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, \(\mathrm{CO}_{3}^{2-},\) \((\mathbf{d})\) ethane \(, \mathrm{C}_{2} \mathrm{H}_{6}.\)

Short answer

(a) Ethylamine: Monodentate ligand; Lewis structure has a lone pair on nitrogen. (b) Trimethylphosphine: Monodentate ligand; Lewis structure has a lone pair on phosphorus. (c) Carbonate: Bidentate ligand; Lewis structure has two negatively charged oxygen atoms. (d) Ethane: Unlikely to act as a ligand; Lewis structure has no lone pairs or electron-rich sites.

Step-by-step solution

(Step 1: Draw the Lewis Structure of Ethylamine)

Ethylamine has the chemical formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}\). The two carbon atoms are bonded to the nitrogen atom, and three hydrogen atoms are bonded to each carbon atom. The nitrogen atom has two single bonds with the carbon atoms and one lone pair of electrons. The Lewis structure of ethylamine can be drawn as: H H H | | | C-C-N-H | | H lone pair on N

(Step 2: Draw the Lewis Structure of Trimethylphosphine)

Trimethylphosphine has the chemical formula \(\mathrm{P}(\mathrm{CH}_{3})_{3}\). Phosphorus is in the center, bonded to three methyl groups (\(\mathrm{CH}_{3}\)). The phosphorus has one lone pair of electrons. The Lewis structure of trimethylphosphine can be drawn as: H H H | | | H-C-P-C-H | | H H lone pair on P

(Step 3: Draw the Lewis Structure of Carbonate)

Carbonate has the chemical formula \(\mathrm{CO}_{3}^{2-}\). The central carbon atom is double-bonded to one oxygen atom and has single bonds to two other oxygen atoms, which carry a negative charge. The Lewis structure of carbonate can be drawn as: O // \ C-O^- \ O^-

(Step 4: Draw the Lewis Structure of Ethane)

Ethane has the chemical formula \(\mathrm{C}_{2}\mathrm{H}_{6}\). The two carbon atoms are bonded to each other, and three hydrogen atoms are bonded to each carbon atom. The Lewis structure of ethane can be drawn as: H H H | | | C-C | | | H H H

(Step 5: Determine if each molecule can act as a monodentate, bidentate, or no ligand)

(a) Ethylamine has a lone pair on the nitrogen atom, so it can act as a monodentate ligand. (b) Trimethylphosphine has a lone pair on the phosphorus atom, so it can act as a monodentate ligand. (c) Carbonate has two negatively charged oxygen atoms, so it can act as a bidentate ligand. (d) Ethane has no electron lone pairs or electron-rich sites, so it is unlikely to act as a ligand at all.

Key concepts

Lewis Structure Drawing

Lewis structures, also known as electron dot structures, are diagrams that show the bonding between atoms in a molecule as well as the lone pairs of electrons that may exist. This visualization plays a crucial role in understanding the chemical bonding and molecular geometry.

When drawing a Lewis structure, the goal is to illustrate how valence electrons are arranged and how they participate in forming bonds. Steps include counting the total number of valence electrons, arranging the atoms to represent bonds, and distributing the remaining electrons to satisfy the octet rule—whereby most atoms prefer to have eight electrons in their valence shell.

To improve understanding of Lewis Structures:

• Always start with skeletal structures, placing the least electronegative atom in the center.
• Use straight lines to represent bonding pairs of electrons and dots for nonbonding lone pairs.
• Check formal charges to ensure the most stable structure.

Monodentate Ligand

A monodentate ligand is a molecule that coordinates to a single site on a metal ion in coordination chemistry. These ligands have a single atom with a lone pair of electrons that can be donated to the metal to form a coordinate bond.

Monodentate ligands such as water, ammonia, or chlorides are like the individual dancers in coordination chemistry—they can bind to the metal center at one point only. Ethylamine and trimethylphosphine, mentioned in our exercise, fall into this category as they each offer a lone pair on nitrogen and phosphorus, respectively, to the metal ion they're coordinating to.

• For clarity, always indicate the bonding site of a monodentate ligand in drawings.
• Remember that monodentate ligands are not limited to neutral molecules; ions can also serve as monodentate ligands.

Bidentate Ligand

Bidentate ligands are the multitaskers of ligands—you can think of them as molecules that have two 'hands' to grab onto a metal atom, forming two coordinate bonds simultaneously. In contrast to monodentate ligands, bidentate ligands have two atoms, each with a lone pair ready to bind to the central metal atom or ion.

Commonly encountered bidentate ligands include ethylenediamine and the carbonate ion from our exercise, which uses its two oxygen atoms with negative charges to connect to the metal ion. The ability to form two bonds makes these ligands very stable, resulting in a chelate complex.

• When illustrating bidentate ligands, show both atoms connected to the metal.
• Highlight their ability to form ring structures with the metal, known as 'chelate rings.'

Ligand Acting Molecules in Coordination Chemistry

In coordination chemistry, ligand acting molecules are the key players that surround and bind to the central metal atom, forming what's known as a coordination complex. These ligands can be water, anions, or organic molecules, each providing lone pairs for bonding.

Identifying ligand acting molecules requires inspecting the molecule for lone pairs or negative charges that can serve as attachment points. For instance, ethylamine and trimethylphosphine are recognized as monodentate ligands due to their lone pairs on nitrogen and phosphorus, while carbonate is a bidentate ligand with its two negatively charged oxygen atoms.

Ligands profoundly influence the properties of the coordination compounds they form, including their color, magnetic properties, and reactivity.

• Explore the coordination number of the metal—the number of ligand atoms connected to a central atom—to anticipate the shape of the complex.
• Study the ligands' electronic properties because they impact the stability and function of the coordination complex.