Question

Determine the oxidation state and coordination number of the metal ion in each complex ion. a. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Br}\right]^{2+}\) b. \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) c. \(\left[\mathrm{Co}(\mathrm{ox})_{3}\right]^{4-}\) d. \(\left[\mathrm{PdCl}_{4}\right]^{2-}\)

Short answer

a. Oxidation state of Co: +3, Coordination number: 6. b. Oxidation state of Fe: +2, Coordination number: 6. c. Oxidation state of Co: +3, Coordination number: 6. d. Oxidation state of Pd: +2, Coordination number: 4.

Step-by-step solution

Analyze the First Complex Ion

For the complex ion \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Br}\right]^{2+}\), we identify ammonium (\(\mathrm{NH}_{3}\)) as a neutral ligand with oxidation state 0 and bromine (Br) usually has an oxidation state of -1. We know that the sum of oxidation states of the metal and ligands should equal the overall charge of the complex. Thus, the oxidation state of Co can be calculated as follows: \(x + 5(0) + (-1) = 2\), solving for \(x\), gives \(x = +3\). The coordination number is given by the number of ligand attachment points to the metal, which is 6 in this case.

Analyze the Second Complex Ion

For the complex ion \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\), the cyanide ligand (\(\mathrm{CN}^-\)) has an oxidation state of -1. Using a similar approach as step 1, for the iron ion we have \(x + 6(-1) = -4\), solving for \(x\) gives \(x = +2\). The coordination number is the number of cyanide ligands attached, which is 6.

Analyze the Third Complex Ion

For the complex ion \(\left[\mathrm{Co}(\mathrm{ox})_{3}\right]^{4-}\), oxalate (\(\mathrm{ox}\)) is a bidentate ligand with an overall charge of -2. The charge sum equation for the cobalt ion will be \(x + 3(-2) = -4\), solving for \(x\) gives \(x = +3\). Each oxalate ligand provides two points of attachment, leading to a coordination number of 6 (2 attachment points per ligand times 3 ligands).

Analyze the Fourth Complex Ion

For the complex ion \(\left[\mathrm{PdCl}_{4}\right]^{2-}\), the chloride ligand (\(\mathrm{Cl}^-\)) has an oxidation state of -1. The palladium ion's oxidation state can be determined by \(x + 4(-1) = -2\), solving for \(x\) gives \(x = +2\). The coordination number is 4, as there are four chloride ions attached to the palladium.

Key concepts

Oxidation State

Understanding the oxidation state of a metal in a complex ion is crucial for grasping the intricacies of coordination chemistry. The oxidation state represents the charge an atom would have if all bonds were ionic, a hypothetical scenario that helps chemists rationalize the distribution of electrons in covalent bonds.

For complex ions, you will typically work out the oxidation state of the central metal by using the known charges of the surrounding ligands and the overall charge of the ion. As you saw in the exercise, to find the oxidation state of cobalt in \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Br}\right]^{2+}\), you considered the neutral ammonia ligands and a single bromine ion, which typically has an oxidation state of -1. Setting up an equation considering these values against the overall charge of the complex ion helps you solve for the metal's oxidation state. This procedure is repeated for each metal ion in various complex ions, providing insight into their electronic configurations and reactivity.

Coordination Chemistry

Coordination chemistry explores the intriguing assembly of complex ions, where a central metal atom or ion is surrounded by a cohort of molecules or ions known as ligands. These ligands are not merely spectators but play a significant role in the stability and properties of the complex by donating electron pairs to the metal, forming coordinate bonds.

In the examples provided, coordination number becomes an important term; it corresponds to the number of ligand attachment points to the metal. These attachment points are where the ligands donate their electrons to form bonds with the central metal ion. The coordination number can offer insight into the geometry of the complex ion—such as octahedral, square planar, or tetrahedral—as seen in the differing coordination numbers for the cobalt, iron, and palladium complexes discussed. Coordination chemistry is a fascinating field where the coordination number not only influences the shape and color of complexes but also their chemical behavior.

Ligand Attachment Points

Ligands are the cornerstone of complex ions, serving as the bridge that connects metal ions into larger, often more functional structures. These ligands attach to the metal via 'attachment points' which are sites where ligands donate their electron pairs to form coordinate covalent bonds with the metal.

The types of ligands are vast, ranging from simple ions like chloride in \(\left[\mathrm{PdCl}_{4}\right]^{2-}\) with a single attachment point, to more complex organic molecules such as oxalate in \(\left[\mathrm{Co}(\mathrm{ox})_{3}\right]^{4-}\), which can have multiple points of attachment. These multidentate ligands can wrap around the metal ion and form several bonds, providing a stronger metal-ligand interaction known as chelation. Understanding ligand attachment points is essential for predicting and explaining the geometry and stability of complex ions.