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) \(\operatorname{trans}-\mathrm{NH}_{4}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\) (d) \(c i s-\left[\operatorname{Ru}(e n)_{2} C l_{2}\right]\)

Short answer

In summary: (a) The complex contains a Co central atom surrounded by 4 NH3 ligands and 2 H2O ligands in a cis-configuration. Sketch the square planar complex with the two water ligands and two ammonia ligands next to each other. (b) The complex contains a Ru central atom surrounded by 1 H2O ligand and 5 Cl ligands. Sketch the octahedral complex with 5 chlorides surrounding the water ligand. Show two Na+ ions outside the complex as counterions. (c) The complex contains a Co central atom surrounded by 2 C2O4 ligands and 2 H2O ligands in a trans-configuration. Sketch the octahedral complex with C2O4 ligands on opposite sides and water ligands on the other two coordination sites, also opposite each other. Show an NH4+ ion outside the complex as the counterion. (d) The complex contains a Ru central atom surrounded by 2 en ligands and 2 Cl ligands in a cis-configuration. Sketch the square planar complex with en ligands next to each other and Cl ligands also next to each other.

Step-by-step solution

(a) Identify central atom and ligands for the given complex c i s-\(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\left(\mathrm{NO}_{3}\right)_{2}\)

In this case, the central metal atom is Co (Cobalt). It is surrounded by 4 NH3 ligands (ammonia) and two H2O ligands (water). The cobalt is in a cis-configuration, which means that the similar ligands are next to each other.

(a) Sketch the structure of the complex

To sketch the complex, start by drawing the central Co atom. Then, place the four NH3 ligands around the Co in a square plane, with two adjacent corners occupied by NH3. Place the two H2O ligands next to each other on the other two corners. Finally, show that there are two NO3- ions outside the complex, not directly bonded to the central metal atom.

(b) Identify central atom and ligands for the given complex \(\mathrm{Na}_{2}\left[\mathrm{Ru}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}_{5}\right]\)

In this case, the central metal atom is Ru (Ruthenium). It is surrounded by one H2O ligand (water) and five Cl ligands (chloride).

(b) Sketch the structure of the complex

To sketch the complex, start by drawing the central Ru atom. Attach the H2O ligand to Ru and then place the five Cl ligands around it in an octahedral fashion. Show that there are two Na+ ions outside the complex as counterions.

(c) Identify central atom and ligands for the given complex \(\operatorname{trans}-\mathrm{NH}_{4}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\)

In this case, the central metal atom is Co (Cobalt). It is surrounded by two C2O4 ligands (oxalate), and two H2O ligands (water). The complex has a trans-configuration, which means the similar ligands are opposite to each other.

(c) Sketch the structure of the complex

To sketch the complex, start by drawing the central Co atom. Attach the two C2O4 ligands in a trans-configuration, which means they are on opposite sides of the Co. Place the two H2O ligands on the other two coordination sites opposite to each other. Finally, show that there is an NH4+ ion outside the complex as the counterion.

(d) Identify central atom and ligands for the given complex \(c i s-\left[\operatorname{Ru}(e n)_{2} C l_{2}\right]\)

In this case, the central metal atom is Ru (Ruthenium). It is surrounded by two en (ethylenediamine) ligands and two Cl (chloride) ligands. The complex has a cis-configuration, which means that the similar ligands are next to each other.

(d) Sketch the structure of the complex

To sketch the complex, start by drawing the central Ru atom. Attach the two en ligands to Ru in a cis-configuration, which means they are next to each other. Place the two Cl ligands on the other two corners, also next to each other.

Key concepts

Cis-Trans Isomerism

In coordination complexes, cis-trans isomerism is a type of stereochemistry that deals with the arrangement of ligands around a central metal atom. The term 'cis' refers to the configuration where two identical or similar ligands are adjacent to each other, while 'trans' indicates that these ligands are positioned opposite to each other. This form of geometric isomerism is significant because it can greatly influence the properties of the complex, including its reactivity, solubility, and even color.

For example, in the complex cis-[Co(NH₃)₄(H2O)₂](NO₃)₂, the two water molecules are positioned next to each other, which is indicative of a cis-arrangement. In contrast, the trans-NH₄[Co(C₂O₄)₂(H2O)₂] complex has its water molecules on opposite sides of the cobalt atom, showcasing a trans configuration. Understanding the differences between cis and trans isomers allows for a deeper comprehension of their chemical behavior and their potential applications in fields like medicinal chemistry.

Ligand Field Theory

Ligand Field Theory (LFT) is an important concept in the chemistry of coordination complexes, providing a way to understand the electronic structure and properties of the compounds formed between transition metals and ligands. This theory extends the principles of crystal field theory by taking into account the covalent as well as ionic aspects of the metal-ligand bond.

LFT describes how the ligands, which are negative or neutral entities surrounding the central metal ion, influence the energy of the metal's d-orbitals. The spatial arrangement of ligands creates an electrostatic field that causes a splitting of the d-orbital energy levels. The nature and extent of this splitting can affect the magnetic and spectroscopic properties of the complex, as well as its color. For example, whether the cobalt in a complex such as [Co(NH₃)₄(H2O)₂](NO₃)₂has a high-spin or low-spin configuration can be analyzed through LFT by considering the strength of the ligand field.

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, is a type of stereoisomerism that involves the arrangement of ligands around a central metal atom in a coordination compound. It's crucial to distinguish geometric isomerism from other isomerism types, such as optical isomerism, because geometric isomers can have vastly different physical and chemical properties.

The way ligands are positioned in relation to each other within the coordination sphere defines the specific isomer. For instance, the compounds cis-[Co(NH₃)₄(H2O)₂](NO₃)₂and trans-[Co(NH₃)₄(H2O)₂](NO₃)₂are geometric isomers of each other. These two isomers, despite having the same molecular formula, can be separated and identified due to their unique geometric arrangements. In industrial applications, one isomer might be more effective or desirable than the other, making the understanding of geometric isomerism essential for the synthesis of specific compounds.

Transition Metal Chemistry

Transition metal chemistry centers on the chemical behavior of the elements found in the d-block of the periodic table. These metals are called transition metals because they exhibit variable oxidation states and typically form colorful compounds, thanks to their d-orbitals. Coordination complexes are a fundamental part of this field, where transition metals are bonded to a variety of ligands.

The ability of transition metals to coordinate with different ligands, like NH₃, H2O, Cl^-, and en (ethylenediamine), leads to a vast array of structures and properties, further enriched by isomerism. The study of transition metal chemistry is essential, not only for understanding basic science but also for practical applications such as catalysis, material science, and medicine. For instance, the metal ruthenium, found in complexes like [Na₂[Ru(H2O)Cl₅]and cis-[Ru(en)₂Cl₂], illustrates the diverse nature of the chemistry entailing transition metals, where subtle changes in coordination can lead to drastic differences in function and reactivity.