Coordination Compounds

Coordination Chemistry – Introduction

Coordination chemistry is the branch of inorganic chemistry that deals with the study of coordination (complex) compounds.

Addition (Molecular) Compounds

When solutions of two or more simple stable salts are mixed in simple molecular proportions and crystallised, the compound formed is called an addition or molecular compound.

Types of Addition Compounds

• Double salts
• Coordination (complex) compounds

Double Salts

• Exist only in solid state
• Dissociate completely into constituent ions in solution
• Lose identity in solution
• Metal ions exhibit normal valency
• Example: KCl·MgCl₂·6H₂O

Coordination (Complex) Compounds

• Exist in solid state
• Do not completely dissociate in solution
• Retain identity in solution
• Properties differ from constituent ions
• Example: K₃[Fe(CN)₆], [Co(NH₃)₆]Cl₃

Werner’s Coordination Theory

• Metal exhibits two valencies:
  • Primary valency (ionisable) → oxidation state
  • Secondary valency (non-ionisable) → coordination number
• Secondary valencies are fixed and directional
• Primary valencies are non-directional
• Secondary valencies determine geometry of the complex

Important Terms

• Central metal atom/ion: metal to which ligands are attached
• Coordination sphere: metal ion + ligands enclosed in square brackets
• Ligands: ions or molecules donating lone pair to metal ion
• Coordination number: number of ligand donor atoms attached to metal

Types of Ligands (Based on Denticity)

Monodentate

• Donate one pair of electrons
• Examples: NH₃, H₂O, Cl⁻, CN⁻, CO

Bidentate

• Donate two pairs of electrons
• Example: Ethylenediamine (en)

Polydentate

• Tridentate: dien
• Tetradentate: trien
• Pentadentate: EDTA³⁻
• Hexadentate: EDTA⁴⁻

Ambidentate Ligands

• Can coordinate through different atoms
• Examples:
  • CN⁻: cyano / isocyano
  • SCN⁻: thiocyanato / isothiocyanato
  • NO₂⁻: nitro / nitrito

Chelation & Chelate Effect

• Polydentate ligands forming ring structures are chelating ligands
• Complexes formed are chelates
• Stability increases with:
  • More chelate rings
  • Five- and six-membered rings
  • Resonance
• Chelated complexes are more stable than non-chelated ones (chelate effect)

Isomerism in Coordination Compounds

Structural Isomerism

• Ionisation isomerism
• Coordination isomerism
• Linkage isomerism
• Hydrate isomerism
• Coordination position isomerism

Stereoisomerism

• Geometrical (cis–trans)
• Optical isomerism

Geometrical Isomerism

• Observed in square planar and octahedral complexes
• Not observed in tetrahedral complexes
• Octahedral Ma₃b₃ complexes show fac and mer forms

Optical Isomerism

• Compounds rotating plane polarised light
• Exist as non-superimposable mirror images (enantiomers)
• Common in octahedral complexes with bidentate ligands
• Rare in square planar complexes

Valence Bond Theory (VBT)

• Explains bonding using hybridisation of metal orbitals
• Inner orbital complexes: d²sp³ (low spin)
• Outer orbital complexes: sp³d² (high spin)
• Explains geometry and magnetic behaviour

Limitations of VBT

• Cannot explain colour and spectra
• Fails to explain stability and kinetics
• No distinction between weak and strong ligands

Crystal Field Theory (CFT)

• Metal–ligand bonding treated as electrostatic
• Explains splitting of d-orbitals
• Octahedral splitting: t_2g and e_g
• Tetrahedral splitting: e and t₂

Factors Affecting Stability of Complexes

• Higher charge on metal ion
• 4d & 5d metals form more stable complexes
• Stronger ligands (spectrochemical series)
• Presence of chelate rings

Organometallic Compounds

• Contain metal–carbon bond

Types

• σ-bonded: Grignard reagents (RMgX)
• π-bonded: Zeise’s salt, ferrocene
• σ + π bonded: metal carbonyls

Metal Carbonyls

• Examples: Ni(CO)₄, Fe(CO)₅
• Metal oxidation state = 0