Study Notes on Coordination Compounds

Coordination Compounds Overview

• Coordination compounds are complex compounds where metal atoms are bound to various anions or neutral molecules through electron sharing.

• These involve new concepts of chemical bonding and molecular structure useful in biological systems, industrial catalysts, and analytical reagents.

• Notable examples include chlorophyll (Mg), haemoglobin (Fe), and vitamin B12 (Co).

Objectives

After studying this unit, students will:

• Appreciate Werner’s theory on coordination compounds.

• Understand key terminologies related to coordination compounds:
    - Coordination entity
    - Central atom/ion
    - Ligand
    - Coordination number
    - Coordination sphere
    - Coordination polyhedron
    - Oxidation number
    - Homoleptic and Heteroleptic

• Learn nomenclature rules for coordination compounds.

• Write formulas and names for mononuclear coordination compounds.

• Define isomerism types in coordination compounds.

• Understand bonding in terms of Valence Bond and Crystal Field theories.

• Recognize applications in daily life.

Alfred Werner's Contributions

• Alfred Werner (1866-1919) developed the first systematic theory of coordination compounds, proposing a primary and secondary valence for metal ions.

• Primary valence: Ionizable, satisfied by negative ions (e.g., in CrCl3, primary valence = 3).

• Secondary valence: Non-ionizable, satisfies through neutral molecules or negative ions, indicating the coordination number.

• Demonstrated through experiments like precipitation of AgCl from cobalt(III) chloride-ammonia complexes.

Werner’s Theory of Coordination Compounds (1898)

1. Metals exhibit two types of linkages: primary and secondary.

2. Primary valences are ionizable and attached to negative ions.

3. Secondary valences are non-ionizable, showing fixed coordination number.

4. Ligands linked by secondary linkages exhibit specific spatial arrangements called coordination polyhedra.

Types of Coordination Shapes

• Common geometrical shapes in coordination compounds:
    - Octahedral: e.g., [Co(NH3)6]3+
    - Tetrahedral: e.g., [Ni(CO)4]
    - Square planar: e.g., [PtCl4]2–

Examples of Cobalt(III) Chloride-Ammonia Complexes

• Colors of complex solutions and AgCl precipitation:
    - Yellow: [Co(NH3)6]3+3Cl– → 1:3 electrolyte.
    - Purple: [CoCl(NH3)5]2+2Cl– → 1:2 electrolyte.
    - Green: [CoCl2(NH3)4]+Cl– → 1:1 electrolyte.
    - Violet: [CoCl2(NH3)4]+Cl– → 1:1 electrolyte.

Definitions of Important Terms Related to Coordination Compounds

a. Coordination Entity

• Central metal atom/ion with fixed ligands attached.

• E.g., [CoCl3(NH3)3].

b. Central Atom/Ion

• The atom/ion that bonds to a defined number of groups.

• E.g., in [NiCl2(H2O)4], it is Ni2+.

c. Ligands

• Atoms/molecules bound to central atom.

• Types:
    - Unidentate: Bonds through a single atom (e.g., Cl–).
    - Didentate: Bonds through two donor atoms (e.g., ethylenediamine).
    - Polydentate: Can bind through multiple donor atoms (e.g., EDTA4–).

d. Coordination Number (CN)

• Defined as the number of donor atoms bonded to the central atom.

• E.g., in [Ni(NH3)4]2+, CN = 4.

e. Coordination Sphere

• Central atom and its ligands enclosed in square brackets.

• E.g., in K4[Fe(CN)6], coordination sphere is [Fe(CN)6]4–.

f. Coordination Polyhedron

• Spatial arrangement of ligand atoms around a central atom.

• Examples include octahedral, tetrahedral, and square planar.

g. Oxidation Number

• Charge of the central atom if all ligands are removed.

• Shown as Roman numeral in parenthesis.

h. Homoleptic vs. Heteroleptic Complexes

• Homoleptic: One type of ligand in the complex (e.g., [Co(NH3)6]3+).

• Heteroleptic: More than one type of ligand (e.g., [Co(NH3)4Cl2]+).

Nomenclature of Coordination Compounds

• Formulas and names follow IUPAC guidelines:

1. Central atom is listed first.

2. Ligands in alphabetical order independent of charge.

3. Polydentate ligands also alphabetized.

4. Use parentheses for ligands when polyatomic.

5. Coefficients for ligands shown as prefix (mono, di, tri).

6. Roman numerals for oxidation state.

7. Anionic ligands end in –o; examples include aqua, ammine, carbonyl.

8. Counter ions listed after coordinating entities: e.g., [Cr(NH3)3(H2O)3]Cl3.

Isomerism in Coordination Compounds

Types of Isomerism

• Isomers: Same formula, different structures.
    - Stereoisomerism includes:
      - Geometrical isomerism (cis/trans).
      - Optical isomerism (enantiomers).
    - Structural isomerism includes:
      - Linkage isomerism.
      - Coordination isomerism.
      - Ionization isomerism.
      - Solvate isomerism.

Geometrical Isomerism Examples

• In complexes of the form [MX2L2], X can be arranged as cis (adjacent) or trans (opposite).

• Various configurations of coordination entities lead to different isomers.

Bonding Theories

Valence Bond Theory (VBT)

• Applies to hybridization in coordination compounds, aiding the prediction of geometry and magnetic behavior.

• Uses metal d, s, p orbitals to find suitable hybridization.

Crystal Field Theory (CFT)

• Treats metal-ligand bonds as ionic, based on electrostatic interactions.

• Splits d orbitals based on ligand field strengths, influencing color and magnetic properties.

Key Points of CFT

• Octahedral complexes split d orbitals into t2g (lower energy) and eg (higher energy) sets due to ligand interactions.

• Tetrahedral complexes have reversed splitting with smaller energy differences.

• Strong vs. weak field ligands are differentiated by their capacity to cause electron pairing.

Applications of Coordination Compounds

• Used in analytical chemistry, qualitative analysis, extraction of metals (e.g., gold with cyanide), and in medicinal chemistry.

• Examples of applications:
    - EDTA for hardness in water.
    - Chelate therapy for metal poisoning.
    - Catalytic roles in industrial processes (e.g., hydrogenation).