Unit 5: Co-ordination Compounds

1. Introduction

The chemistry of transition metals is largely dominated by the formation of **co-ordination compounds** (or complexes). These are a class of compounds in which a central metal atom or ion is bonded to a number of ions or neutral molecules by coordinate covalent bonds.

Co-ordination compounds are distinct from simple salts and double salts. For example, when KCl, MgCl₂, and H₂O are mixed in a molar ratio, they form the double salt Carnallite (KCl·MgCl₂·6H₂O), which dissociates completely into its constituent ions (K⁺, Mg²⁺, Cl⁻) in solution. In contrast, when KCN is mixed with Fe(CN)₂, they form a complex salt, potassium ferrocyanide (K₄[Fe(CN)₆]), which dissociates to give K⁺ ions and a complex ion, [Fe(CN)₆]⁴⁻. This complex ion does not further dissociate into Fe²⁺ and CN⁻ ions, indicating a stable, distinct chemical entity.

These compounds are of immense importance in various fields, including analytical chemistry, metallurgy, biological systems (e.g., chlorophyll, hemoglobin, vitamin B12), and industry (catalysis, electroplating).

2. Werner's Theory of Co-ordination Compounds (1893)

Alfred Werner, the "father of co-ordination chemistry," was the first to propose a successful theory to explain the bonding and properties of these compounds. He studied the complexes formed between cobalt(III) chloride and ammonia.

For example, in the complex [Co(NH₃)₆]Cl₃:

• The six ammonia (NH₃) molecules are attached to the central Co³⁺ ion by **secondary valencies**. These are non-ionisable and define the co-ordination sphere. The co-ordination number is 6.
• The three chloride (Cl⁻) ions are attached by **primary valencies**. These are ionisable and balance the +3 charge of the cobalt ion.

3. Definitions of Important Terms

3.1 Co-ordination Entity

A co-ordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl₃(NH₃)₃] is a co-ordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions.

3.2 Central Atom/Ion

The atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it is called the central atom or ion. In co-ordination compounds, the central atom/ion acts as a **Lewis acid** (electron pair acceptor).

3.3 Ligands

Classification of Ligands:

• Unidentate/Monodentate: Ligands that bind to the metal ion through a single donor atom. Examples: H₂O, NH₃, Cl⁻, CN⁻.
• Didentate/Bidentate: Ligands that can bind through two donor atoms. Examples: Ethane-1,2-diamine (en), Oxalate ion (ox).
• Polydentate: Ligands that can bind through several donor atoms. Example: Ethylenediaminetetraacetate ion (EDTA⁴⁻) is a hexadentate ligand.
• Chelating Ligands: When a di- or polydentate ligand uses two or more donor atoms to bind a single metal ion, it is said to be a chelating ligand, forming a ring structure called a chelate complex. Chelate complexes are generally more stable than complexes with unidentate ligands.
• Ambidentate Ligands: Unidentate ligands that can bind to the central metal atom through two different atoms. Examples: The nitrite ion (NO₂⁻) can coordinate through either the nitrogen (nitro) or the oxygen (nitrito) atom. The thiocyanate ion (SCN⁻) can coordinate through sulfur or nitrogen.

3.4 Co-ordination Number

3.5 Co-ordination Sphere

The central atom/ion and the ligands attached to it are enclosed in square brackets and are collectively termed the co-ordination sphere. The ionisable groups written outside the bracket are the counter ions. For example, in K₄[Fe(CN)₆], the co-ordination sphere is [Fe(CN)₆]⁴⁻ and the counter ion is K⁺.

3.6 Co-ordination Polyhedron

The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a co-ordination polyhedron. The most common co-ordination polyhedra are **octahedral**, **tetrahedral**, and **square planar**.

3.7 Oxidation Number of Central Atom

The oxidation number of the central atom in a complex is the charge it would carry if all the ligands were removed along with the electron pairs that are shared with the central atom. It is calculated based on the charges of the ligands and the overall charge of the complex ion.

4. IUPAC Nomenclature of Mononuclear Co-ordination Compounds

A systematic method of naming is required to describe these complex compounds unambiguously. The rules are as follows:

1. The **cation** is named first, whether it is simple or complex, followed by the **anion**.
2. Naming the Co-ordination Sphere:
   • Ligands are named first, in **alphabetical order**, followed by the name of the central metal atom/ion.
   • The prefixes di-, tri-, tetra-, etc., are used to indicate the number of individual ligands. These prefixes are not considered when determining the alphabetical order. For complex ligands that already contain a prefix (like ethylenediamine), bis-, tris-, tetrakis- are used.
3. Naming Ligands:
   • **Anionic ligands** end in '-o'. For example, Cl⁻ (chlorido), CN⁻ (cyanido), SO₄²⁻ (sulphato), NO₂⁻ (nitrito-N or nitrito-O).
   • **Neutral and cationic ligands** are given their usual names, with some exceptions: H₂O (aqua), NH₃ (ammine), CO (carbonyl), NO (nitrosyl).
4. Naming the Central Metal Atom:
   • If the complex ion is a **cation** or is **neutral**, the metal is named by its usual element name. Example: [Co(NH₃)₆]³⁺ is named...cobalt...
   • If the complex ion is an **anion**, the name of the metal ends with the suffix **'-ate'**. Example: [Fe(CN)₆]⁴⁻ is named...ferrate... Some metals use their Latin names (e.g., Fe: ferrate, Cu: cuprate, Ag: argentate, Au: aurate).
5. The **oxidation state** of the central metal is indicated by a Roman numeral in parentheses immediately following its name.

5. Bonding in Co-ordination Compounds

Several theories have been proposed to explain the nature of bonding between the central metal atom and the ligands. We will discuss three key theories.

5.1 Valence Bond Theory (VBT)

Proposed by Linus Pauling, VBT explains the formation of co-ordination compounds in terms of hybridization of metal orbitals. The central metal atom/ion makes available a number of empty orbitals (equal to its co-ordination number) for the formation of coordinate bonds with ligand orbitals.

Co-ordination Number	Hybridization	Geometry (Shape)
4	sp³	Tetrahedral
4	dsp²	Square Planar
5	sp³d	Trigonal Bipyramidal
6	sp³d²	Octahedral (Outer orbital)
6	d²sp³	Octahedral (Inner orbital)

5.2 Crystal Field Theory (CFT)

Crystal Field Theory (CFT) provides a more sophisticated model for bonding in co-ordination compounds. It treats the interaction between the metal ion and the ligands as purely electrostatic.

Crystal Field Splitting in Octahedral Complexes:

In an octahedral complex, six ligands approach the central metal ion along the x, y, and z axes. The d-orbitals that point directly along the axes (the **e_g** set: d_x²-y² and d_z²) experience more repulsion from the ligands and are raised to a higher energy level. The d-orbitals that lie between the axes (the **t_2g** set: d_xy, d_yz, d_zx) experience less repulsion and are lowered to a lower energy level.

The energy separation between these two sets is called the **crystal field splitting energy (Δ_o)**. The magnitude of Δ_o depends on the nature of the metal ion and the ligands. Ligands that cause a large splitting are called **strong-field ligands** (e.g., CN⁻, CO), while those that cause a small splitting are called **weak-field ligands** (e.g., I⁻, Br⁻).

This splitting of d-orbitals explains the **color** (due to d-d electron transitions) and **magnetic properties** of co-ordination compounds. For example, in a d⁴ configuration, a weak-field ligand (small Δ_o) will lead to a high-spin complex (4 unpaired electrons), while a strong-field ligand (large Δ_o) will lead to a low-spin complex (2 unpaired electrons).

Crystal Field Splitting in Tetrahedral Complexes:

In a tetrahedral complex, the ligands approach between the axes. This results in an inverted splitting pattern compared to octahedral complexes: the t_2g set is at a higher energy level and the e_g set is at a lower energy level. The splitting energy (Δ_t) is much smaller than in octahedral complexes (Δ_t ≈ 4/9 Δ_o). Consequently, tetrahedral complexes are almost always high-spin.

6. Isomerism in Co-ordination Compounds

Isomers are two or more compounds that have the same chemical formula but a different arrangement of atoms. This difference in arrangement leads to different physical and/or chemical properties. Co-ordination compounds exhibit two main types of isomerism: structural isomerism and stereoisomerism.

6.1 Structural Isomerism

This type of isomerism arises due to differences in the structure of the co-ordination compound, i.e., different bonding connectivity.

1. Ionisation Isomerism: Arises when the counter ion in a complex salt is itself a potential ligand and can exchange places with a ligand in the co-ordination sphere. The isomers give different ions in solution.
   Example: [Co(NH₃)₅Br]SO₄ (gives SO₄²⁻ ions in solution) and [Co(NH₃)₅SO₄]Br (gives Br⁻ ions in solution).
2. Solvate (or Hydrate) Isomerism: Arises when water molecules (or other solvent molecules) exchange places between being a ligand and being free solvent molecules in the crystal lattice.
   Example: [Cr(H₂O)₆]Cl₃ (violet) and [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green).
3. Linkage Isomerism: Arises in complexes containing an ambidentate ligand. The ligand can bind to the metal through different donor atoms.
   Example: [Co(NH₃)₅(NO₂)]²⁺ (nitro, bonded through N) and [Co(NH₃)₅(ONO)]²⁺ (nitrito, bonded through O).
4. Co-ordination Isomerism: Arises in compounds containing both cationic and anionic complex ions. It involves the interchange of ligands between the cationic and anionic co-ordination spheres.
   Example: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

6.2 Stereoisomerism (Space Isomerism)

This type of isomerism occurs when compounds have the same chemical formula and the same bonding connectivity, but differ in the spatial arrangement of their atoms. There are two main types:

1. Geometrical Isomerism (cis-trans isomerism): Arises due to different possible geometric arrangements of the ligands around the central metal atom.
   • Cis-isomer: Similar groups occupy adjacent positions.
   • Trans-isomer: Similar groups occupy opposite positions.

   This isomerism is common in square planar ([MA₂B₂] type) and octahedral ([MA₄B₂] type) complexes.

   Image Placeholder: Diagrams showing cis and trans isomers of [Pt(NH₃)₂(Cl)₂] (square planar) and [Co(NH₃)₄(Cl)₂]⁺ (octahedral).
2. Optical Isomerism: Arises when a complex and its mirror image are non-superimposable. Such isomers are called **enantiomers**. They are optically active, meaning they can rotate the plane of plane-polarized light.
   • Dextrorotatory (d-): Rotates light to the right.
   • Levorotatory (l-): Rotates light to the left.

   This isomerism is common in octahedral complexes containing bidentate ligands, such as [Co(en)₃]³⁺.

   Image Placeholder: Diagrams showing the non-superimposable mirror images (enantiomers) of [Co(en)₃]³⁺.

7. Importance of Co-ordination Compounds

Co-ordination compounds are vital in numerous fields of science and technology.

• In Qualitative Analysis: Many analytical tests for metal ions involve the formation of colored co-ordination complexes. For example, the detection of Ni²⁺ ions involves the formation of a bright red precipitate of [Ni(dmg)₂]. The deep blue color of [Cu(NH₃)₄]²⁺ is used to detect Cu²⁺ ions.
• In Extraction of Metals (Metallurgy): Co-ordination compounds are used in the extraction and purification of metals. For example, in the cyanide process for extracting gold and silver, the metal is first dissolved to form a soluble cyano complex (e.g., [Au(CN)₂]⁻). The metal is then recovered by displacement with a more reactive metal like zinc.
• In Biological Systems: Life would not be possible without co-ordination compounds.
  • Chlorophyll, the pigment responsible for photosynthesis in plants, is a co-ordination compound of magnesium.
  • Hemoglobin, the oxygen-carrier in the blood of animals, is a co-ordination compound of iron.
  • Vitamin B₁₂ (cyanocobalamin), essential for the production of red blood cells, is a co-ordination compound of cobalt.
  • Many enzymes, which are biological catalysts, are metalloenzymes containing metal ions in their active sites.
• In Medicine: Chelating ligands are used in chelation therapy to treat metal poisoning by forming stable, soluble complexes with toxic heavy metals (like lead or mercury), which can then be excreted from the body. The platinum-containing compound **cisplatin** is an important anti-cancer drug.
• In Industry: Co-ordination compounds are widely used as catalysts for many industrial processes, such as the hydrogenation of alkenes using Wilkinson's catalyst ([RhCl(PPh₃)₃]). They are also used in electroplating to achieve smooth and uniform deposits of metals.

Important Questions and Answers

Conceptual Questions

Q1: What is the difference between a double salt and a co-ordination compound?

A: A **double salt** (like Mohr's salt or Carnallite) exists only in the solid state and dissociates completely into its constituent simple ions when dissolved in water. A **co-ordination compound** contains a complex ion that retains its identity in solution and does not dissociate completely into its constituent ions.

Q2: State the main postulates of Werner's theory of co-ordination compounds.

A: Werner proposed that central metal atoms in co-ordination compounds exhibit two types of valencies:
1. **Primary Valency:** Corresponds to the oxidation state, is ionisable, and satisfied by anions.
2. **Secondary Valency:** Corresponds to the co-ordination number, is non-ionisable, and satisfied by ligands. It is directional and determines the geometry of the complex.

Q3: What is a chelating ligand? Why are chelate complexes more stable than complexes with unidentate ligands?

A: A chelating ligand is a bi- or polydentate ligand that can form a ring structure by bonding to a single central metal ion through two or more donor atoms. Chelate complexes are more stable due to the **chelate effect**, which is an entropic effect. The formation of a chelate complex from unidentate ligands leads to an increase in the number of free particles in the system, resulting in a favorable increase in entropy, which makes the complex more stable.

Q4: What is the spectrochemical series?

A: The spectrochemical series is an arrangement of ligands in order of their increasing ability to cause crystal field splitting (increasing Δ_o). A simplified series is:
I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO.
Ligands on the right end (like CN⁻, CO) are strong-field ligands, while those on the left (like I⁻, Br⁻) are weak-field ligands.

Q5: Explain why [NiCl₄]²⁻ is paramagnetic while [Ni(CN)₄]²⁻ is diamagnetic.

A: In both complexes, Ni is in the +2 oxidation state (3d⁸ configuration).
✔ In **[NiCl₄]²⁻**, Cl⁻ is a weak-field ligand. It does not cause pairing of the 3d electrons. The hybridization is sp³, leading to a tetrahedral geometry with two unpaired electrons. Hence, it is **paramagnetic**.
✔ In **[Ni(CN)₄]²⁻**, CN⁻ is a strong-field ligand. It forces the two unpaired 3d electrons to pair up. The hybridization is dsp² (using one inner d-orbital), leading to a square planar geometry with no unpaired electrons. Hence, it is **diamagnetic**.

Q6: Why are co-ordination compounds generally coloured?

A: The color of co-ordination compounds (especially of transition metals) is due to **d-d electron transitions**. Crystal field theory explains that the d-orbitals of the central metal ion split into two energy levels (e.g., t_2g and e_g). When the complex absorbs light of a certain wavelength (energy) from the visible spectrum, an electron is promoted from a lower energy d-orbital to a higher energy d-orbital. The color we see is the complementary color of the light that was absorbed.

Q7: What is an ambidentate ligand? Give an example.

A: An ambidentate ligand is a unidentate ligand that can coordinate to a central metal atom through two different donor atoms.
Example: The nitrite ion, NO₂⁻, can bind through the nitrogen atom to form a nitro complex (-NO₂) or through an oxygen atom to form a nitrito complex (-ONO).

Q8: What is the difference between geometrical and optical isomerism?

A: Geometrical isomerism (cis-trans) arises from different spatial arrangements of ligands around the central atom, leading to different geometric shapes (e.g., similar groups adjacent vs. opposite). Optical isomerism arises when a complex is chiral, meaning its mirror image is non-superimposable. These isomers (enantiomers) have identical physical properties except for their ability to rotate plane-polarized light.

Q9: What is the role of hemoglobin in the body?

A: Hemoglobin is a co-ordination compound of iron found in red blood cells. Its primary role is to act as an **oxygen carrier**. It binds with oxygen in the lungs to form oxyhemoglobin and transports it to various tissues throughout the body, where it releases the oxygen for cellular respiration.

Q10: What is the coordination number of the central metal ion in [Cr(en)₂(C₂O₄)]Cl?

A: The ligands are 'en' (ethylenediamine) and 'C₂O₄' (oxalate).
✔ 'en' is a bidentate ligand (donates 2 electron pairs). There are two 'en' ligands, contributing 2 × 2 = 4 coordinate bonds.
✔ 'C₂O₄' is also a bidentate ligand (donates 2 electron pairs). There is one 'C₂O₄' ligand, contributing 2 coordinate bonds.
Total coordinate bonds = 4 + 2 = 6.
Therefore, the coordination number of Cr is 6.

Numerical & Application-based Problems

Q11: Calculate the oxidation state of the central metal ion in the following complexes: (a) [Co(NH₃)₄Cl₂]⁺, (b) [PtCl₄]²⁻, (c) K₂[Zn(OH)₄].

A:
(a) [Co(NH₃)₄Cl₂]⁺: Let oxidation state of Co be 'x'.
x + 4(0) + 2(-1) = +1 → x - 2 = +1 → x = +3. Oxidation state of Co is +3.
(b) [PtCl₄]²⁻: Let oxidation state of Pt be 'x'.
x + 4(-1) = -2 → x - 4 = -2 → x = +2. Oxidation state of Pt is +2.
(c) K₂[Zn(OH)₄]: Let oxidation state of Zn be 'x'.
2(+1) + x + 4(-1) = 0 → 2 + x - 4 = 0 → x = +2. Oxidation state of Zn is +2.

Q12: Write the IUPAC name for the complex [Pt(en)₂(Cl)(NO₂)]SO₄.

A:
1. Cation is the complex, anion is sulphate.
2. Ligands: 'en' (ethylenediamine), 'Cl' (chlorido), 'NO₂' (nitro).
3. Alphabetical order: chlorido, ethylenediamine, nitro.
4. Since 'en' is complex, use 'bis(ethylenediamine)'.
5. Metal is Platinum (Pt) in a cationic complex.
6. Oxidation state of Pt: Let it be 'x'.
x + 2(0) + 1(-1) + 1(-1) = +2 (since SO₄ is -2).
x - 2 = +2 → x = +4.
Name: Chloridobis(ethylenediamine)nitroplatinum(IV) sulphate.

Q13: Write the formula for the following compound: Hexaamminecobalt(III) sulphate.

A:
1. Central metal: Cobalt(III) → Co³⁺.
2. Ligands: Hexaammine → 6 NH₃ (neutral ligand).
3. Co-ordination sphere: [Co(NH₃)₆].
4. Charge of the complex ion: +3 (from Co) + 6(0) = +3. So, [Co(NH₃)₆]³⁺.
5. Anion: sulphate → SO₄²⁻.
6. Balance the charges: We need two [Co(NH₃)₆]³⁺ ions (total charge +6) to balance three SO₄²⁻ ions (total charge -6).
Formula: [Co(NH₃)₆]₂(SO₄)₃.

Q14: The complex [Co(NH₃)₆]³⁺ is an inner orbital octahedral complex. Predict its hybridization and magnetic behavior.

A:
Central metal ion is Co³⁺. Electronic configuration of Co (Z=27) is [Ar] 3d⁷4s².
Configuration of Co³⁺ is [Ar] 3d⁶.
Since it is an inner orbital complex, the hybridization involves the inner (n-1)d orbitals, which is 3d. To make two 3d orbitals available, the six electrons in the 3d orbitals must pair up in three orbitals.
The hybridization will be d²sp³.
Since all the electrons are paired up, the complex will have no unpaired electrons.
Magnetic Behavior: Diamagnetic.

Q15: A coordination compound CrCl₃·6H₂O gives a precipitate with AgNO₃ corresponding to two equivalents of AgCl. What is the structural formula of the compound and its IUPAC name?

A:
Since two equivalents of AgCl are precipitated, it means that two Cl⁻ ions are outside the co-ordination sphere and are ionisable. The remaining one Cl⁻ ion and all six H₂O molecules are inside the co-ordination sphere.
Structural Formula: [Cr(H₂O)₆]Cl₂ · Cl is incorrect. 2 Cl- are outside. So, one Cl must be inside. [Cr(H₂O)₅Cl]Cl₂·H₂O. Let's re-read. 2 equivalents of AgCl precipitate. So 2 Cl- are counter ions.
The formula must be [Cr(H₂O)₅Cl]Cl₂·H₂O.
IUPAC Name:
Central metal: Chromium(III). Ligands: aqua, chlorido.
Name: Pentaaquachloridochromium(III) chloride monohydrate.

Q16: Using VBT, predict the geometry and magnetic character of [Fe(CN)₆]⁴⁻.

A:
1. Central metal ion: Fe²⁺. Configuration of Fe (Z=26) is [Ar] 3d⁶4s². Configuration of Fe²⁺ is [Ar] 3d⁶.
2. Ligand: CN⁻ is a strong-field ligand. It will cause the pairing of the 3d electrons.
3. The six 3d electrons will pair up in three d-orbitals, leaving two 3d orbitals empty.
4. For a co-ordination number of 6, we need six empty hybrid orbitals. These will be formed by the hybridization of two 3d, one 4s, and three 4p orbitals.
5. Hybridization: d²sp³.
6. Geometry: Octahedral.
7. Since all electrons are paired, the complex has no unpaired electrons.
8. Magnetic Character: Diamagnetic.

Q17: The spin-only magnetic moment of [MnBr₄]²⁻ is 5.9 BM. Predict the geometry of the complex.

A:
The spin-only magnetic moment (μ) is given by `μ = √[n(n+2)]` BM, where 'n' is the number of unpaired electrons.
5.9 = √[n(n+2)]. Squaring both sides: (5.9)² ≈ 34.8 = n(n+2).
By inspection, if n=5, n(n+2) = 5(7) = 35, which is very close. So, there are 5 unpaired electrons.
The central metal ion is Mn²⁺ (Configuration of Mn (Z=25) is [Ar] 3d⁵4s²; Mn²⁺ is [Ar] 3d⁵).
A 3d⁵ configuration with 5 unpaired electrons means that the ligand (Br⁻) is a weak-field ligand and has not caused any pairing. For a co-ordination number of 4, the available empty orbitals for hybridization are the outer 4s and 4p orbitals.
Hybridization: sp³.
Geometry: Tetrahedral.

Q18: Draw the geometrical isomers of [Co(NH₃)₃(NO₂)₃].

A: This is an octahedral complex of the type [MA₃B₃]. It exists as two geometrical isomers:
1. Facial (fac) isomer: The three identical ligands (e.g., the three NO₂ groups) occupy the corners of one face of the octahedron.
2. Meridional (mer) isomer: The three identical ligands occupy positions around the meridian of the octahedron (as if on a semi-circle).

Q19: Which of the following is more stable: [Cu(NH₃)₄]²⁺ or [Cu(en)₂]²⁺? Why?

A: The complex [Cu(en)₂]²⁺ is more stable.
Reason: This is due to the **chelate effect**. Ethylenediamine (en) is a bidentate ligand that forms a stable five-membered ring with the central Cu²⁺ ion. This chelation (ring formation) leads to a significant increase in the stability of the complex compared to the analogous complex with unidentate ligands (NH₃). The formation of chelate rings is thermodynamically more favorable.

Q20: A solution of [Ni(H₂O)₆]²⁺ is green, but a solution of [Ni(CN)₄]²⁻ is colorless. Explain.

A:
✔ **[Ni(H₂O)₆]²⁺:** Ni²⁺ has a 3d⁸ configuration. H₂O is a weak-field ligand, causing a small crystal field splitting (Δ_o). The complex absorbs light in the red region of the visible spectrum to promote a d-d electron transition. The transmitted light is the complementary color, which is **green**.
✔ **[Ni(CN)₄]²⁻:** CN⁻ is a very strong-field ligand. It causes a very large crystal field splitting (Δ_o). The energy required for a d-d transition is very high, corresponding to light in the ultraviolet (UV) region. Since the complex does not absorb any light from the visible spectrum, it appears **colorless**.