Coordination Chemistry Overview

Overview

This lecture covers the key topics of coordination compounds, focusing on definitions, nomenclature, isomerism, valence bond theory (VBT), and crystal field theory (CFT), all crucial for board exams and further study in inorganic chemistry.

Introduction to Coordination Compounds

• Coordination compounds consist of a central metal atom/ion bonded to surrounding ligands via coordinate bonds.
• These compounds often appear complex (e.g., K₄[Fe(CN)₆], [Co(NH₃)₆]Cl₃).
• The structure includes a central metal atom, ligands (donating electron pairs), and possible counter ions.
• The coordination sphere is the central metal ion with ligands attached.
• The coordination number is the number of ligand atoms directly bonded to the metal.

Werner’s Theory

• Proposed two types of valencies: primary (ionizable, satisfied by anions) and secondary (non-ionizable, equals coordination number, satisfied by ligands).
• Only primary valency breaks in solution; secondary remains intact.
• Demonstrated using cobalt-ammonia-chloride complexes and their differing reactions with AgNO₃.

Types and Classification of Compounds

• Double salts completely ionize in water, unlike coordination compounds which retain their identity.
• Complexes are classified as homoleptic (same type of ligands) or heteroleptic (different ligands).
• Ligands classified by denticity: monodentate, bidentate, tridentate, and polydentate (e.g., EDTA is hexadentate).
• Ambidentate ligands can bond through two different atoms (e.g., NO₂⁻ via N or O).

Nomenclature of Coordination Compounds

• Name cation first, then anion, following the sequence: ligand(s) (alphabetically), then central metal with its oxidation number in Roman numerals.
• Ligand prefixes: mono-, di-, tri-, tetra-, penta-, hexa- (use bis-, tris- for polydentate ligands).
• For anionic complexes, add “-ate” to the metal name.
• Negative ligands end with “o,” positives with “ium.”
• Oxidation state is calculated by considering charges on all components.

Isomerism in Coordination Compounds

• Isomers: compounds with the same molecular formula but different arrangements.
• Structural isomerism types: ionization, coordination, linkage, and hydrate (solvate) isomerism.
• Stereoisomerism types: geometrical (cis/trans, fac/mer) and optical (d- and l- forms, non-superimposable mirror images).

Valence Bond Theory (VBT)

• Explains bond formation by overlap of metal ion orbitals and ligand lone pairs.
• Types of hybridization: sp³ (tetrahedral), dsp² (square planar), d²sp³/sp³d² (octahedral).
• Ligand field strength influences electron pairing: strong ligands cause low spin, weak ligands cause high spin.
• Magnetic behavior: paramagnetic (unpaired electrons), diamagnetic (all paired).
• Magnetic moment (μ) calculated as √n(n+2), where n = number of unpaired electrons.

Crystal Field Theory (CFT)

• Focuses on splitting of d orbitals upon ligand approach, resulting in energy differences (e.g., t₂g and e_g in octahedral fields).
• Explains color and magnetism in coordination compounds via d–d electron transitions.
• Spectrochemical series ranks ligands by field strength from weak (I⁻) to strong (CN⁻, CO).
• The color observed is the complementary color to the one absorbed due to d–d transitions.

Key Terms & Definitions

• Coordination Number — Number of ligand donor atoms bonded to the central metal.
• Ligand — Ion or molecule that donates electron pairs to the metal ion.
• Homoleptic Complex — Complex with only one type of ligand.
• Heteroleptic Complex — Complex with more than one type of ligand.
• Monodentate/Bidentate Ligand — Ligand donating one/two pairs of electrons, respectively.
• Ambidentate Ligand — Ligand with two possible donor atoms.
• Primary Valency — Ionizable valency (oxidation state).
• Secondary Valency — Non-ionizable valency (coordination number).
• Isomerism — Existence of compounds with same formula but different structure/arrangement.
• Cis/Trans Isomerism — Type of geometrical isomerism in coordination compounds.
• Low Spin/High Spin — Refers to the electron pairing induced by ligand strength.

Action Items / Next Steps

• Revise ligand names, types, and denticity for quick recall.
• Practice naming and writing formulas for coordination compounds.
• Solve homework: Write IUPAC names for given formulas, and analyze electronic configurations, hybridization, and magnetic properties using VBT and CFT.
• Review examples and test your understanding of isomerism types and identification methods.
• Memorize the spectrochemical series and key structural diagrams for octahedral and tetrahedral splitting.