Question

Sketch the structure of the complex in each of the following compounds and give the full compound name: (a) cis- \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{Na}_{2}\left[\mathrm{Ru}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}_{5}\right]\) (c) trans- \(\mathrm{NH}_{4}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\) (d) cis- \(\left[\mathrm{Ru}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]\)

Short answer

\( \begin{array}{l} \textbf{(a)} \qquad \text{cis-Tetraamminecobalt(III) hexahydrate dinitrate} \\ \vspace{3pt} \textbf{(b)} \qquad \text{Sodium hexachlororuthenate(III) dihydrate} \\ \vspace{3pt} \textbf{(c)} \qquad \text{trans-Diaquabis(oxalato)cobaltate(III) tetrahydrate} \\ \vspace{3pt} \textbf{(d)} \qquad \text{cis-Dichlorobis(ethylenediamine)ruthenium(III)} \end{array} \)

Step-by-step solution

Identify the Metal and Ligands

The central metal ion in this complex is Cobalt (Co), and it is coordinated to four Ammonia (NH3) ligands and two Water (H2O) ligands.

Determine the Coordination Number and Geometry

The Coordination Number in this complex is 6, which is the sum of all ligands bonded to the central metal ion. The Coordination Geometry for Coordination Number 6 is typically Octahedral.

Sketch the Structure

In the cis isomer, the two Water (H2O) ligands are positioned adjacent to each other. The remaining four ligands occupied in the octahedral structure are Ammonia (NH3).

Compound Name

The full compound name is cis-Tetraamminecobalt(III) hexahydrate dinitrate. (b) Sketch the structure and give the full compound name for \(\mathrm{Na}_{2}\left[\mathrm{Ru}\left(\mathrm{H}_{2} \mathrm{O}\right)\mathrm{Cl}_{5}\right]\).

Identify the Metal and Ligands

The central metal ion in this complex is Ruthenium (Ru), and it is coordinated to one Water (H2O) ligand and five Chloride (Cl) ligands.

Determine the Coordination Number and Geometry

The Coordination Number in this complex is 6. The Coordination Geometry for Coordination Number 6 is typically Octahedral.

Sketch the Structure

The structure will have an octahedral arrangement, with one Water ligand and five Chloride ligands connected to the Ruthenium center.

Compound Name

The full compound name is Sodium hexachlororuthenate(III) dihydrate. (c) Sketch the structure and give the full compound name for trans- \(\mathrm{NH}_{4}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\).

Identify the Metal and Ligands

The central metal ion in this complex is Cobalt (Co), and it is coordinated to two Oxalate (C2O4) ligands and two Water (H2O) ligands.

Determine the Coordination Number and Geometry

The Coordination Number in this complex is 6 since each Oxalate ligand occupies two coordination sites. The Coordination Geometry for Coordination Number 6 is typically Octahedral.

Sketch the Structure

In the trans isomer, the two Water (H2O) ligands are positioned opposite to each other. The remaining four ligands in the octahedral structure are occupied by two Oxalate (C2O4) ligands.

Compound Name

The full compound name is trans-Diaquabis(oxalato)cobaltate(III) tetrahydrate. (d) Sketch the structure and give the full compound name for cis- \(\left[\mathrm{Ru}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]\).

Identify the Metal and Ligands

The central metal ion in this complex is Ruthenium (Ru), and it is coordinated to two Ethylenediamine (en) ligands and two Chloride (Cl) ligands.

Determine the Coordination Number and Geometry

The Coordination Number in this complex is 6 since each Ethylenediamine ligand occupies two coordination sites. The Coordination Geometry for Coordination Number 6 is typically Octahedral.

Sketch the Structure

In the cis isomer, the two Chloride (Cl) ligands are positioned adjacent to each other. The remaining four ligands in the octahedral structure are occupied by two Ethylenediamine (en) ligands.

Compound Name

The full compound name is cis-Dichlorobis(ethylenediamine)ruthenium(III).

Key concepts

Coordination Chemistry

Coordination chemistry focuses on the interaction between metal ions and ligands. Metal ions act as central units that attract and bind various ligands in a structure known as a coordination complex. Ligands are molecules or ions that can donate a pair of electrons to the central metal ion, forming a stable system.

The strength and type of interaction between the central metal ion and the ligands can affect the properties of the coordination compound. The bonds in these complexes are often considered coordinate covalent bonds because the electron pair is typically donated by the ligand to the metal ion. This relationship forms the foundation of coordination chemistry, influencing properties such as color, magnetism, and reactivity.

Isomerism

Isomerism in coordination compounds occurs when compounds have the same chemical formula but different arrangements of atoms. This can lead to different physical and chemical properties.

• **Geometrical Isomers:** These occur due to the different possible spatial arrangements of the ligands. For example, in squared planar and octahedral complexes, the ligands can either be adjacent (cis) or opposite each other (trans).
• **Optical Isomers:** These are non-superimposable mirror images, much like left and right hands. This kind of isomerism gives rise to chirality in some octahedral or tetrahedral complexes where there are asymmetrical arrangements of the ligands.

Understanding isomerism is crucial for predicting the behavior and reactivity of coordination compounds in chemical reactions.

Naming Compounds

Naming coordination compounds follows a systematic approach to convey detailed information about the complex. The rules for naming these compounds are extensive but can be summarized for easier understanding:

• Name the ligands first in alphabetical order, followed by the name of the central metal ion.
• If a ligand appears multiple times, prefixes like mono-, di-, tri- are used.
• If the overall charge of the complex is anion, the metal takes an "-ate" suffix.
• Use the oxidation state of the metal, indicated in Roman numerals, in parentheses immediately following the metal name.

Understanding these rules is essential for communication in chemistry and provides insight into the structure and behavior of the compound.

Coordination Number

The coordination number indicates the number of ligand bonds to the central metal ion in a coordination compound. It is a crucial parameter that influences the overall geometry and stability of the compound.

• **Common Coordination Numbers:** Six and four are the most common, leading to octahedral and tetrahedral geometries, respectively. However, some complexes can have coordination numbers ranging from two to twelve.
• **Factors Affecting Coordination Number:** These include the size of the metal ion, its electronic configuration, and the size and electronic nature of the ligands.

By determining the coordination number, you can predict the likely geometry and potential reactions of the coordination compound.

Octahedral Geometry

Octahedral geometry is a common spatial arrangement in coordination chemistry for complexes with a coordination number of six. In this configuration, six ligands symmetrically surround the central metal ion, forming an octahedron.

This geometry is significant because it influences both the physical properties and the reactivity of coordination compounds. Octahedral complexes can participate in processes such as ligand substitution reactions, where one ligand switches places with another in the geometry.

Understanding octahedral geometry helps in predicting the electronic interactions within the compound, which can influence properties like color due to d-orbital splitting, and magnetic behavior based on electron pairing.