Question

Draw structures of each of the following. a. cis-dichloroethylenediamineplatinum(II) b. trans-dichlorobis(ethylenediamine) cobalt(II) c. cis-tetraamminechloronitrocobalt(III) ion d. trans-tetraamminechloronitritocobalt(III) ion e. trans-diaquabis(ethylenediamine) copper(II) ion

Short answer

a. cis-dichloroethylenediamineplatinum(II): Pt with square planar coordination, adjacent Cl^- ions, and adjacent ethylenediamine ligands. b. trans-dichlorobis(ethylenediamine) cobalt(II): Co with square planar coordination, opposite Cl^- ions, and opposite ethylenediamine ligands. c. cis-tetraamminechloronitrocobalt(III) ion: Co with octahedral coordination, adjacent Cl^- and NO2^- ligands, and four ammine ligands around them. d. trans-tetraamminechloronitritocobalt(III) ion: Co with octahedral coordination, opposite Cl^- and NO2^- ligands, and four ammine ligands around them. e. trans-diaquabis(ethylenediamine) copper(II) ion: Cu with square planar coordination, opposite H2O ligands, and opposite ethylenediamine ligands.

Step-by-step solution

a. cis-dichloroethylenediamineplatinum(II)

1. Identify the central metal ion and ligands: In this complex, platinum (Pt) is the central metal ion with oxidation state +2, and the ligands are ethylenediamine (en) and two chloride ions (Cl^-). 2. Determining coordination number: Platinum(II) forms a square planar complex, which means it will have four ligands around it forming 90° angles. 3. Draw the cis- configuration: In cis- configuration, the same type of ligands should be adjacent to each other. Place the two chloride (Cl^-) ions adjacent, and ethylenediamine (en) ligands adjacent as well.

b. trans-dichlorobis(ethylenediamine) cobalt(II)

1. Identify the central metal ion and ligands: In this complex, cobalt (Co) is the central metal ion with oxidation state +2, and the ligands are two ethylenediamine (en) and two chloride ions (Cl^-). 2. Determining coordination number: Cobalt(II) also forms a square planar complex, so it will have four ligands around it at 90° angles. 3. Draw the trans- configuration: In trans- configuration, the same type of ligands should be opposite to each other. Place the two chloride (Cl^-) ions opposite, and ethylenediamine (en) ligands opposite as well.

c. cis-tetraamminechloronitrocobalt(III) ion

1. Identify the central metal ion and ligands: In this complex, cobalt (Co) is the central metal ion with oxidation state +3, and the ligands are four ammine (NH3), one chloro (Cl^-), and one nitro (NO2^-) ligands. 2. Determining coordination number: Cobalt(III) forms an octahedral complex, which means it will have six ligands around it forming 90° angles in the plane and 180° angles across the complex. 3. Draw the cis- configuration: In cis- configuration, the same type of ligands should be adjacent to each other. Place chloro (Cl^-) and nitro (NO2^-) ligands adjacent, and arrange the ammine (NH3) ligands around them.

d. trans-tetraamminechloronitritocobalt(III) ion

1. Identify the central metal ion and ligands: In this complex, cobalt (Co) is the central metal ion with oxidation state +3, and the ligands are four ammine (NH3), one chloro (Cl^-), and one nitrito (NO2^-) ligands. 2. Determining coordination number: Cobalt(III) forms an octahedral complex, which means it will have six ligands around it forming 90° angles in the plane and 180° angles across the complex. 3. Draw the trans- configuration: In trans- configuration, the same type of ligands should be opposite to each other. Place chloro (Cl^-) and nitrito (NO2^-) ligands opposite, and arrange the ammine (NH3) ligands around them.

e. trans-diaquabis(ethylenediamine) copper(II) ion

1. Identify the central metal ion and ligands: In this complex, copper (Cu) is the central metal ion with oxidation state +2, and the ligands are two aqua (H2O) and two ethylenediamine (en) ligands. 2. Determining coordination number: Copper(II) forms a square planar complex, so it will have four ligands around it at 90° angles. 3. Draw the trans- configuration: In trans- configuration, the same type of ligands should be opposite to each other. Place the two aqua (H2O) ligands opposite, and ethylenediamine (en) ligands opposite as well.

Key concepts

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism found in certain metal complexes with square planar or octahedral geometries. These isomers contain the same components, but differ in the spatial arrangement of the ligands around the central metal atom.

In cis isomers, two identical or related ligands are positioned adjacent to each other on the same side of the metal, often leading to a higher possibility of intramolecular interactions. Conversely, in trans isomers, similar or identical ligands are located opposite to each other, often resulting in a more symmetrical structure and sometimes altering the compound's reactivity and physical properties.

For a student's clear understanding, it's crucial to visually differentiate cis and trans isomers. Taking the exercise's example of cis-dichloroethylenediamineplatinum(II), the two chloride ions are adjacent, whereas in the trans counterpart, they are positioned across from each other. Similarly, with cobalt complexes in the exercise, paying close attention to the ligands' positions relative to one another is essential for depicting cis or trans forms correctly.

Ligands and Metal Complexes

In coordination chemistry, ligands are ions or molecules that bind to a central metal atom to form a complex. A ligand can be of various types, such as water (aqua), ammonia (ammine), chloride, or organic molecules like ethylenediamine.

The nature of the ligand, including its size, electronic properties, and the number of points it attaches to the metal, plays a significant role in determining the overall structure and properties of the metal complex. Ligands can be classified as monodentate, bonding to the metal at a single attachment point, or polydentate, offering multiple bonds to the metal, which are also known as chelating agents.

It's important for students to identify ligands within a metal complex, as demonstrated in the textbook exercise. Recognizing each ligand's bonding site helps in predicting the geometry of the complex and in understanding its chemical behavior. For example, ethylenediamine (en) is a bidentate ligand that occupies two coordination sites in complexes such as trans-diaquabis(ethylenediamine) copper(II) ion.

Coordination Number

The coordination number of a central metal ion in a complex refers to the number of ligand attachment points to the metal. It's a critical concept because it gives insight into the geometry of the complex and influences its stability and chemical reactivity.

Metal ions can have various coordination numbers, typically ranging from 2 to 12, with 4 (square planar or tetrahedral) and 6 (octahedral) being among the most common in inorganic chemistry. For instance, a platinum(II) ion in a square planar complex, as presented in the exercise, has a coordination number of 4. The cobalt(III) ion, with a coordination number of 6, forms an octahedral complex.

By comprehending the concept of coordination number, students can proficiently predict the spatial arrangement of ligands around the central metal ion. As seen in the examples of various complexes provided in the exercise, the coordination number directly corresponds to the number of ligands illustrated in each structure.