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Understanding Coordination Compounds and Their Properties
May 22, 2025
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Coordination Compounds Lecture Notes
Introduction to Coordination Compounds
D Block Elements
: Examples include iron, copper, zinc.
These metals typically lose electrons, but in coordination compounds, they accept electron pairs.
Reasons Metals Accept Electron Pairs
:
Metals have empty shells.
Metals achieve stability by completing the octet rule.
Formation of Coordination Compounds
Example
: Copper and NH₃.
Copper (metal atom) accepts a pair of electrons from NH₃ (neutral molecule with a lone pair).
Coordinate Bond
: Formed between NH₃ and copper.
Definitions
Coordination Compounds
: Complex compounds where a metal atom is bonded to ions or molecules.
Ligand
: An ion or molecule that donates a pair of electrons to a central metal atom.
Composition
: Metal atom + ligand(s).
Basic Terms
Central Atom
: The metal atom in a coordination compound (e.g., chromium in [Cr(NH₃)₆]Cl₃).
Coordination Number
: Number of electron pairs donated (e.g., 6 for [Cr(NH₃)₆]Cl₃).
Coordination Sphere
: The central atom and ligands enclosed in brackets.
Counter Ion
: Ion outside the coordination sphere (e.g., Cl in [Cr(NH₃)₆]Cl₃).
Werner’s Theory
Werner
: First to study coordination compounds.
Developed [Co(NH₃)₆]Cl₃, showing different types of bonding with ammonia and chloride.
Types of Valencies
:
Primary Valency
: Satisfied by negative ions; represents oxidation state; ionizable.
Secondary Valency
: Satisfied by neutral molecules or positive ions; represents coordination number; non-ionizable.
Calculating Valencies
Primary Valency
: Oxidation number of central atom.
Secondary Valency
: Coordination number.
Structure and Examples
Werner’s Structures
: Showcases positioning of primary and secondary valencies.
Types of Ligands
Denticity
: Number of lone pairs donated by a ligand.
Types of Ligands
:
Unidentate/Monodentate
: Donate one pair (e.g., Cl⁻).
Bidentate
: Donate two pairs (e.g., ethylenediamine - en).
Ambidentate
: Can donate via different atoms (e.g., SCN⁻).
Polydentate
: Donate multiple pairs (e.g., EDTA).
Complexes
Homoleptic Complex
: Single type of ligand.
Heteroleptic Complex
: Multiple types of ligands.
Nomenclature of Coordination Compounds
Ligand Names
:
Neutral
: e.g., NH₃ (ammine), H₂O (aqua).
Anionic
: Add "-o" (e.g., Cl⁻ = chloro).
Cationic
: Add "-ium" (e.g., NO⁺ = nitrosonium).
Naming Rules
:
Left to right.
Alphabetical order for ligands.
Add -ate for negative coordination spheres.
Crystal Field Theory (CFT)
Concept
: Explains splitting of d-orbitals when ligands approach a metal.
Orbital Splitting
:
Octahedral
: Ligands attack along axes; splits into e_g and t_2g.
Tetrahedral
: Ligands attack between axes.
Energy Levels
: Different for e_g and t_2g in octahedral; reversed in tetrahedral.
Valence Bond Theory (VBT) and Hybridization
Hybridization
: Process of orbitals combining to form new hybrid orbitals.
Examples
:
Inner Orbital Complex
: e.g., d²sp³ (octahedral).
Magnetic Properties
: Depend on pairing of electrons (low-spin vs. high-spin).
Summary
Coordination compounds involve metal atoms accepting electron pairs from ligands.
Werner's work laid the foundation for understanding structure and valency.
CFT and VBT provide insights into bonding and properties of coordination compounds.
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