the overall stability of the compound, a concept known as the chelate effect. Bidentate and other polydentate ligands are often used in medicinal chemistry and materials science to create targeted compounds with specific desired properties.