Problem 51 - Chemistry: The Central Science

Question

Sketch the structure of the complex in each of the following compounds: (a) \(c i s-\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\left(\mathrm{NO}_{3}\right)_{2}\) (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-[Ru(en) \(\left._{2} \mathrm{Cl}_{2}\right]\)

Step-by-Step Solution

Answer

(a) In cis-[Co(NH3)4(H2O)2](NO3)2, the central atom (Co) is surrounded by 4 NH3 ligands and 2 H2O ligands in a cis configuration, with the water ligands adjacent to each other. (b) In Na2[Ru(H2O)Cl5], the central atom (Ru) is surrounded by 1 H2O ligand and 5 Cl- ligands. (c) In trans-NH4[Co(C2O4)2(H2O)2], the central atom (Co) is surrounded by 2 C2O4 ligands and 2 H2O ligands in a trans configuration, with the water ligands opposite each other. (d) In cis-[Ru(en)2Cl2], the central atom (Ru) is surrounded by 2 en ligands and 2 Cl- ligands in a cis configuration, with the chloride ligands adjacent to each other.

Step 1: Identify the central atom and ligands

The central atom in this complex is cobalt (Co). The ligands are 4 ammonia (NH3) and 2 water (H2O) molecules.

Step 2: Determine arrangement of ligands around central atom

The given structure is cis. This means the same types of ligands (in this case, the two water molecules) are adjacent to each other around the central atom.

Step 3: Draw the structure

In order to sketch the structure of the complex, place cobalt at the center and arrange the ligands around it in a cis configuration. Make sure to place the two water molecules next to each other, with the four ammonia molecules occupying the remaining positions. (b) \(\mathrm{Na}_{2}\left[\mathrm{Ru}\left(\mathrm{H}_{2} \mathrm{O}\right)\mathrm{Cl}_{5}\right]\)

Step 4: Identify the central atom and ligands

The central atom in this complex is ruthenium (Ru). The ligands are 1 water (H2O) molecule and 5 chloride ions (Cl-).

Step 5: Determine arrangement of ligands around central atom

Arrangement information is not given, but we can draw the structure showing a total of six positions around the ruthenium atom being occupied by the ligands.

Step 6: Draw the structure

Place ruthenium at the center of the sketch with water and the chloride ions around it. Put the water molecule in one position and the chloride ions in the other five positions. (c) trans- \(\mathrm{NH}_{4}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\)

Step 7: Identify the central atom and ligands

The central atom in this complex is cobalt (Co). The ligands are 2 oxalate ions (C2O4) and 2 water (H2O) molecules.

Step 8: Determine arrangement of ligands around central atom

The given structure is trans. This means the same types of ligands (in this case, the two water molecules) are opposite each other around the central atom.

Step 9: Draw the structure

Place cobalt at the center and arrange the ligands around it in a trans configuration. Make sure to place the two water molecules opposite each other, with the two oxalate ions occupying the other, opposite positions. (d) cis-[Ru(en) \(\left._{2} \mathrm{Cl}_{2}\right]\)

Step 10: Identify the central atom and ligands

The central atom in this complex is ruthenium (Ru). The ligands are 2 ethylenediamine (en) molecules and 2 chloride ions (Cl-).

Step 11: Determine arrangement of ligands around central atom

The given structure is cis. This means the same types of ligands (in this case, the two chloride ions) are adjacent to each other around the central atom.

Step 12: Draw the structure

Place ruthenium at the center and arrange the ligands around it in a cis configuration. Make sure to place the two chloride ions next to each other, with the two ethylenediamine molecules occupying the remaining positions.

Key Concepts

Cobalt Complexes

Cobalt complexes are fascinating structures in coordination chemistry, where cobalt typically acts as the central metal atom. In these complexes, cobalt can adopt different oxidation states, but +2 and +3 are the most common. These complexes often feature various ligands that bind to the cobalt through coordination bonds. Ligands such as ammonia (NH₃) and water (H₂O) interact with cobalt to form stable configurations. Such complexes can exhibit different geometries, like octahedral, common for a coordination number of six.
For example, in a complex like cis-[Co(NH₃)₄(H₂O)₂](NO₃)₂, cobalt is surrounded by six ligands, forming an octahedral shape. This specific coordination leads to interesting chemical properties and plays an important role in applications like catalysis and electronics.

Ligand Arrangement

In coordination complexes, ligand arrangement refers to the spatial positioning of ligands around a central metal atom. This arrangement is a crucial factor determining the compound's properties and functionality. Ligands can be monodentate, which means they bind through a single atom, or polydentate, binding through multiple atoms.
The arrangement can be specified as cis or trans. In cis configurations, ligands of the same type are adjacent, like in cis-platin, a well-known chemotherapy drug. On the other hand, in trans configurations, similar ligands are located opposite to each other. The difference in these arrangements can lead to different chemical, electrical, and biological properties, which is critical in tailoring materials and pharmaceutical compounds.

Ruthenium Complexes

Ruthenium complexes constitute an important class in coordination chemistry due to their versatile applications. Ruthenium can exhibit various oxidation states, often encompassing +2, +3, or +4, enabling it to form varying coordination complexes. These complexes have significant roles in areas like catalysis, where ruthenium catalysts aid in hydrogenation reactions.
Compounds like Na₂[Ru(H₂O)Cl₅] illustrate the diversity of ruthenium's coordination capabilities, featuring mixed-ligand environments that offer unique reactivity. Typically, these complexes contain mixed ligand types, such as chloride ions or water molecules, arranged around the central ruthenium atom in various geometries, often leading to octahedral or square-planar configurations.

Cis-Trans Isomerism

Cis-trans isomerism is a form of stereoisomerism where the same atoms are connected differently in three-dimensional space. This type of isomerism is particularly important in coordination chemistry for compounds that form geometric shapes, such as octahedral or square planar systems.
In the case of complexes like trans-[Co(C₂O₄)₂(H₂O)₂]²⁻, the isomerism comes from the placement of similar ligands across the metal center. Trans isomers have identical ligands placed opposite each other, while in cis isomers, they are adjacent. This spatial difference can significantly affect the compound's chemical properties, solubility, and, in biological contexts, its activity, impacting things like drug administration and effectiveness.