Study Notes on Coordination Compounds

Chemistry 118: Coordination Compounds

Introduction to Coordination Compounds

  • Definition: Coordination compounds are complexes formed when metal atoms share electrons with surrounding anions or neutral molecules.

  • Significance: They serve as vital components in biological systems, such as chlorophyll (Mg), hemoglobin (Fe), and vitamin B12 (Co).

  • Applications: Used in metallurgical processes, industrial catalysis, analytical chemistry, electroplating, textile dyeing, and medicinal chemistry.

Learning Objectives

After studying this unit, you will be able to:

  • Appreciate the postulates of Werner’s theory of coordination compounds.

  • Understand key terms 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 types of isomerism in coordination compounds.

  • Understand bonding nature in coordination compounds through Valence Bond Theory and Crystal Field Theory.

  • Recognize importance and applications of coordination compounds.

Werner’s Theory of Coordination Compounds

  • Alfred Werner (1866-1919): Pioneer in coordination chemistry; proposed concepts regarding metal ion coordination, including primary and secondary valences.

  • Primary Valence: Indicates the number of ions that can be ionized; satisfied by negative ions.

  • Secondary Valence: Indicates the number of groups bound to a metal ion, corresponding to its coordination number; satisfied by neutral molecules or negative ions.

    • Example: In CrCl3, CoCl2, and PdCl2, their primary valences are 3, 2, and 2, respectively.

Observations in Coordination Chemistry

  • Certain chloride ions remain in solution while others precipitate as AgCl when treated with excess AgNO3.

  • Insight: Coordination numbers deduced from the reactions; for cobalt compounds, secondary valences are 6.

Postulates of Werner’s Theory

  1. Metals in coordination compounds exhibit two types of linkages:

    • Primary (ionizable)

    • Secondary (non-ionizable)

  2. The primary valences are generally ionizable and satisfied by negative ions.

  3. The secondary valences correspond to the coordination number and are satisfied by neutral molecules or negative ions.

  4. The geometric shapes formed have characteristic arrangements related to coordination numbers, termed coordination polyhedra.

Coordination Entities

  • Definition: Comprise a central metal atom/ion bonded to a definitive number of ions or molecules.

    • Example: [CoCl3(NH3)3] includes cobalt surrounded by ammonia and chloride ions.

Key Definitions

  • Central Atom/Ion: The atom/ion around which ligands are arranged geometrically, serves as a Lewis acid.

  • Ligands: Ions/molecules bound to the central atom. Can be:

    • Unidentate: binds through one atom (e.g., Cl−, H2O, NH3)

    • Didentate: binds through two atoms (e.g., ethylene diamine)

    • Polydentate: binds through multiple atoms (e.g., EDTA4−)

    • Chelate Ligands: Binds through multiple donor atoms to form stable complexes.

  • Coordination Number (CN): Number of donor atoms bonded to the central atom (e.g., CN of [PtCl6]2− is 6).

  • Coordination Sphere: Enclosed in square brackets, includes the central atom and ligands.

  • Coordination Polyhedron: Spatial arrangement of ligand atoms attached to the central atom/ion.

Nomenclature of Coordination Compounds

  • General Rules:

    1. Cation is named first.

    2. Ligands are listed in alphabetical order.

    3. Complex formulas enclosed in square brackets, charge noted outside.

    4. Ligands indicated by suffix alterations (e.g., -o for anionic ligands).

    5. Prefixes for ligand counts (mono-, di-, tri-, etc.).

    6. Charges of metal centers represented in Roman numerals in parentheses.

Examples of Nomenclature

  • [Cr(NH3)3(H2O)3]Cl3 is named triamminetriaquachromium(III) chloride.

  • [Co(H2NCH2CH2NH2)3]2(SO4)3 is named tris(ethane-1,2-diamine)cobalt(III) sulfate.

Types of Isomerism

  • Stereoisomerism: Isomers with the same molecular formula but different spatial arrangements.

    • Geometrical Isomerism: Cis/trans arrangements in complexes.

    • Optical Isomerism: Non-superimposable mirror images (enantiomers).

  • Structural Isomerism: Different bonding arrangements.

    • Linkage Isomerism: Different attachment points of ligands.

    • Coordination Isomerism: Interchanging ligands from cationic to anionic species.

    • Ionization Isomerism: Different ions dissociating in solution.

    • Solvate Isomerism: Presence/absence of solvent molecules.

Bonding in Coordination Compounds

  1. Valence Bond Theory (VBT): Explains bonding using hybridization of metal orbitals influenced by ligands.

    • Octahedral complexes: Hybridization schemes and orbital distribution.

    • Tetrahedral complexes: Different hybridization and configurations leading to paramagnetic/diamagnetic characteristics.

  2. Crystal Field Theory (CFT): Focuses on electrostatic interactions between the metal and ligands.

    • Orbital Splitting: Describes changes in energy levels due to ligand approach.

    • Spectrochemical Series: Ranks ligands by strength of field they produce.

Color Implications

  • Colors in coordination compounds arise from d-d transitions due to crystal field stabilization effects.

    • Example: Color of [Ti(H2O)6]3+ explained by energy absorption in d orbitals.

Importance and Applications of Coordination Compounds

  • Essential in biological systems (e.g., chlorophyll, hemoglobin).

  • Widely utilized in analytical chemistry for detection and quantitative measurements.

  • Key roles in extraction and purification processes in metallurgy.

  • Useful as catalysts in industrial processes (e.g., Wilkinson catalyst, electroplating).

  • Chelate therapy in medicinal applications for detoxification.

Summary

  • Coordination compounds are pivotal in modern inorganic chemistry, with wide-ranging applications and significant roles in various fields.