Question

The complex ion \(\left[\mathrm{Ni}(\mathrm{CN})_{2} \mathrm{Br}_{2}\right]^{2-}\) has a square-planar geometry. Draw the structures of the geometric isomers of this complex.

Short answer

The complex has two isomers: cis and trans.

Step-by-step solution

Understand the Structure and Geometry

The complex ion \([\mathrm{Ni}(\mathrm{CN})_{2}\mathrm{Br}_{2}]^{2-}\) is square-planar. This geometry means that the nickel ion (Ni) is surrounded by four ligands at the corners of a square.

Identify Ligands

In the given complex, the ligands are two cyanide ions (CN\(^-\)) and two bromide ions (Br\(^-\)). These ligands will occupy the positions around the nickel ion in a square-planar arrangement.

Find Geometric Isomers

In a square-planar complex, geometric isomers occur when pairs of ligands can be arranged differently around the central atom. The ligands can either be next to each other (cis) or opposite each other (trans).

Draw the Cis Isomer

For the cis isomer, draw the structure with the two CN\(^-\) ions adjacent to each other on the square plane and the two Br\(^-\) ions adjacent to each other as well.

Draw the Trans Isomer

For the trans isomer, draw the structure with CN\(^-\) and Br\(^-\) ions opposite each other on the square plane.

Key concepts

Square-Planar Complex

A square-planar complex is a specific type of coordination compound where a central metal atom is surrounded by four ligands positioned at the corners of a square. This geometry is common for certain d extsuperscript{8} metal ions, such as nickel(II), platinum(II), and palladium(II). In the context of our exercise, the complex \([\mathrm{Ni}(\mathrm{CN})_{2}\mathrm{Br}_{2}]^{2-}\) adopts this square-planar geometry. In this arrangement:

• The metal, in this case nickel, is at the center of the square plane.
• The four ligands are positioned symmetrically around the metal.

This arrangement leads to unique chemical properties and allows for the occurrence of geometric isomerism, as the position of ligands relative to each other can vary.

Ligands

Ligands are ions or molecules that bind to a central metal atom to form a coordination complex. They can range from simple ions, like chloride \(\mathrm{Cl}^-\), to more complex organic molecules. Ligands are crucial in determining the shape and properties of the complex.

• In the complex \([\mathrm{Ni}(\mathrm{CN})_{2}\mathrm{Br}_{2}]^{2-}\), the ligands are two cyanide ions \(\mathrm{CN}^-\) and two bromide ions \(\mathrm{Br}^-\).
• These ligands act as Lewis bases, donating electrons to the metal center.

Ligands not only influence the geometry of the complex but also its reactivity, color, and magnetic properties.

Cis and Trans Isomers

Cis and trans isomerism is a form of geometric isomerism particularly relevant to square-planar complexes. This type of isomerism involves different placement of ligands around the central atom, resulting in different spatial arrangements. In a square-planar complex:

• The cis isomer occurs when similar ligands are adjacent to each other. For example, in the complex \([\mathrm{Ni}(\mathrm{CN})_{2}\mathrm{Br}_{2}]^{2-}\), both cyanide ligands being next to each other is a cis configuration.
• The trans isomer occurs when similar ligands are opposite each other. In this complex, each cyanide ion is opposite a bromide ion in the trans configuration.

These different arrangements can drastically impact the chemical properties and potential applications of the complexes.

Coordination Chemistry

Coordination chemistry is the study of complex compounds formed between central metal atoms and surrounding molecules or ions called ligands. This field explores the structures, reactivity, and synthesis of these complexes.

• The primary focus is on how these central atoms bind to ligands and the resulting electronic and geometric configurations.
• Coordination chemistry is not only about creating stable complexes but also understanding their behavior, which is integral to fields like catalysis, bioinorganic chemistry, and materials science.

Square-planar complexes and their isomers provide a fascinating glimpse into how slight changes in geometry can lead to significant differences in chemical properties and reactivity, highlighting the importance of coordination chemistry in scientific research and industry.