Problem 27 - Chemistry: The Central Science in SI Units, Global Edition

Question

For each of the following molecules or polyatomic ions, draw the Lewis structure and indicate if it can act as a monodentate ligand, a bidentate ligand, or is unlikely to act as a ligand at all: (a) ethylamine, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\), (b) trimethylphosphine, \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\), (c) carbonate, \(\mathrm{CO}_{3}^{2-}\) (d) ethane, \(\mathrm{C}_{2} \mathrm{H}_{6}\).

Step-by-Step Solution

Answer

(a) Ethylamine: Monodentate ligand; Lewis structure: H-C-C-N with one lone pair on N. (b) Trimethylphosphine: Monodentate ligand; Lewis structure: P surrounded by three CH3 groups with one lone pair on P. (c) Carbonate: Bidentate ligand; Lewis structure: O=C-O^--O^- with lone pairs on both negatively charged O atoms. (d) Ethane: Unlikely to act as a ligand; Lewis structure: H-C-C-H with no available lone pairs.

Step 1: (a) Ethylamine, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\): Lewis Structure

To draw the Lewis structure for ethylamine, first, we can build the skeleton structure as follows: a nitrogen (N) atom connected to a -CH2 connected to another -CH3 group. Then we can complete the octet by adding the electron pairs around the atoms. \( \mathrm{H} \\ \:| \\ \mathrm{H}\mathrm{C} - \mathrm{C-H} \\ \:| \\ \mathrm{H} - \mathrm{N} |\\ \:| \\ \mathrm{H}\)

Step 2: (a) Ethylamine, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\): Ligand Type

In the Lewis structure of ethylamine, the nitrogen atom has one lone pair of electrons that can be donated to a central metal atom, making it a monodentate ligand.

Step 3: (b) Trimethylphosphine, \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\): Lewis Structure

To draw the Lewis structure for trimethylphosphine, place a phosphorus (P) atom in the center and connect it to three methyl groups (-CH3). Then, complete the octets by adding the corresponding electron pairs around the atoms. \( \\ \mathrm{CH}_{3} \\ | \\ \mathrm{P} - \mathrm{CH}_{3} \\ | \\ \mathrm{CH}_{3}\)

Step 4: (b) Trimethylphosphine, \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\): Ligand Type

In the Lewis structure of trimethylphosphine, the phosphorus atom has one lone pair of electrons that can be donated to a central metal atom. Therefore, it can act as a monodentate ligand.

Step 5: (c) Carbonate, \(\mathrm{CO}_{3}^{2-}\): Lewis Structure

To draw the Lewis structure for carbonate, place a carbon (C) atom in the center and connect it to three oxygen (O) atoms with double bonds to one oxygen and single bonds to the other two oxygen atoms. Add electron pairs around the oxygen atoms to complete their octets. The 2- charge comes from the two extra electrons in the system. \[ \mathrm{O} = \mathrm{C} - \mathrm{O}^- - \mathrm{O}^- \]

Step 6: (c) Carbonate, \(\mathrm{CO}_{3}^{2-}\): Ligand Type

Carbonate ion has two electron-rich oxygens, each with a lone pair that can be donated to a central metal atom. Therefore, it can act as a bidentate ligand.

Step 7: (d) Ethane, \(\mathrm{C}_{2} \mathrm{H}_{6}\): Lewis Structure

To draw the Lewis structure for ethane, connect two carbon (C) atoms in the center and complete the octets by adding three hydrogen (H) atoms to each carbon atom. \( \mathrm{H} \\ \:| \\ \mathrm{H} - \mathrm{C} - \mathrm{C} - \mathrm{H} \\ \:| \\ \mathrm{H}\)

Step 8: (d) Ethane, \(\mathrm{C}_{2} \mathrm{H}_{6}\): Ligand Type

In the Lewis structure of ethane, there are no lone pairs of electrons available for donation to a central metal atom. Thus, ethane is unlikely to act as a ligand.

Key Concepts

Monodentate Ligand

A monodentate ligand is a type of ligand that uses a single atom to bind to a central metal in a complex. This means it donates just one pair of electrons in the coordination bond. Here are some key features:

• Single point of attachment: Provides only one atom with a lone pair that attaches to the metal.
• Common atoms: Typically involves atoms like nitrogen or phosphorus that have lone pairs of electrons.
• Examples: Ethylamine (\(\mathrm{CH_3CH_2NH_2}\)), where the nitrogen atom donates its lone pair to the metal atom.

In ethylamine, the nitrogen atom is the point of contact, helping it to securely attach in coordination compounds but only at one point. This intrinsic property is what defines it as a monodentate ligand.

Bidentate Ligand

A bidentate ligand is capable of forming two bonds with a central metal. It possesses two donor atoms, each with a lone pair of electrons that can attach to the metal. Here are the essential aspects:

• Dual attachment: Two atoms within the same ligand molecule coordinate with the metal.
• Flexibility: Often exhibit some degree of flexibility to accommodate binding at two sites.
• Example: Carbonate ions (\( \mathrm{CO_3^{2-}} \)), where two oxygen atoms can each donate a lone pair of electrons to the central metal.

In the case of carbonate, the structure allows it to wrap around metal centers, effectively attaching at two different sites. This makes bidentate ligands very effective at stabilizing metal complexes due to the creation of a chelate ring, which is notably stronger.

Polyatomic Ions

Polyatomic ions are charged entities composed of two or more atoms covalently bonded, or of a metal complex that acts as a single unit. Some key points include:

• Composite unit: Consists of multiple atoms, usually with at least one full charge due to extra electrons.
• Charge distribution: The charge is often distributed across multiple atoms, contributing to stability.
• Examples: Carbonate (\( \mathrm{CO_3^{2-}} \)), where the polyatomic ion carries a \(2-\) charge spread over the entire molecule.

Polyatomic ions like carbonate are integral in various chemical reactions and solutions, frequently participating in forming salts and complexes. Because they act as single charged units, they exhibit unique chemical behaviors, making them significant not only as ligands but also in broader chemical contexts.