Problem 25 - Chemistry: The Central Science in SI Units, Global Edition

Question

Indicate the coordination number and the oxidation number of the metal for each of the following complexes: (a) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA})]\) (b) \(\mathrm{KMnO}_{4}\) (c) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2}\) (d) \(\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_{6}\) (e) \(\mathrm{Rh}\left(\mathrm{PPh}_{3}\right)_{3} \mathrm{Cl}\) (f) \(\mathrm{Zn}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{NH}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

(a) Coordination number: 6, Oxidation number: +2 (b) Coordination number: 4, Oxidation number: +7 (c) Coordination number: 4, Oxidation number: +2 (d) Coordination number: 6, Oxidation number: +3 (e) Coordination number: 4, Oxidation number: +1 (f) Coordination number: 3, Oxidation number: +2

Step 1: a) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA})]\)

The metal in this complex is Co. The ligand is EDTA, which is a hexadentate ligand (it can bind to the metal through six donor sites). Therefore, the coordination number is 6. As for the oxidation number, the overall charge of the complex is -2 (due to the 2 sodium ions), and the charge of EDTA is -4. Therefore, the oxidation number of the Co is +2.

Step 2: b) \(\mathrm{KMnO}_{4}\)

The metal in this compound is Mn. The ligands in this compound are four oxygen atoms. Therefore, the coordination number is 4. As for the oxidation number, K has a +1 charge and each oxygen atom has a -2 charge. Thus, \(1 + x + 4(-2) = 0\), where x is the oxidation number of Mn. Solving for x, we get the oxidation number of Mn as +7.

Step 3: c) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2}\)

In this complex, the metal is Pt. The ligands are four ammonia molecules. Therefore, the coordination number is 4. As for the oxidation number, the overall charge of the complex is 0, and each NH3 molecule has a charge of 0. Therefore, the oxidation number of the Pt must be +2, as the overall charge of the chloride ions is -2.

Step 4: d) \(\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_{6}\)

In this complex, the metal is Fe. The ligands are six CN- ions. Therefore, the coordination number is 6. As for the oxidation number, K has a +1 charge and each CN- ion has a -1 charge. So, \(3 + x - 6 = 0\), where x is the oxidation number of Fe. Solving for x, the oxidation number of Fe is +3.

Step 5: e) \(\mathrm{Rh}\left(\mathrm{PPh}_{3}\right)_{3} \mathrm{Cl}\)

In this complex, the metal is Rh. There are three PPh3 ligands and one Cl- ligand. Therefore, the coordination number is 4. As for the oxidation number, since all PPh3 ligands have a charge of 0, we only need to consider the charge of Cl-, which is -1. The charge of the Rh must be +1 to balance this charge. So, the oxidation number of Rh is +1.

Step 6: f) \(\mathrm{Zn}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\left(\mathrm{NH}_{3}\right)_{2}\)

In this complex, the metal is Zn. There are one C2O4 ligand and two NH3 ligands. Therefore, the coordination number is 3. As for the oxidation number, since C2O4 has a charge of -2 and each NH3 has a charge of 0, the charge of the Zn must be +2 to balance the charge of the complex. Therefore, the oxidation number of Zn is +2.

Key Concepts

Coordination Number

In coordination chemistry, the coordination number represents the number of ligand atoms that are directly bonded to the central metal atom or ion in a complex. It tells us how many points of attachment or connections the ligands make with the metal. Coordination numbers can vary depending on the size and charge of the metal ion and the ligands involved. Common coordination numbers are 2, 4, and 6.

• For example, in the complex \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA})]\), EDTA is a hexadentate ligand, meaning it can attach to cobalt at six different sites, giving this complex a coordination number of 6.
• On the other hand, \(\mathrm{KMnO}_{4}\), with four oxygen atoms around manganese, has a coordination number of 4.

Understanding how to determine the coordination number is fundamental in predicting the shape and reactivity of a coordination compound.

Oxidation Number

The oxidation number of a metal in a complex indicates the electrical charge the metal would have if all the bonds to ligands were ionic. This number helps in identifying the degree of oxidation, which is crucial for understanding chemical reactions. Calculating the oxidation number involves balancing the total charge of the complex with the charges of the ligands and any counterions.

• In \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA})]\), the coordination complex has a charge of \(-2\), while EDTA, a negatively charged ligand, contributes \(-4\), leading to an oxidation number of \(+2\) for cobalt.
• \(\mathrm{KMnO}_{4}\), overall neutral, has each oxygen with a charge of \(-2\), which implies a \(+7\) oxidation state for manganese after considering the \(+1\) charge from potassium.

Understanding oxidation numbers is essential for predicting how complexes participate in redox reactions.

Ligands

Ligands are ions or molecules that bind to a central metal atom to form a coordination complex. They donate electron pairs to the metal, acting as Lewis bases. Ligands can be neutral like water or ammonia, or negatively charged like chloride or cyanide ions. Ligands vary in the number of donor sites:

• Monodentate ligands, like \(\mathrm{NH}_3\), use one atom to bind to the metal.
• Polydentate ligands, like EDTA, can bind through multiple donor sites.

In \([\mathrm{Pt}(\mathrm{NH}_3)_{4}] \mathrm{Cl}_2\), ammonia is the ligand that surrounds platinum, creating a stable complex. The nature and number of ligands influence the properties and stability of the metal complexes.

Metal Complexes

Metal complexes consist of a central metal atom or ion surrounded by molecules or anions, known as ligands. These complexes can have specific geometries, dictated by the coordination number and the type of ligands. The structure of metal complexes is essential for understanding their chemical behavior:

• For instance, \([\mathrm{Pt}(\mathrm{NH}_3)_{4}] \mathrm{Cl}_2\) forms a squared planar geometry due to its coordination number of 4.
• \(\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_6\) features an octahedral shape as a result of its coordination number of 6.

Metal complexes play significant roles in catalysis, medicine, and materials science, often demonstrating unique chemical properties based on their structure.