ChemistryChapter 6: The Transition Metals
6.3.6 Geometries of Complexes
- Geometry of a Complex:
Explanation level
In the previous sections, we learned that transition metal ions form complexes with ligands. The arrangement of these ligands in three-dimensional space around the central metal ion is called the geometry of the complex. The geometry is crucial because it influences the properties of the complex, including its color and magnetic behavior. This section explains the common geometries of transition metal complexes and how they are determined by the coordination number and the nature of the ligands.
Definition
- Geometry of a Complex: The three-dimensional arrangement of ligands around the central metal ion.
- Coordination Number: The number of coordinate covalent bonds formed by the metal ion with ligands.
Core Concept
The core concept is that the geometry of a complex is determined primarily by its coordination number and the nature of the ligands (their size and charge). The ligands arrange themselves around the metal ion to minimize repulsion between them. This is an application of the Valence Shell Electron Pair Repulsion (VSEPR) theory, which states that electron pairs (in this case, the electron pairs shared in the coordinate bonds) repel each other and will arrange themselves as far apart as possible.
Detailed Explanation
The textbook describes the geometries associated with the most common coordination numbers: 6, 4, and 2.
1. Complexes with Coordination Number 6: Octahedral Geometry
- Shape: The complexes with coordination number 6 adopt octahedral shapes, in which six donor atoms of ligands (both monodentate and polydentate) are positioned at the vertices of an octahedron (oct means eight and hedron means faces).
- Reason for Geometry: According to VSEPR theory, this three-dimensional arrangement of ligands experiences minimum ligand-ligand repulsion. The six ligands are as far apart from each other as possible.
- Commonality: Octahedral complexes are the most common and stable complexes, and almost all transition metals can form octahedral complexes.
- Ligands: The octahedral complexes can have both monodentate and polydentate ligands.
- Bond Angles: The angles between all ligands is 90°.
- Examples:
- Hexaaquanickel(II) ion, [Ni(H_2O)_6]2+
- Tris(1,2-diaminoethane)nickel(II) ion, [Ni(en)_3]2+
2. Complexes with Coordination Number 4
There are two possible arrangements of ligands around central metal ions in complexes with coordination number 4. One is the tetrahedral arrangement of dative bonds following the VSEPR argument, and the other is the square planar geometry which violates VSEPR theory.
i. Tetrahedral Complexes
- Shape: Four ligands are arranged at the corners of a tetrahedron.
- Reason for Geometry: The tetrahedral geometry is supported by VSEPR theory because the four ligands are at maximum distances, making 109.5° angles.
- Commonality: They are less common than octahedral complexes and you will often see them with chloride ligands, like tetrachlorocuprate(II), [CuCl_4]2-.
- Metal Ions: Tetrahedral complexes are mostly formed by metal ions with d10 electron configuration such as Cu⁺ and Zn^{2+}.
- Examples: [Cu(CN)_4]3- and [Zn(NH_3)_4]2+.
ii. Square Planar Complexes
- Shape: Four ligands are directed at the four corners of a square, making 90° angles with one another.
- Reason for Geometry: This arrangement of ligands in this type of complex is against VSEPR theory. This is because VSEPR theory would predict a tetrahedral arrangement to minimize repulsion. The square planar geometry is favored by certain electronic configurations and ligand field effects.
- Commonality: They are less common than tetrahedral complexes.
- Metal Ions: Most square planar complexes are made by platinum, like diamminedichloroplatinum(II) complex.
- Examples: [Ni(CN)_4]2-, [AuCl_4]-, and [Pt(NH_3)_4]2+.
3. Complexes with Coordination Number 2: Linear Geometry
- Shape: Linear complexes are less stable. They have two monodentate ligands lying at 180° angle.
- Example: Diamminesilver(I) ion, [H_3N-Ag-NH_3]+.
- Other Examples: [CuCl_2]- and [Au(CN)_2]-.
Summary of Geometries
| Coordination Number | Geometry | Bond Angles | Example |
|---|---|---|---|
| 2 | Linear | 180° | [Ag(NH_3)_2]+ |
| 4 | Tetrahedral | 109.5° | [CuCl_4]2- |
| 4 | Square Planar | 90° | [Pt(NH_3)_2Cl_2] |
| 6 | Octahedral | 90° | [Ni(H_2O)_6]2+ |
Intermediate Level
The geometry of a complex is determined by the coordination number and the nature of the ligands. The VSEPR theory explains that ligands will arrange themselves to minimize repulsion. For coordination number 6, the octahedral geometry is the most common and stable, with bond angles of 90°. For coordination number 4, there are two possible geometries: tetrahedral (bond angles 109.5°) and square planar (bond angles 90°). Tetrahedral is more common for d10 ions like Zn^{2+}, while square planar is often seen with d8 ions like Pt^{2+}. For coordination number 2, the geometry is linear with a bond angle of 180°. The geometry affects the properties of the complex, such as its color and magnetic behavior.
Important Points
- - Geometry: The 3D arrangement of ligands around the metal ion.
- - VSEPR Theory: Ligands arrange to minimize repulsion.
- - Coordination Number 6: Octahedral (most common), 90° angles.
- - Coordination Number 4: Tetrahedral (109.5°) or Square Planar (90°).
- - Coordination Number 2: Linear (180°), less stable.
- - Tetrahedral: Common for d10 ions (e.g., Zn^{2+}).
- - Square Planar: Common for d8 ions (e.g., Pt^{2+}).
- - Examples: [Ni(H_2O)_6]2+ (octahedral), [CuCl_4]2- (tetrahedral), [Pt(NH_3)_2Cl_2] (square planar), [Ag(NH_3)_2]+ (linear).
Remember
- - Octahedral: Coordination number 6, 90° angles.
- - Tetrahedral: Coordination number 4, 109.5° angles.
- - Square Planar: Coordination number 4, 90° angles.
- - Linear: Coordination number 2, 180° angles.
- - VSEPR: Ligands minimize repulsion.
- - Tetrahedral: Common for d10 ions.
- - Square Planar: Common for d8 ions (e.g., Pt).
- - Most common geometry: Octahedral.
Common Mistakes
- - Thinking that all four-coordinate complexes are tetrahedral. Some are square planar (e.g., platinum complexes).
- - Forgetting that the geometry of a complex affects its properties. The geometry influences the d orbital splitting and therefore the color and magnetic behavior.
- - Confusing coordination number with the number of ligands. For monodentate ligands, they are the same, but for polydentate ligands, the coordination number is higher than the number of ligand molecules.
- - Assuming that VSEPR theory always predicts the correct geometry. For square planar complexes, VSEPR theory would predict tetrahedral, but the actual geometry is square planar due to electronic factors.
- - Not recognizing the common geometries for specific metal ions. For example, d10 ions prefer tetrahedral, while d8 ions often prefer square planar.
Quick Revision
- Octahedral (coordination number 6): The most common geometry, with ligands at 90° angles.
- Tetrahedral (coordination number 4): Ligands at 109.5° angles, common for d10 ions like Zn^{2+}.
- Square Planar (coordination number 4): Ligands at 90° angles, common for d8 ions like Pt^{2+}.
- Linear (coordination number 2): Ligands at 180° angles, less stable.
The arrangement is governed by VSEPR theory (minimizing repulsion) and the electronic configuration of the metal ion.
60-Second Revision
The geometry of a complex depends on its coordination number. Coordination number 6 gives octahedral (90°), the most common. Coordination number 4 gives tetrahedral (109.5°) or square planar (90°). Tetrahedral is common for d10 ions, and square planar for d8 ions like Pt. Coordination number 2 gives linear (180°). VSEPR theory explains the arrangement by minimizing ligand repulsion.
Frequently Asked Questions (FAQs)
- Octahedral (coordination number 6): Most common, with 90° bond angles.
- Tetrahedral (coordination number 4): With 109.5° bond angles.
- Square Planar (coordination number 4): With 90° bond angles.
- Linear (coordination number 2): With 180° bond angles.
Q2. Why is the octahedral geometry the most common for transition metal complexes? Answer: The octahedral geometry is the most common because it allows six ligands to arrange themselves around the metal ion in a way that minimizes repulsion between them. This arrangement is very stable and is favored by the electronic configuration of most transition metal ions.
Q3. What is the difference between tetrahedral and square planar geometries? Answer: Both have a coordination number of 4. In a tetrahedral geometry, the four ligands are at the corners of a tetrahedron with bond angles of 109.5°. In a square planar geometry, the four ligands are at the corners of a square with bond angles of 90°. Tetrahedral is more common for d10 ions, while square planar is more common for d8 ions like Pt^{2+}.
Q4. Which geometry is favored by d8 metal ions like Pt^{2+}? Answer: d8 metal ions like Pt^{2+} strongly favor a square planar geometry. This is due to the large ligand field stabilization energy associated with this arrangement for a d8 configuration.
Q5. Which geometry is favored by d10 metal ions like Zn^{2+}? Answer: d10 metal ions like Zn^{2+} favor a tetrahedral geometry. This is because they have no ligand field stabilization energy preference and tetrahedral geometry minimizes steric repulsion between the ligands.
- Q6. What is the bond angle in an octahedral complex?
- Answer: The bond angle between adjacent ligands in an octahedral complex is 90°.
- Q7. What is the bond angle in a tetrahedral complex?
- Answer: The bond angle between adjacent ligands in a tetrahedral complex is 109.5°.
- Q8. What is the bond angle in a square planar complex?
- Answer: The bond angle between adjacent ligands in a square planar complex is 90°.
- Q9. What is the geometry of the [CuCl_4]2- complex?
- Answer: The [CuCl_4]2- complex is tetrahedral.
- Q10. What is the geometry of the [Pt(NH_3)_2Cl_2] complex?
- Answer: The [Pt(NH_3)_2Cl_2] complex is square planar.
Multiple Choice Questions (MCQs)
- Q1. What is the geometry of a complex with coordination number 6?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: D
- Explanation: Complexes with coordination number 6 adopt an octahedral shape.
- Q2. What is the bond angle in a tetrahedral complex?
- A. 90°
- B. 109.5°
- C. 120°
- D. 180°
- Correct Answer: B
- Explanation: The bond angle in a tetrahedral complex is 109.5°.
- Q3. Which geometry is most common for transition metal complexes?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: D
- Explanation: Octahedral complexes are the most common and stable complexes.
- Q4. What is the geometry of the [CuCl_4]2- complex?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: B
- Explanation: The [CuCl_4]2- complex is tetrahedral.
- Q5. Which type of metal ions favor square planar geometry?
- A. d0
- B. d5
- C. d8
- D. d10
- Correct Answer: C
- Explanation: d8 metal ions like Pt^{2+} often favor square planar geometry.
- Q6. What is the bond angle in a square planar complex?
- A. 90°
- B. 109.5°
- C. 120°
- D. 180°
- Correct Answer: A
- Explanation: The bond angle in a square planar complex is 90°.
- Q7. What is the geometry of a complex with coordination number 2?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: A
- Explanation: Complexes with coordination number 2 are linear.
- Q8. Which of the following complexes is octahedral?
- A. [CuCl_4]2-
- B. [Ni(H_2O)_6]2+
- C. [Pt(NH_3)_2Cl_2]
- D. [Ag(NH_3)_2]+
- Correct Answer: B
- Explanation: [Ni(H_2O)_6]2+ is an octahedral complex.
- Q9. Which geometry is favored by d10 metal ions like Zn^{2+}?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: B
- Explanation: d10 metal ions favor tetrahedral geometry.
Q10. Why does the square planar geometry violate VSEPR theory? A. Because it has 109.5° bond angles. B. Because it has 90° bond angles, which is not the arrangement that minimizes repulsion. C. Because it has a coordination number of 4. D. Because it is only formed by platinum. Correct Answer: B Explanation: VSEPR theory predicts that four ligands should arrange tetrahedrally to minimize repulsion. The square planar geometry, with 90° angles, is not the predicted VSEPR arrangement, so it violates VSEPR theory.
Multiple Choice Questions (MCQs)
- Q1. What is the geometry of a complex with coordination number 6?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: D
- Explanation: Complexes with coordination number 6 are octahedral.
- Q2. What is the bond angle in a tetrahedral complex?
- A. 90°
- B. 109.5°
- C. 120°
- D. 180°
- Correct Answer: B
- Explanation: The bond angle in a tetrahedral complex is 109.5°.
- Q3. Which geometry is most common for transition metal complexes?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: D
- Explanation: Octahedral complexes are the most common and stable.
- Q4. What is the geometry of the [CuCl_4]2- complex?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: B
- Explanation: The [CuCl_4]2- complex is tetrahedral.
- Q5. Which type of metal ions favor square planar geometry?
- A. d0
- B. d5
- C. d8
- D. d10
- Correct Answer: C
- Explanation: d8 metal ions like Pt^{2+} often favor square planar geometry.
- Q6. What is the bond angle in a square planar complex?
- A. 90°
- B. 109.5°
- C. 120°
- D. 180°
- Correct Answer: A
- Explanation: The bond angle in a square planar complex is 90°.
- Q7. What is the geometry of a complex with coordination number 2?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: A
- Explanation: Complexes with coordination number 2 are linear.
- Q8. Which of the following complexes is octahedral?
- A. [CuCl_4]2-
- B. [Ni(H_2O)_6]2+
- C. [Pt(NH_3)_2Cl_2]
- D. [Ag(NH_3)_2]+
- Correct Answer: B
- Explanation: [Ni(H_2O)_6]2+ is an octahedral complex.
- Q9. Which geometry is favored by d10 metal ions like Zn^{2+}?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: B
- Explanation: d10 metal ions favor tetrahedral geometry.
Q10. Why does the square planar geometry violate VSEPR theory? A. Because it has 109.5° bond angles. B. Because it has 90° bond angles, which is not the arrangement that minimizes repulsion. C. Because it has a coordination number of 4. D. Because it is only formed by platinum. Correct Answer: B Explanation: VSEPR theory predicts a tetrahedral arrangement to minimize repulsion. The square planar geometry, with 90° angles, is not the predicted VSEPR arrangement.
Multiple Choice Questions (MCQs)
- Q1. Which of the following complexes has a linear geometry?
- A. [CuCl_4]2-
- B. [Ag(NH_3)_2]+
- C. [Ni(H_2O)_6]2+
- D. [Pt(NH_3)_2Cl_2]
- Correct Answer: B
- Explanation: [Ag(NH_3)_2]+ has a linear geometry with two ligands at 180°.
- Q2. What is the coordination number of a complex with an octahedral geometry?
- A. 2
- B. 4
- C. 6
- D. 8
- Correct Answer: C
- Explanation: An octahedral complex has a coordination number of 6.
- Q3. Which of the following complexes is square planar?
- A. [CuCl_4]2-
- B. [Ni(H_2O)_6]2+
- C. [Pt(NH_3)_2Cl_2]
- D. [Ag(NH_3)_2]+
- Correct Answer: C
- Explanation: [Pt(NH_3)_2Cl_2] is a square planar complex.
- Q4. What is the bond angle in a linear complex?
- A. 90°
- B. 109.5°
- C. 120°
- D. 180°
- Correct Answer: D
- Explanation: The bond angle in a linear complex is 180°.
- Q5. Which of the following is a common geometry for coordination number 4?
- A. Octahedral
- B. Linear
- C. Tetrahedral
- D. Trigonal bipyramidal
- Correct Answer: C
- Explanation: Tetrahedral is a common geometry for coordination number 4, along with square planar.
- Q6. What is the geometry of the [Ni(H_2O)_6]2+ complex?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: D
- Explanation: [Ni(H_2O)_6]2+ is an octahedral complex.
Q7. Which of the following statements about the geometry of complexes is true? A. All four-coordinate complexes are tetrahedral. B. All six-coordinate complexes are octahedral. C. All two-coordinate complexes are linear. D. All of the above are true. Correct Answer: C Explanation: Two-coordinate complexes are always linear. Four-coordinate complexes can be tetrahedral or square planar, and six-coordinate complexes are octahedral.
- Q8. Which geometry is more common for d8 metal ions like Pt^{2+}?
- A. Linear
- B. Tetrahedral
- C. Square planar
- D. Octahedral
- Correct Answer: C
- Explanation: d8 metal ions like Pt^{2+} often form square planar complexes.
- Q9. What is the coordination number of a complex with a tetrahedral geometry?
- A. 2
- B. 4
- C. 6
- D. 8
- Correct Answer: B
- Explanation: A tetrahedral complex has a coordination number of 4.
- Q10. Which of the following is NOT a common geometry for transition metal complexes?
- A. Octahedral
- B. Tetrahedral
- C. Square planar
- D. Trigonal bipyramidal
- Correct Answer: D
- Explanation: Trigonal bipyramidal is less common for transition metal complexes compared to octahedral, tetrahedral, and square planar geometries.
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Revision Zone
Review the key facts, formulas and checks before you move on.6.3.6 Geometries of Complexes
Sticky Notes · Quick Revision
Key Point 1
- Coordination number 6: Octahedral, 90°.
Key Point 2
- Coordination number 4: Tetrahedral (109.5°) or Square Planar (90°).
Key Point 3
- Coordination number 2: Linear, 180°.
Key Point 4
- Octahedral: Most common.
Key Point 5
- Tetrahedral: d10 ions (e.g., Zn^{2+}).
Key Point 6
- Square Planar: d8 ions (e.g., Pt^{2+}).
Key Point 7
- VSEPR: Minimizes repulsion.
Key Point 8
- Example: [CuCl_4]2- is tetrahedral.
Key Point 9
- Example: [Pt(NH_3)_2Cl_2] is square planar.
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