Question

Formulas of Coordination Compounds Which of the following ligands is expected to be monodentate and which might be polydentate? (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (d) en (b) \(\mathrm{CH}_{3} \mathrm{CN}\) (e) \(\mathrm{Br}^{-}\) (c) \(\mathrm{N}_{3}^{-}\) (f) phen

Short answer

\( \mathrm{CH}_3 \mathrm{NH}_2 \), \( \mathrm{CH}_3 \mathrm{CN} \), \( \mathrm{N}_3^- \), and \( \mathrm{Br}^- \) are monodentate; "en" and "phen" are polydentate.

Step-by-step solution

Identify Ligands

Examine each given chemical structure or formula to identify whether a ligand is monodentate or polydentate. Monodentate ligands have one donor atom, while polydentate ligands have multiple donor atoms capable of bonding to a central metal ion in a coordination compound.

Analyze Ligand (a) \\\mathrm{CH}_{3} \\\mathrm{NH}_{2}

- The molecule \( \mathrm{CH}_3 \mathrm{NH}_2 \) has one nitrogen atom with a lone pair of electrons, making it a monodentate ligand. Only the nitrogen can coordinate with a metal center.

Analyze Ligand (b) \\\mathrm{CH}_{3} \\\mathrm{CN}

- The ligand \( \mathrm{CH}_3 \mathrm{CN} \) has a nitrile group (CN) where the nitrogen atom can donate a lone pair of electrons. Therefore, this ligand is monodentate.

Analyze Ligand (c) \\\mathrm{N}_{3}^{-}

- The \( \mathrm{N}_3^- \) ligand (azide) can act as a bridging ligand due to the multiple nitrogen atoms, but typically acts as a monodentate ligand in complexes.

Analyze Ligand (d) en

- "en" stands for ethylenediamine, which has two nitrogen atoms, each with a lone pair of electrons. Therefore, it acts as a bidentate ligand, coordinating to the metal center at two points.

Analyze Ligand (e) \\\mathrm{Br}^{-}

- The ligand \( \mathrm{Br}^- \) is a simple halide with one donor atom, making it a monodentate ligand.

Analyze Ligand (f) phen

- "phen" refers to 1,10-phenanthroline, which has two nitrogen atoms available for coordination. This makes phen a bidentate ligand.

Key concepts

Monodentate Ligands

Monodentate ligands are essential players in coordination chemistry. They attach to a central metal atom via a single atom with a lone pair of electrons. This singular attachment point can be envisioned almost like a fishing hook linking them to the metal. - Examples of monodentate ligands include simple molecules like \(\mathrm{CH}_3\mathrm{NH}_2\), where only one nitrogen atom is available to form a bond. - Other examples include \(\mathrm{CH}_3\mathrm{CN}\) and \(\mathrm{Br}^{-}\), both of which have only one site available for attachment (a nitrogen in the nitrile group and bromine, respectively). - The azide ion, \(\mathrm{N}_3^{-}\), typically acts as a monodentate ligand despite having multiple nitrogen atoms, mainly due to its typical coordination preference. Monodentate ligands provide a straightforward way of studying coordination compounds and are often used to create complexes due to their simplicity. They are like the simple building blocks of much more complex systems in chemistry.

Polydentate Ligands

In coordination chemistry, polydentate ligands are like multitaskers, connecting to the central metal atom or ion through multiple atoms with lone pairs. This multiple attachment ability forms more stable complexes, often referred to as chelates. - Ethylenediamine ("en") is a common example of a bidentate ligand, which means it can attach at two sites on the metal ion via its two nitrogen atoms, each equipped with a lone pair. - Similarly, 1,10-phenanthroline ("phen") can bind at two points through its nitrogen atoms, enhancing the stability of the resulting complex. Polydentate ligands are invaluable for creating more robust and stable complexes in coordination chemistry. They essentially 'hug' the metal ion with several arms, providing enhanced stability and resistance to decomposition. This capability is crucial in a wide range of applications, including catalysis and medicinal chemistry.

Coordination Chemistry

Coordination chemistry revolves around the interactions between metal atoms or ions and ligands. It plays a fundamental role in various scientific fields, from developing new materials to understanding biological systems. - In these complexes, the ligand provides lone electron pairs to the central metal, forming coordinate bonds and resulting in a coordination compound. - Coordination numbers vary and can be defined as the number of ligand atoms bonded directly to the central atom, commonly six or lower, although higher numbers exist. The variety in coordination chemistry is immense, influenced by several factors including the nature of the metal, the ligand itself (either monodentate or polydentate), and the charge balance in the complex. Beyond direct applications, a fundamental understanding of coordination chemistry provides insights into chemical bonding, molecular structure, and reactivity. This knowledge informs industries that rely on complex formation, like pharmaceuticals and materials science.