Lecture 8

• Coordination compound


  A central metal ion M, surrounded by a shell of ions/molecules known as ligands L
• M


  Normally an electron acceptor/Lewis acid
• L


  Normally a nucleophile/Lewis base
• Types of ligands


  • σ-donor ligand: donates electron density, often using s-orbitals or sp^n hybrids
  • σ-acceptor ligand: accepts electron density, using s-orbitals
  • π-donor ligand: donates electron density using p-orbitals
  • π-acceptor ligand: accepts electron density using p-orbitals
• σ-donor ligand


  • The most common
• π-donor and -acceptor ligands


  • Have both σ and π components
• We won't cover σ-acceptor in this part of the course, because it is too complicated
• Hapticity/'eta' (η)


  The number of neighbouring atoms in a ligand which are coordinated simultaneously to a metal centre
• Denticity/'kappa' (κ)


  The number of non-neighbouring atoms in a ligand which are coordinated simultaneously to a metal centre
• Formation constants (K$_f$)


  Used to express metal complex stability
• Complex ion equilibrium: aA + bB ⇌ cC

  The formation constant is given by: K$_f$=[C]^c/([A]^a[B]^b)
• Formation constant (K$_f$)


  Allows calculations of thermodynamic values ΔG, ΔS and ΔH: ΔG^0=-RT ln{K}=ΔH^0-TΔS^0
• Stepwise formation constants


  Used to given an overall formation constant, β$_n$: β$_n$=K$_1$×K$_2$×K$_3$×...×K$_n$, log β$_n$=log K$_1$+log K$_2$+log K$_3$+...+log K$_n$
• Formation constant (K$_f$)


  • High values indicate high complex stability
  • Values typically decrease with each successive substitution
  • If they don't, then it's likely a change in geometry occurred, particularly with small metal ions
• Factors affecting metal complex stability (monodentate only)


  • Ionic size and charge
  • HSAB
• Ionic size and charge


  Stability usually decreases with increasing cation size, but this does not apply to the transition metals
• HSAB


  • Hard cations form more stable complexes with hard ligands, due to ionic bonding (entropy driven)
  • Soft cations form more stable complexes with soft ligands, due to covalent bonding (enthalpy driven)
  • Hard-soft mismatches are not favoured as not enough energy is released to overcome the high solvation energy of the hard species
• Hard acids


  Small and highly-charged = high charge density = less polarisable
• Soft acids


  Less electropositive metals, with a lower charge-to-radius ratio = low charge density = more polarisable
• Hard bases


  Small anions/neutral molecules = high charge density = less polarisable
• Soft bases


  Larger anions/neutral molecules = low charge density = more polarisable
• This explains why early first-row transition metals are isolated from oxide ores, but Pb and Hg occur mostly in sulfide ores
• The soft character increases particularly towards the bottom left of the d-block
• Most second and third-row transition metals occur in nature as suflide ores (soft-soft match)
• Denticity (κ)


  • 1 donor atom = monodentate/unidentate
  • 2 donor atom = didientate/bidentate
  • 3 donor atom = tridentate/terdentate
  • 4 donor atom = tetradentate
  • 5 donor atom = pentadentate
  • 6 donor atom = hexadentate
• Chelate


  A ligand bonded to the central metal atom at two or more points
• Chelate effect


  • Confers increased stability on a complex
  • Can be thought of through probability, sterics, and entropy
• Chelation


  Increases stability, which can be measured and compared with non-chelated analogues through formation constants (K$_f$) and entropy values (ΔS^0)