Problem 63 - Chemistry The Molecular Nature of Matter

Question

What is the coordination number of nickel in \(\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{NO}_{2}\right)_{2}\right]^{4-} ?\)

Step-by-Step Solution

Answer

The coordination number of nickel in \(\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{NO}_{2}\right)_{2}\right]^{4-}\) is 6.

Step 1: Identify the ligands and their dentate nature

Examine the complex \(\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{NO}_{2}\right)_{2}\right]^{4-}\) to identify the ligands and determine their dentate nature. Oxalate \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) is a bidentate ligand, which means each oxalate can coordinate to the nickel ion through two donor atoms. The nitro group \(\mathrm{NO}_{2}^{-}\) can act as either a monodentate or bidentate ligand, but in this complex, it's commonly monodentate, coordinating through just one nitrogen atom.

Step 2: Count the number of attachment points

Calculate the total number of atoms that coordinate to the nickel ion. There are two oxalate ligands, each offering two attachment points, and there are two nitro groups each offering one attachment point. Therefore, there are \(2\times2 + 2\times1 = 6\) total points of attachment.

Step 3: Determine the coordination number

The coordination number is the total number of points of attachment (ligand donor atoms) to the central metal ion. In this case, the coordination number of nickel in \(\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{NO}_{2}\right)_{2}\right]^{4-}\) is 6.

Key Concepts

Ligands in Coordination Chemistry

In coordination chemistry, ligands are molecules or ions that bind to a central metal atom to form a coordination complex. They possess at least one donor atom with a lone pair of electrons that can be used to establish a coordinative bond with the metal. Depending on the number of these donor atoms, ligands can be classified as monodentate, where only one donor atom is available, or polydentate, which includes bidentate, tridentate, and so forth, with multiple donor atoms.

For instance, in the complex ion \[\begin{equation}\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{NO}_{2}\right)_{2}\right]^{4-},\end{equation}\] water (\[\begin{equation}H_2O\end{equation}\]) and ammonia (\[\begin{equation}NH_3\end{equation}\]) would be considered typical examples of monodentate ligands, though they are not present in this particular complex. The understanding of different ligand types and how they interact with central metal ions is key to grasping the structure and behavior of coordination compounds.

Dentate Nature of Ligands

The term 'dentate' refers to the 'toothed' manner in which ligands attach to the metal center, characterized by the number of coordination connections they can make. Monodentate ligands, as the name suggests, bind through a single 'tooth' or donor atom. In contrast, polydentate ligands, such as bidentate ligands, have multiple binding sites. The bidentate ligands can form chelates, which involve the formation of a five- or six-membered ring including the metal ion.

In our example, the oxalate ion (\[\begin{equation}\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\end{equation}\]) is bidentate and therefore binds to the nickel ion at two points, creating a more stable ring structure. Understanding the dentate nature is crucial for predicting the stability and reactivity of the complex ion created. The more points of attachment a ligand has, the more stable the complex tends to be due to the chelate effect.

Complex Ion Structure

The structure of a complex ion encompasses both the spatial arrangement of the ligands around the central metal ion and the coordination number, which is the number of points at which the ligands are attached to the metal ion. The arrangement of the ligands is governed by principles of molecular geometry, such as the VSEPR theory, which help predict shapes based on repulsions between electron pairs.

For the complex ion \[\begin{equation}\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{NO}_{2}\right)_{2}\right]^{4-},\end{equation}\] we determined a coordination number of six. Common geometries for this coordination number include octahedral, where the ligands are evenly distributed as if at the corners of an octahedron, and can produce symmetric and distorted isomers. It's important for students to visualize these structures, as they are significant in understanding the complex's properties and behavior in various chemical reactions and applications.