Problem 17 - Chemistry: Principles and Reactions

Question

Sketch the geometry of (a) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) (tetrahedral) (b) cis \(-\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (c) trans- \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Br}_{2}\right]^{2+}\) (d) trans- \(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\) (e) \([\mathrm{Au}(\mathrm{CN}) \mathrm{Br}]^{+}\)

Step-by-Step Solution

Answer

Question: Sketch the geometry of the following coordination complexes: (a) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) (tetrahedral) (b) cis \(-\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\mathrm{Cl}_{2}\right]^{+}\) (c) trans- \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Br}_{2}\right]^{2+}\) (d) trans- \(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\) (e) \([\mathrm{Au}(\mathrm{CN}) \mathrm{Br}]^{+}\)

Step 1: Identify the central atom

The central atom in this complex is Zn.

Step 2: Count the number of ligands

There are 4 ligands: two ammonia (NH3) molecules and two chloride (Cl) ions.

Step 3: Sketch the tetrahedral structure

Since the geometry is specified as tetrahedral, we can now place the Zn central atom in the middle, with the four ligands (two NH3 and two Cl) positioned around the central atom at approximately 109.5° bond angles from each other. (b) cis \(-\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\mathrm{Cl}_{2}\right]^{+}\)

Step 1: Identify the central atom

The central atom in this complex is Co.

Step 2: Count the number of ligands

There are 6 ligands: four water (H2O) molecules and two chloride (Cl) ions.

Step 3: Determine the geometry and sketch the structure

With six ligands, the Co complex forms an octahedral geometry. Place the Co central atom in the middle and surround it with the four H2O ligands and two Cl ligands. Since the complex is cis, both Cl ligands will be adjacent to each other. (c) trans- \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Br}_{2}\right]^{2+}\)

Step 1: Identify the central atom

The central atom in this complex is Pt.

Step 2: Count the number of ligands

There are 4 ligands: two ammonia (NH3) molecules and two bromide (Br) ions.

Step 3: Determine the geometry and sketch the structure

With four ligands, the Pt complex forms a square planar geometry. Place the Pt central atom in the center and surround it with the four ligands in a square arrangement. In the trans-isomer, the two Br ligands will be opposite each other. (d) trans- \(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\)

Step 1: Identify the central atom

The central atom in this complex is Ni.

Step 2: Count the number of ligands

There are 4 ligands: two oxalate (ox) ions and two hydroxide (OH) ions.

Step 3: Determine the geometry and sketch the structure

With four ligands, the Ni complex forms a square planar geometry. Place the Ni central atom in the center and surround it with the four ligands in a square arrangement. In the trans-isomer, the two OH ligands will be opposite each other. (e) \([\mathrm{Au}(\mathrm{CN}) \mathrm{Br}]^{+}\)

Step 1: Identify the central atom

The central atom in this complex is Au.

Step 2: Count the number of ligands

There are 2 ligands: one cyanide (CN) ion and one bromide (Br) ion.

Step 3: Determine the geometry and sketch the structure

With two ligands, the Au complex forms a linear geometry. Place the Au central atom in the center, with the CN ligand on one side and the Br ion on the other side of Au at a 180° bond angle.

Key Concepts

Coordination Chemistry

Coordination chemistry is the study of compounds formed between metal ions and various organic or inorganic ligands. These compounds, known as coordination compounds or complex ions, feature a central metal atom or ion bonded to surrounding molecules or ions, called ligands. Ligands possess at least one donor atom with an electron pair used to form a coordinate covalent bond with the metal. The interesting property of coordination complexes is that they exhibit a variety of colors, magnetic properties, and reactivity, which find applications in many areas such as catalysis, material sciences, and medicine.

The number of ligands bonded to the central atom, along with the charge and size of the metal ion, determines the coordination number of the complex. For example, in \[\mathrm{Zn}(\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}\], the coordination number of Zn is 4 because there are four ligands attached to it.

Molecular Geometry

The molecular geometry of a coordination compound refers to the three-dimensional arrangement of the ligands attached to the central metal ion. This arrangement is crucial as it influences the reactivity, color, and magnetic properties of the complex. The geometry is determined by the coordination number, which is directly related to the number of ligands bonded to the central metal.

Common geometries include linear, tetrahedral, octahedral, and square planar. For instance, in \[\mathrm{Zn}(\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}\], the geometry is tetrahedral, as indicated in the exercise solution. On the other hand, a coordination compound with a coordination number of 6, like cis-\[\mathrm{Co}(\mathrm{H}_{2} \mathrm{O})_{4}\mathrm{Cl}_{2}\]^{+}, generally forms an octahedral shape.

Ligands in Coordination Compounds

Ligands play a central role in the formation of coordination compounds. They can be simple ions like Cl- or complex molecules like the oxalate ion (ox) or ammonia (NH3). Based on the number of donor atoms, ligands can be classified as monodentate, bidentate, or polydentate. Monodentate ligands, such as CN- and Cl-, provide one donor atom to bond with the metal. Bidentate ligands, such as oxalate in the \[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\]^{3-} complex, can form two bonds to the metal, and polydentate ligands can form multiple bonds.

Understanding the diversity of ligands and their bonding helps predict the structure and properties of the resulting complexes.

Isomerism in Coordination Compounds

Isomerism is a phenomenon where compounds with the same composition have different arrangements of atoms, resulting in distinct properties. In coordination chemistry, isomerism can occur in several forms, including structural isomers and stereoisomers. Structural isomers differ in the connectivity of atoms, whereas stereoisomers have the same connectivity but different spatial arrangements.

Geometric isomerism, a type of stereoisomerism, is seen in coordination compounds where ligands can assume different arrangements around the central atom. For example, in the given exercise, cis-\[\mathrm{Co}(\mathrm{H}_{2} \mathrm{O})_{4}\mathrm{Cl}_{2}\]^{+} and trans-\[\mathrm{Pt}(\mathrm{NH}_{3})_{2}\mathrm{Br}_{2}\]^{2+} represent geometric isomers. In the cis form, identical ligands are adjacent, whereas in the trans form, they are opposite each other. Isomerism is an essential concept as it can have significant effects on the chemical and physical properties of coordination compounds.