Question

Indicate the coordination number and the oxidation number of the metal for each of the following complexes: (a) \(\mathrm{Na}_{2}\left[\mathrm{CdCl}_{4}\right]\) (b) \(\mathrm{K}_{2}\left[\mathrm{MoOCl}_{4}\right]\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (d) \(\left[\mathrm{Ni}(\mathrm{CN})_{5}\right]^{3-}\) (e) \(\mathrm{K}_{3}\left[\mathrm{~V}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]\) (f) \(\left[\mathrm{Zn}(\mathrm{en})_{2}\right\rceil \mathrm{Br}_{2}\)

Short answer

a) Coordination number: 4; Oxidation number: 2+ b) Coordination number: 5; Oxidation number: 4+ c) Coordination number: 6; Oxidation number: 3+ d) Coordination number: 5; Oxidation number: 2+ e) Coordination number: 6; Oxidation number: 3+ f) Coordination number: 4; Oxidation number: 2+

Step-by-step solution

Coordination number

The ligand in this complex is \(\mathrm{Cl}^{-}\) and there are 4 of them bonded to \(\mathrm{Cd}\), so the coordination number is 4.

Oxidation number

Since all 4 ligands have negative charges \(1-\) and two sodium ions \(Na\) have positive charges \(1+\) each, the oxidation number of \(\mathrm{Cd}\) is 2+ to balance those charges. b) For the complex \(\mathrm{K}_{2}\left[\mathrm{MoOCl}_{4}\right]\):

Coordination number

For this complex, there are 5 ligands around the central metal atom. Thus, the coordination number is 5.

Oxidation number

The ligand \(\mathrm{O}^{2-}\) has a charge of \(2-\) and each \(\mathrm{Cl}^{-}\) has a charge of \(1-\). The total negative charge of the ligands is \(4-\) and each of the 2 potassium ions have a positive charge \(1+\) each, balancing the total negative charge. Thus, the oxidation number of \(\mathrm{Mo}\) should be 4+. c) For the complex \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]\mathrm{Cl}\):

Coordination number

There are 6 ligands bonded to the central metal \(\mathrm{Co}\). So, the coordination number is 6.

Oxidation number

The ligand \(\mathrm{NH}_{3}\) is neutral, so their oxidation number contribution is zero. Since there are 3 \(\mathrm{Cl}\) ions with a total charge of \(3-\), the oxidation number of \(\mathrm{Co}\) must be 3+ to balance the charge. d) For the complex \(\left[\mathrm{Ni}(\mathrm{CN})_{5}\right]^{3-}\):

Coordination number

There are 5 \(\mathrm{CN}\) ligands bonded to the central metal \(\mathrm{Ni}\). Hence, the coordination number is 5.

Oxidation number

Each \(\mathrm{CN}^{-}\) ligand has a charge of \(1-\) and there are 5 of them, making a total of \(5-\) charge. With the complex ion's total charge of \(3-\), the oxidation number of \(\mathrm{Ni}\) is 2+. e) For the complex \(\mathrm{K}_{3}\left[\mathrm{~V}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{3}\right]\):

Coordination number

There are 3 \(\mathrm{C}_{2}\mathrm{O}_{4}\) ligands, each having 2 oxygen atoms bonded to the metal, totaling 6 atoms bonded to \(\mathrm{V}\). This makes the coordination number 6.

Oxidation number

Each \(\mathrm{C}_{2}\mathrm{O}_{4}\) is a bidentate ligand and has a charge of \(2-\), making a total of \(6-\) charge for the 3 ligands. With 3 potassium ions each having \(1+\) charge, the balance is achieved with \(\mathrm{V}\) having an oxidation number of 3+. f) For the complex \(\left[\mathrm{Zn}(\mathrm{en})_{2}\right] \mathrm{Br}_{2}\):

Coordination number

There are 2 \(\mathrm{en}\) ligands bonded to \(\mathrm{Zn}\) and since each ligand can form bonds with 2 central atoms, the coordination number is 4.

Oxidation number

The ligand \(\mathrm{en}\) is neutral, so their oxidation number contribution is zero. As there are 2 \(\mathrm{Br}^{-}\) ions with a total charge of \(2-\), the oxidation number of \(\mathrm{Zn}\) must be 2+ to balance the charges.

Key concepts

Coordination Number

The coordination number in coordination chemistry refers to the number of ligand atoms directly bonded to the central metal atom in a complex. This is crucial for understanding the geometry and potential reactivity of the complex. For example, in the complex \([\mathrm{CdCl}_4]^{2-}\), the central cadmium metal is bonded to four chloride ions, giving it a coordination number of 4.

Coordination numbers can vary widely, from 2 to as high as 12, depending on the size, charge, and electron configuration of the metal and the ligands involved. Typically:

• Coordination number 2 often results in a linear shape.
• Coordination number 4 can be square planar or tetrahedral.
• Coordination number 6 is common and usually results in an octahedral shape.

Understanding coordination numbers helps predict the structure and properties of the complex, which is vital in fields like bioinorganic chemistry and catalysis.

Oxidation State

The oxidation state of a metal in a complex ion is the charge that metal would have if all the ligands and electron pairs shared with the metal were removed. It provides insights into the electron configuration and overall charge distribution within the complex. For instance, in \([\mathrm{MoOCl}_4]^{2-}\), the central molybdenum has an oxidation state of 4+. This accounts for the total charge from the ligands and external balancing ions like potassium in this complex.

To determine the oxidation state:

• Consider the known charges of ligands. For example, \mathrm{Cl}^{-}\ is -1, and \mathrm{NH}_3\ is neutral.
• Combine these with the known charge of the entire complex.
• The difference provides the metal's oxidation state.

Knowing the oxidation state helps in understanding the metal's potential reactivity and its electron-donating or -accepting behavior.

Transition Metals

Transition metals are elements from the d-block of the periodic table, known for their ability to form various complex ions. They have unique properties due to their unfilled d orbitals, allowing them to form multiple oxidation states and coordination complexes, like \([\mathrm{Co(NH}_3)_4 \mathrm{Cl}_2] \mathrm{Cl}\), where cobalt can adjust its electron configuration to bond with different ligands.

Key characteristics of transition metals include:

• They often exhibit variable oxidation states, enabling multiple complex formations.
• They commonly form colored compounds, a result of d-d electron transitions.
• They can act as catalysts in many chemical reactions.

Understanding how transition metals interact with ligands provides insights into their roles in both industrial processes and biological systems.

Complex Ions

Complex ions, or coordination complexes, consist of a central metal ion bonded to surrounding ligands. These ions are foundational in coordination chemistry. An example is \([\mathrm{Ni(CN)}_5]^{3-}\), where nickel is surrounded by five cyanide ions, forming a stable structure.

Characteristics of complex ions include:

• Central metal ions are often positively charged transition metals.
• Ligands can be neutral (like water or ammonia) or negatively charged (like chloride or cyanide).
• The stability and formation of complex ions depend on factors such as ligand types and the charge of the metal.

Complex ions play roles in catalysis, material science, and biochemistry, highlighting their importance across various fields.

Ligands

Ligands are ions or molecules that donate an electron pair to form a coordinate bond with a central metal atom. They are crucial in forming coordination complexes. In \([\mathrm{Zn(en)}_2] \mathrm{Br}_2\), ethylenediamine (en) acts as a bidentate ligand, meaning it can attach at two points to the zinc center, stabilizing the complex.

Types of ligands include:

• Monodentate: Bind through one atom, like \mathrm{Cl}^{-}\ or \mathrm{NH}_3\.
• Bidentate: Bind through two atoms, like ethylenediamine.
• Polydentate: Can bind through multiple atoms, such as EDTA.

Understanding ligands is key to predicting the shape, reactivity, and application of coordination complexes in varied scientific disciplines.