6.3 Formation of Complex Compounds

Transition metals are unique in their ability to form complex compounds (also called coordination compounds). These compounds are essential in many areas of chemistry, biology, and industry. For example, chlorophyll (which is vital for photosynthesis) is a complex containing magnesium, and hemoglobin (which carries oxygen in the blood) is a complex containing iron. This section introduces the concept of complex compounds, the role of ligands, and the reasons why transition metals are so good at forming them.

Definition

• Complex Compound (Coordination Compound): A compound formed between transition metals and ligand(s) through coordinate covalent bonds. The ligands donate lone pairs of electrons into the vacant orbitals of the transition metal.
• Ligand: An atom or group of atoms that is electron-rich and can donate a lone pair of electrons to a transition metal ion, forming a dative covalent bond. Ligands are also called Lewis bases or nucleophiles.
• Central Metal Ion: The transition metal ion that accepts electron pairs from the ligands. It acts as a Lewis acid.
• Coordination Sphere: The central metal ion together with its attached ligands, enclosed in square brackets.
• Coordination Number: The number of coordinate covalent bonds formed by the metal ion with ligands.

Simple Explanation: A complex compound is like a central metal ion surrounded by a group of molecules or ions (ligands) that are attached to it. The ligands "donate" electron pairs to the metal ion, which "accepts" them.

Core Concept

The core concept is that transition metals have vacant d orbitals that are energetically accessible and can accept electron pairs from ligands. The small size and high charge density of transition metal ions allow them to strongly attract ligands. This leads to the formation of coordinate covalent bonds, creating a complex. The ligands act as Lewis bases (electron-pair donors), while the transition metal ion acts as a Lewis acid (electron-pair acceptor).

Detailed Explanation

What is a Ligand?

A ligand is an atom or group of atoms which is electron-rich and can donate lone pairs of electrons to the transition metal ions, making dative covalent bonds with them. Ligands may be molecules or ions made of non-metal atoms. They are also called Lewis bases or nucleophiles because they can donate electrons.

Based on the number of lone pairs they can contribute to coordinate covalent bond formation, ligands have the following classes:

• The ligand which can form one dative covalent bond with a transition metal atom/ion is called monodentate (Latin word meaning 'one-toothed').
• They may have more than one lone pair of electrons but can donate only one.
• Examples: aqua (H_2O), halo (X^-), hydroxo (OH^-), ammine (NH_3), and cyano (CN^-).

• The ligand that can donate more than one lone pair of electrons is called a polydentate ligand (many-toothed).
• Bidentate Ligands: Can form two coordinate covalent bonds at a time with the central transition metal ion. Examples: 1,2-diaminoethane (NH_2CH_2CH_2NH_2) and ethanedioate (C_2O_4^{2-}).
• Hexadentate Ligands: Can donate six lone pairs of electrons. Example: ethanediaminetetraethanoic acid (EDTA).

Why Do Transition Metals Form Complex Compounds?

• They have a small size and high charge density, enabling them to strongly attract ligands.
• They have a 3d sub-shell which has low energy and can bond to p-orbitals of ligands easily.

Transition metals lose electrons and become cations. They have vacant d orbitals that undergo hybridization to produce the same number of new orbitals with equal energy. This process is followed by reaction of ligands with the transition metal ion to form a complex. The ligands donate electron pairs to the hybridized vacant 3d orbitals of the central metal ion. The vacant 3d orbitals of first row transition metals are energetically accessible because they have low energy, and ligands can easily donate lone pairs of electrons to them making dative bonds with them. After this process, ligands come and donate electron pairs to those hybridized orbitals forming a complex ion.

Complexes of Copper (II) with Water and Chloride Ligands

The textbook provides a detailed example of how copper(II) ions form complexes with different ligands.

• The copper(II) ion (Cu^{2+}) has the electronic configuration [Ar]3d⁹.
• Six water ligands approach the copper(II) ion and form six coordinate covalent bonds with it.
• Before making bonds, the six vacant orbitals of the copper(II) ion undergo hybridization, generating six sp^3d^2 hybrid orbitals.
• These orbitals are then available for six water molecules to coordinatively bond and form the complex [Cu(H_2O)_6]²⁺, which has an octahedral geometry.

• Copper(II) ions intermix four vacant orbitals (4s and 4p) giving four sp^3 hybrid orbitals.
• Four chloride ligands coordinatively bond with those four hybrid orbitals.
• With chloride ligands, copper(II) forms a tetrahedral complex, [CuCl_4]^2-, because chloride is a big ligand and six chloride ligands cannot surround a small copper(II) ion.

Complexes of Cobalt(II) ion with Water and Chloride Ligands

• The condensed electronic configuration of cobalt(II) ion is [Ar]3d⁷.
• It forms an octahedral complex where six water ligands form dative bonds with the empty orbitals available with cobalt(II) ion. The complex is [Co(H_2O)_6]²⁺.
• Similarly, cobalt(II) ion reacts with four chloride ligands, making four dative bonds and giving a tetrahedral complex, [CoCl_4]^2-.

Intermediate Level

Complex compounds are formed when transition metal ions (acting as Lewis acids) accept electron pairs from ligands (acting as Lewis bases) to form coordinate covalent bonds. The transition metals have vacant d orbitals that can accept these electron pairs. The number of coordinate bonds formed is the coordination number. Ligands can be monodentate (forming one bond, e.g., H_2O, NH_3, Cl^-), bidentate (forming two bonds, e.g., 1,2-diaminoethane), or polydentate (forming many bonds, e.g., EDTA, which is hexadentate). The geometry of the complex depends on the coordination number and the nature of the ligands. For example, copper(II) forms an octahedral complex with water ([Cu(H_2O)_6]²⁺) but a tetrahedral complex with chloride ([CuCl_4]^2-) because the chloride ion is larger.

Important Points

• - Complex Compound: Formed between a transition metal ion (Lewis acid) and ligands (Lewis bases).
• - Ligand: Electron-pair donor; can be monodentate, bidentate, or polydentate.
• - Coordination Number: The number of coordinate bonds formed by the metal ion.
• - Vacant d Orbitals: Key to the ability of transition metals to form complexes.
• - Hybridization: Determines the geometry of the complex (e.g., octahedral, tetrahedral).
• - Examples: [Cu(H_2O)_6]²⁺ (octahedral), [CuCl_4]^2- (tetrahedral), [Co(H_2O)_6]²⁺ (octahedral).
• - Monodentate Ligands: H_2O, NH_3, Cl^-, CN^-.
• - Bidentate Ligands: 1,2-diaminoethane, ethanedioate.
• - Polydentate Ligands: EDTA (hexadentate).

Remember

• - Complex: Metal ion + ligands.
• - Ligand: Lewis base (electron donor).
• - Metal ion: Lewis acid (electron acceptor).
• - Coordination number: Number of bonds to ligands.
• - Monodentate: 1 bond (e.g., H_2O).
• - Bidentate: 2 bonds (e.g., 1,2-diaminoethane).
• - Polydentate: Many bonds (e.g., EDTA).
• - Transition metals have vacant d orbitals for complex formation.
• - Geometry depends on coordination number and ligand size.

Common Mistakes

• - Thinking that all ligands form the same number of bonds. Ligands can be monodentate, bidentate, or polydentate.
• - Forgetting the role of vacant d orbitals. These are essential for accepting electron pairs from ligands.
• - Confusing coordination number with the number of ligands. For a monodentate ligand, they are the same, but for polydentate ligands, the coordination number is higher than the number of ligand molecules.
• - Assuming that all complexes are octahedral. The geometry depends on the coordination number and the nature of the ligands (e.g., tetrahedral for [CuCl_4]^2-).
• - Not recognizing the difference between Lewis acids and bases. The metal ion is the Lewis acid, and the ligand is the Lewis base.

Quick Revision

Complex compounds are formed when transition metal ions (Lewis acids) accept electron pairs from ligands (Lewis bases) to form coordinate covalent bonds. The transition metals have vacant d orbitals that can accept these electron pairs. Ligands can be monodentate (forming one bond, e.g., H_2O, Cl^-), bidentate (forming two bonds, e.g., 1,2-diaminoethane), or polydentate (forming many bonds, e.g., EDTA). The coordination number is the number of coordinate bonds formed by the metal ion. The geometry of the complex (e.g., octahedral, tetrahedral) depends on the coordination number and the nature of the ligands.

60-Second Revision

A complex compound forms when a transition metal ion (Lewis acid) accepts electron pairs from ligands (Lewis bases). Transition metals have vacant d orbitals for this. Ligands can be monodentate (1 bond, e.g., H_2O, Cl^-), bidentate (2 bonds, e.g., 1,2-diaminoethane), or polydentate (many bonds, e.g., EDTA). The coordination number is the number of coordinate bonds. Geometry (octahedral, tetrahedral) depends on coordination number and ligand size.

Frequently Asked Questions (FAQs)

Q1. What is a complex compound? Answer: A complex compound (or coordination compound) is a compound formed between a central transition metal ion (or atom) and one or more ligands. The ligands donate lone pairs of electrons to the metal ion, forming coordinate covalent bonds. The metal ion acts as a Lewis acid (electron-pair acceptor), and the ligands act as Lewis bases (electron-pair donors).

• Monodentate ligands form one bond: e.g., water (H_2O), ammonia (NH_3), chloride (Cl^-), cyanide (CN^-).
• Bidentate ligands form two bonds: e.g., 1,2-diaminoethane (NH_2CH_2CH_2NH_2), ethanedioate (C_2O_4^{2-}).
• Polydentate ligands form many bonds: e.g., EDTA (hexadentate).

• They have a small size and high charge density, which enables them to strongly attract ligands.
• They have vacant d orbitals that are energetically accessible and can accept electron pairs from ligands, forming coordinate covalent bonds.

Q4. What is the coordination number? Answer: The coordination number is the number of coordinate covalent bonds formed by the central metal ion with its ligands. For example, in [Cu(H_2O)_6]²⁺, the coordination number of copper is 6. In [CuCl_4]^2-, the coordination number is 4.

Q5. Explain the difference between the complexes formed by copper(II) with water and with chloride. Answer: Copper(II) forms an octahedral complex with water, [Cu(H_2O)_6]²⁺, because six water molecules (which are small) can fit around the copper ion. However, with chloride ions, it forms a tetrahedral complex, [CuCl_4]^2-, because chloride ions are larger, and only four can fit around the small copper(II) ion.

Q6. What is the difference between a monodentate and a bidentate ligand? Answer: A monodentate ligand can form only one coordinate covalent bond with a metal ion (e.g., H_2O, NH_3, Cl^-). A bidentate ligand can form two coordinate covalent bonds with a metal ion (e.g., 1,2-diaminoethane, ethanedioate).

Q7. What is the role of hybridization in complex formation? Answer: Hybridization is the process by which the vacant orbitals of the transition metal ion mix to produce a new set of equivalent orbitals with the correct geometry to accept electron pairs from the ligands. For example, in [Cu(H_2O)_6]²⁺, the metal ion uses sp^3d^2 hybridization to form an octahedral complex. In [CuCl_4]^2-, it uses sp^3 hybridization to form a tetrahedral complex.

Q8. What is EDTA? What type of ligand is it? Answer: EDTA stands for ethanediaminetetraethanoic acid (ethylenediaminetetraacetic acid). It is a hexadentate ligand, meaning it can donate six lone pairs of electrons to a central metal ion, forming six coordinate covalent bonds. It is commonly used in analytical chemistry and in medicine (e.g., to treat heavy metal poisoning).

Q9. Why is the coordination number of copper(II) different in [Cu(H_2O)_6]²⁺ and [CuCl_4]^2-? Answer: The difference is due to the size of the ligands. Water molecules are small, so six can fit around the copper(II) ion, forming an octahedral complex with a coordination number of 6. Chloride ions are larger, so only four can fit around the copper(II) ion, forming a tetrahedral complex with a coordination number of 4.

Multiple Choice Questions (MCQs)

• Q1. What is a ligand?
• A. A species that accepts an electron pair.
• B. A species that donates an electron pair.
• C. A central metal ion.
• D. A type of catalyst.
• Correct Answer: B
• Explanation: A ligand is a Lewis base that donates a lone pair of electrons to a central metal ion to form a coordinate covalent bond.

Q2. Which of the following is a monodentate ligand? A. 1,2-diaminoethane B. EDTA C. Water D. Ethanedioate Correct Answer: C Explanation: Water (H_2O) is a monodentate ligand because it can form only one coordinate covalent bond. 1,2-diaminoethane and ethanedioate are bidentate, and EDTA is hexadentate.

• Q3. What is the coordination number of copper in [Cu(H_2O)_6]²⁺?
• A. 2
• B. 4
• C. 6
• D. 8
• Correct Answer: C
• Explanation: In [Cu(H_2O)_6]²⁺, the copper ion is bonded to six water molecules, giving a coordination number of 6.

• Q4. What is the geometry of the [CuCl_4]^2- complex?
• A. Linear
• B. Square planar
• C. Tetrahedral
• D. Octahedral
• Correct Answer: C
• Explanation: The [CuCl_4]^2- complex is tetrahedral because four chloride ligands surround the copper ion.

Q5. Why do transition metals form complexes? A. Because they have a small size and high charge density, and vacant d orbitals. B. Because they have large atomic radii and low charge density. C. Because they are poor conductors of electricity. D. Because they have low melting points. Correct Answer: A Explanation: Transition metals form complexes due to their small size, high charge density, and the presence of vacant d orbitals that can accept electron pairs from ligands.

• Q6. Which of the following is a bidentate ligand?
• A. H_2O
• B. NH_3
• C. 1,2-diaminoethane
• D. Cl^-
• Correct Answer: C
• Explanation: 1,2-diaminoethane is a bidentate ligand because it has two nitrogen atoms, each with a lone pair of electrons, that can coordinate to a metal ion.

• Q7. What is the role of the transition metal ion in a complex?
• A. Lewis base
• B. Lewis acid
• C. Ligand
• D. Solvent
• Correct Answer: B
• Explanation: The transition metal ion acts as a Lewis acid by accepting electron pairs from the ligands.

• Q8. What type of ligand is EDTA?
• A. Monodentate
• B. Bidentate
• C. Tridentate
• D. Hexadentate
• Correct Answer: D
• Explanation: EDTA is a hexadentate ligand, meaning it can form six coordinate covalent bonds with a central metal ion.

• Q9. Which of the following is the correct coordination number for the complex [Co(H_2O)_6]²⁺?
• A. 2
• B. 4
• C. 6
• D. 8
• Correct Answer: C
• Explanation: In [Co(H_2O)_6]²⁺, the cobalt ion is bonded to six water molecules, giving a coordination number of 6.

Q10. Why does copper(II) form a tetrahedral complex with chloride but an octahedral complex with water? A. Because chloride is a smaller ligand than water. B. Because chloride is a larger ligand than water. C. Because water is a stronger ligand than chloride. D. Because copper has different oxidation states in the two complexes. Correct Answer: B Explanation: Chloride ions are larger than water molecules, so only four can fit around the copper(II) ion, forming a tetrahedral complex. Water molecules are smaller, so six can fit, forming an octahedral complex.

Multiple Choice Questions (MCQs)

• Q1. What is a ligand?
• A. A species that accepts an electron pair.
• B. A species that donates an electron pair.
• C. A central metal ion.
• D. A type of catalyst.
• Correct Answer: B
• Explanation: A ligand is a Lewis base that donates a lone pair of electrons to a central metal ion.

• Q2. Which of the following is a monodentate ligand?
• A. 1,2-diaminoethane
• B. EDTA
• C. Water
• D. Ethanedioate
• Correct Answer: C
• Explanation: Water (H_2O) is a monodentate ligand.

• Q3. What is the coordination number of copper in [Cu(H_2O)_6]²⁺?
• A. 2
• B. 4
• C. 6
• D. 8
• Correct Answer: C
• Explanation: In [Cu(H_2O)_6]²⁺, the copper ion is bonded to six water molecules.

• Q4. What is the geometry of the [CuCl_4]^2- complex?
• A. Linear
• B. Square planar
• C. Tetrahedral
• D. Octahedral
• Correct Answer: C
• Explanation: The [CuCl_4]^2- complex is tetrahedral.

• Q5. Why do transition metals form complexes?
• A. Because they have a small size and high charge density, and vacant d orbitals.
• B. Because they have large atomic radii and low charge density.
• C. Because they are poor conductors of electricity.
• D. Because they have low melting points.
• Correct Answer: A
• Explanation: Transition metals form complexes due to their small size, high charge density, and vacant d orbitals.

• Q6. Which of the following is a bidentate ligand?
• A. H_2O
• B. NH_3
• C. 1,2-diaminoethane
• D. Cl^-
• Correct Answer: C
• Explanation: 1,2-diaminoethane is a bidentate ligand.

• Q7. What is the role of the transition metal ion in a complex?
• A. Lewis base
• B. Lewis acid
• C. Ligand
• D. Solvent
• Correct Answer: B
• Explanation: The transition metal ion acts as a Lewis acid.

• Q8. What type of ligand is EDTA?
• A. Monodentate
• B. Bidentate
• C. Tridentate
• D. Hexadentate
• Correct Answer: D
• Explanation: EDTA is a hexadentate ligand.

• Q9. Which of the following is the correct coordination number for the complex [Co(H_2O)_6]²⁺?
• A. 2
• B. 4
• C. 6
• D. 8
• Correct Answer: C
• Explanation: In [Co(H_2O)_6]²⁺, the cobalt ion is bonded to six water molecules.

• Q10. Why does copper(II) form a tetrahedral complex with chloride but an octahedral complex with water?
• A. Because chloride is a smaller ligand than water.
• B. Because chloride is a larger ligand than water.
• C. Because water is a stronger ligand than chloride.
• D. Because copper has different oxidation states in the two complexes.
• Correct Answer: B
• Explanation: Chloride ions are larger than water molecules, so only four can fit around the copper(II) ion, forming a tetrahedral complex.