Question

Sketch the geometry of (a) \(c i s-\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Br}_{4}\right]^{2-}\). (b) trans-[Ni(NH \(\left.\left._{3}\right)_{2}(\mathrm{en})_{2}\right]^{2+}\).

Short answer

Both complexes exhibit octahedral geometry: \( cis-\) for Cu complex, \( trans-\) for Ni complex.

Step-by-step solution

Determine the Coordination Geometry for (a)

For the complex ion \( cis-\left[\mathrm{Cu}\left(\mathrm{H}_{2}\right)_{2}\mathrm{Br}_{4}\right]^{2-} \), the central metal ion is copper (Cu), which typically exhibits coordination numbers of 4 or 6. Given there are 4 bromide ions and 2 water molecules surrounding the copper, the most likely coordination number is 6, which usually leads to an octahedral geometry.

Sketch the Octahedral Geometry for (a)

In an octahedral complex, the ligands are arranged at 90-degree angles from each other about the central atom. For the \( cis-\) configuration, two identical ligands (in this case, water molecules) are adjacent:1. Place Cu at the center.2. Position two H2O ligands adjacent to each other on one face.3. Place the four Br- ions in the remaining positions of the octahedron, ensuring that each is opposite another Br-.

Determine the Coordination Geometry for (b)

For the complex ion \( trans-[Ni(NH_{3})_{2}(en)_{2}]^{2+} \), nickel (Ni) typically exhibits a coordination number of 6. The ethylenediamine (en) acts as a bidentate ligand while ammonia (NH3) is monodentate. This gives us a total of 2 (from two NH3) and 4 (from two en) donor atoms, resulting in an octahedral geometry.

Sketch the Octahedral Geometry for (b)

In the octahedral \( trans- \) configuration, pairs of identical ligands are positioned across from each other. The sketch process includes:1. Place Ni at the center.2. Place two NH3 ligands directly opposite each other.3. Place the two ethylenediamine ligands in the remaining positions, ensuring each en ligand can coordinate through two donor atoms, forming 3-membered rings with Ni at the center.

Key concepts

Octahedral Geometry

In the realm of coordination chemistry, octahedral geometry is a common and pivotal structure. This geometry is characterized by a central atom surrounded by six ligands, positioned symmetrically.
Imagine the structure as two four-sided pyramids (with square bases) joined at the base. The central metal atom or ion, such as copper or nickel, sits at the very center of this arrangement.

• Coordination Number: The number 6 is crucial, due to six bonds formed with ligands.
• Angles: Each angle between the ligands is 90 degrees, providing maximal separation.

These features make the octahedral geometry particularly stable and prevalent, such as seen in the complex ions \([\text{Cu}(\text{H}_{2}\text{O})_{2}\text{Br}_{4}]^{2-}\) and \([\text{Ni}(\text{NH}_{3})_{2}(\text{en})_{2}]^{2+}\). Both possess this admirable symmetry, making them excellent examples of how octahedral structures work in practice.

Ligand Arrangement

Ligand arrangement is another crucial aspect when dealing with coordination complexes. The position of ligands around the central metal ion significantly impacts the properties and isomers of the complex.
In an octahedral complex, there are specific arrangements like 'cis' and 'trans'. When you see 'cis', it means two identical ligands are adjacent, such as the two water molecules in the \([\text{Cu}(\text{H}_{2}\text{O})_{2}\text{Br}_{4}]^{2-}\) complex.

• 'Cis' Configuration: Ligands are next to each other; think of it as neighboring positions.
• 'Trans'' Configuration: Identical ligands are positioned opposite each other, as seen with the ammonia ligands in the \([\text{Ni}(\text{NH}_{3})_{2}(\text{en})_{2}]^{2+}\) complex.

These arrangements determine the overall structure and its properties, influencing things like symmetry and how the complex interacts with other molecules.

Bidentate Ligand

The term 'bidentate ligand' refers to a ligand that forms two bonds with a central metal ion. This occurs when a single ligand molecule uses two different donor atoms to attach to the metal atom. Ethylenediamine, or "en," is a classic example of a bidentate ligand because it uses its two nitrogen atoms to establish connections with the metal ion. This process increases the stability of the complex, creating a 'chelating' effect.

• Bidentate ligands form multiple rings, adding rigidity and stability.
• They often result in more robust metal-ligand interactions compared to monodentate ligands.

In coordination complexes like \([\text{Ni}(\text{NH}_{3})_{2}(\text{en})_{2}]^{2+}\), the presence of bidentate ligands creates three-membered chelate rings. This structural formation is essential for achieving the desired geometry and reinforcing the complex's stability.