Problem 3 - Chemistry: The Central Science

Question

Draw the Lewis structure for the ligand shown here. (a) Which atoms can serve as donor atoms? Classify this ligand as monodentate, bidentate, or polydentate. (b) How many of these ligands are needed to fill the coordination sphere in an octahedral complex? [Section 23.2] $$ \mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NHCH}_{2} \mathrm{CO}_{2}{\underline{\phantom{xx}}}^{-} $$

Step-by-Step Solution

Answer

The Lewis structure of the ligand is: ``` H H | | H-N-C-C-C-N-C-C-O-O | | | | | H H H H H ``` The nitrogen atom (N) on the left and the negatively charged oxygen atom (O) on the right serve as donor atoms, making it a bidentate ligand. To fill the coordination sphere in an octahedral complex, three of these ligands are needed.

Step 1: Drawing the Lewis structure

To draw the Lewis structure for the given ligand, first, write down the chemical formula: \(NH_2CH_2CH_2NHCH_2CO_2^{-}\) Now, draw the Lewis structure by connecting the atoms with covalent bonds and adding lone pairs to fulfill the octet rule for each atom: ``` H H | | H-N-C-C-C-N-C-C-O-O | | | | | H H H H H ```

Step 2: Identifying donor atoms

Donor atoms are those that can donate a lone pair of electrons to form a coordinate covalent bond with a metal ion. In our ligand, two atoms can serve as donor atoms: the nitrogen atom (N) on the left (connected to the two hydrogens) and the negatively charged oxygen atom (O) on the right (connected to the C=O bond). Both nitrogen and oxygen have lone pairs that can be donated to a metal ion.

Step 3: Classifying the ligand

Since the ligand has two donor atoms that can donate electron pairs to a metal ion, it can be classified as a bidentate ligand.

Step 4: Determining the number of ligands needed for the octahedral complex

In an octahedral complex, there are six sites available for ligands to attach to the metal ion. Given that our ligand is bidentate and can attach at two sites, we need three of these ligands to completely fill the coordination sphere in an octahedral complex (6 sites / 2 sites per ligand = 3 ligands).

Key Concepts

Lewis Structure

Lewis structures are a visual representation of molecules. They help us understand how atoms are bonded together and the location of lone pairs of electrons. In coordination chemistry, drawing the Lewis structure for a ligand is the first step to understanding its bonding behavior.
This process usually involves:

• Connecting atoms in the molecular formula with covalent bonds.
• Adding lone pairs to atoms to satisfy the octet rule, except for hydrogen.

In the original exercise, the ligand \( ext{NH}_2 ext{CH}_2 ext{CH}_2 ext{NHCH}_2 ext{CO}_2^{-}\) was drawn with nitrogen and oxygen atoms featuring lone pairs. These lone pairs are crucial for forming bonds in coordination complexes.

Donor Atoms

Donor atoms are pivotal in the realm of coordination chemistry. They are atoms within a ligand that donate lone pairs of electrons to a metal ion, forming coordinate covalent bonds.
Two primary characteristics of donor atoms include:

• Presence of lone pairs that can be shared with a metal ion.
• Being a part of a stable structure post-donation.

In the provided ligand, the nitrogen and negatively charged oxygen atoms serve as donor atoms. Nitrogen, with its lone pair, and oxygen, with its extra electron, can both donate these electrons to form stable bonds with metal ions.

Bidentate Ligand

A bidentate ligand is one that can attach to a metal ion at two sites, utilizing two donor atoms. This forms a more stable five-membered or six-membered chelate ring with the metal center.
Understanding bidentate ligands involves recognizing:

• The ability of the ligand to form two coordinate covalent bonds.
• The presence of two donor atoms capable of simultaneous bonding.

The ligand in focus ( ext{e.g., NH}_2 ext{CH}_2 ext{CH}_2 ext{NHCH}_2 ext{CO}_2^{-}) has two donor atoms, making it a bidentate ligand. Nitrogen and oxygen work in tandem to form a more robust attachment to the metal ion.

Octahedral Complex

An octahedral complex is a central metal ion surrounded by six ligands, forming an octahedral geometry. This spatial arrangement is significant in coordination chemistry due to its symmetry and the resultant stability.
Key points about octahedral complexes include:

• It involves six coordination sites around the metal ion.
• Can utilize bidentate ligands, requiring fewer ligands overall.

In an octahedral structure, if bidentate ligands are used, only three ligands are needed to fill all six sites completely, as each ligand provides two sites. Thus, three bidentate ligands, like the one in our example, are sufficient to saturate the coordination sphere of the metal ion.