Coordination Numbers & Geometry: Videos & Practice Problems

Coordination number indicates the number of ligands bonded to a central metal cation, commonly 2, 4, or 6. Complexes exhibit predictable geometries: linear for coordination number 2, octahedral for 6, and either tetrahedral (d₁₀) or square planar (d₈) for 4. Understanding these geometries is crucial for predicting the behavior of coordination compounds in chemical reactions, including acid-base reactions and complex ion formation.

Coordination Numbers

The coordination number is a crucial concept in coordination chemistry, representing the number of ligands that are bonded to a central metal cation. Common coordination numbers include 2, 4, and 6, which correspond to different geometrical arrangements of the ligands around the metal ion. Understanding these coordination numbers is essential when studying coordination complexes, as they influence the properties and reactivity of the complex ions formed.

In coordination complexes, ligands can vary in size, charge, and bonding characteristics, which can affect the overall stability and geometry of the complex. For instance, a coordination number of 2 typically leads to a linear arrangement, while a coordination number of 4 can result in either a tetrahedral or square planar geometry. A coordination number of 6 usually results in an octahedral geometry, which is one of the most common arrangements in transition metal complexes.

By examining the coordination numbers and the types of ligands involved, one can gain deeper insights into the behavior and characteristics of various coordination complexes, enhancing our understanding of their applications in fields such as catalysis, materials science, and biochemistry.

Coordination Numbers and Geometry Example

To determine the coordination number of a complex ion, focus on the ligands attached to the central metal cation. In this case, the complex ion consists of chromium surrounded by four water molecules and two bromide ions. The coordination number is calculated by counting the total number of ligands bonded to the metal. Here, we have:

4 (from water) + 2 (from bromide) = 6 ligands.

Thus, the coordination number for this complex ion is 6, indicating that the central metal ion is coordinated to six ligands in total.

Molecular Geometry of Coordination Complexes

Coordination complexes exhibit specific geometries that are determined by their coordination number and electron configuration. The coordination number indicates the number of ligands attached to a metal cation, with common values being 2, 4, and 6 in general chemistry.

For a coordination number of 2, the geometry is linear, as two ligands create a straight line. A helpful mnemonic is to remember that two points define a line. In contrast, a coordination number of 6 results in an octahedral geometry, which can be visualized with the analogy of an octopus having eight tentacles, minus two for its eyes, leading to six effective coordination sites.

When dealing with a coordination number of 4, the electron configuration of the metal cation becomes crucial. If the configuration ends with d^{10}, the geometry is tetrahedral. This can be remembered by associating "ten" with "tetrahedral," as both contain the number 10. Conversely, if the electron configuration ends with d^{8}, the geometry is square planar. A useful way to recall this is that a square has four sides, and a plane can be thought of as having two wings, leading to the multiplication of 4 and 2, which equals 8.

In summary, understanding the relationship between coordination numbers and electron configurations allows for the prediction of the geometries of coordination complexes. For coordination numbers of 2, 4, and 6, the corresponding geometries are linear, tetrahedral or square planar, and octahedral, respectively. Utilizing these memory aids can facilitate the learning process in identifying the correct geometry for various complex ions.

Coordination Numbers and Geometry Example

In coordination chemistry, the geometry of a complex ion is determined by the number of ligands surrounding the central metal atom. In this case, we have chromium as the central metal, coordinated to four ammonia (NH₃) molecules and two chloride (Cl^-) ions. The total number of ligands is six, which is a key factor in determining the geometry of the complex.

When a metal ion is surrounded by six ligands, the resulting geometry is typically octahedral. This arrangement allows for optimal spatial distribution of the ligands around the central atom, minimizing repulsion between them. Therefore, for the complex ion with chromium, four ammonia molecules, and two chloride ions, the geometry is confirmed to be octahedral.

Correctly label all the components of the coordination complex and its coordination number: Na₂[SnCl₆].

• A. Sn²⁺: metal cation
  Cl^–: ligands
  Na⁺: counterions
• B. Sn²⁺: metal cation
  Cl^–: counterions
  Na⁺: ligands
• C. Sn²⁺: counterion
  Cl^–: ligands
  Na⁺: metal cation
• D. Sn⁴⁺: metal cation
  Cl^–: ligands
  Na⁺: counterions
• E. Sn⁴⁺: metal cation
  Cl^–: counterion
  Na⁺: ligands
• F. Sn⁴⁺: counterion
  Cl^–: ligands
  Na⁺: metal cation

Answer: (D)

Determine the geometry for the following complex ion: [Ni(OH)₄]^2–.

• A. Tetrahedral
• B. Square planar
• C. Octahedral
• D. Linear

Answer: (B)

Determine the geometry for the following complex molecule: Pd(H₂O)₄.

• A. Octahedral
• B. Square Planar
• C. Linear
• D. Tetrahedral

Answer: (D)

Here’s what students ask on this topic:

What is the coordination number in a coordination complex?

The coordination number in a coordination complex refers to the number of ligands that are bonded to the central metal cation. Common coordination numbers are 2, 4, and 6. For example, if a metal cation is bonded to two ligands, its coordination number is 2. This concept is crucial in understanding the structure and geometry of coordination complexes, as the coordination number directly influences the spatial arrangement of the ligands around the central metal ion.

How does the coordination number affect the geometry of a coordination complex?

The coordination number significantly affects the geometry of a coordination complex. For a coordination number of 2, the geometry is linear, visualized as two points forming a straight line. A coordination number of 4 can result in either a tetrahedral or square planar geometry, depending on the electron configuration of the metal cation. Specifically, a d₁₀ configuration leads to a tetrahedral geometry, while a d₈ configuration results in a square planar geometry. A coordination number of 6 typically results in an octahedral geometry, which can be humorously remembered as an octopus with 8 tentacles minus 2 eyes.

What is the geometry of a coordination complex with a coordination number of 4 and a d₁₀ electron configuration?

A coordination complex with a coordination number of 4 and a d₁₀ electron configuration will have a tetrahedral geometry. This can be remembered using the memory aid that both 'd₁₀' and 'tetrahedral' contain the letters 'T' and 'E'. In this geometry, the four ligands are positioned at the corners of a tetrahedron around the central metal cation.

What is the geometry of a coordination complex with a coordination number of 6?

A coordination complex with a coordination number of 6 typically has an octahedral geometry. This can be visualized as six ligands positioned at the corners of an octahedron around the central metal cation. A helpful memory aid is to think of an octopus with 8 tentacles, and by subtracting 2 eyes, you get 6, which corresponds to the coordination number.

How can you remember the geometry of a coordination complex with a coordination number of 4 and a d₈ electron configuration?

To remember the geometry of a coordination complex with a coordination number of 4 and a d₈ electron configuration, you can use the memory aid that a square has 4 sides and a plane has 2 wings. Multiplying these together (4 sides × 2 wings) gives 8, which corresponds to the d₈ configuration. Therefore, the geometry is square planar.