Coordinate Compounds

  • Definition: Compounds in which a central metal atom is bonded to one or more ligands through coordinate covalent bonds.
  • Types: Octahedral, tetrahedral, square planar, etc.
  • Spectrochemical series: Arranges ligands in order of increasing crystal field splitting energy.
  • Weak field ligands: Low crystal field splitting energy.
  • Strong field ligands: High crystal field splitting energy.

Octahedral Coordination Complexes

  • Octahedral geometry: Central metal atom surrounded by six ligands.
  • Examples: [Co(NH3)6]3+, [Cr(H2O)6]3+
  • Ligand: An ion or molecule that donates a pair of electrons to form a coordinate bond with the central metal atom.
  • Coordinated atom: Central metal ion.

Weak Field Ligands

  • Result in small crystal field splitting energy (Δ).
  • Examples: H2O, NH3, Cl-, F-, SCN-, OH-, NO2-
  • Low Δ leads to low energy difference between d orbitals.
  • Greater chance of pairing electrons in low energy orbitals.

Strong Field Ligands

  • Result in large crystal field splitting energy (Δ).
  • Examples: CN-, CO, en, ox2-, NH2-, NO, NO3-
  • High Δ leads to high energy difference between d orbitals.
  • Less chance of pairing electrons in low energy orbitals.

Pairing in Low Field Ligands

  • Low field ligands have small crystal field splitting energy.
  • Result in weak splitting of d orbitals.
  • Eg orbitals become higher in energy than t2g orbitals.
  • Pairing occurs in t2g orbitals when electrons fill d orbitals.

Pairing in Strong Field Ligands

  • Strong field ligands have large crystal field splitting energy.
  • Result in strong splitting of d orbitals.
  • T2g orbitals become higher in energy than eg orbitals.
  • Pairing occurs in eg orbitals when electrons fill d orbitals.

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Spectrochemical Series

  • Arrangement of ligands based on increasing crystal field splitting energy (Δ).
  • Determines the splitting of d orbitals in coordination compounds.
  • Low Δ: Weak field ligands.
  • High Δ: Strong field ligands.
  • Useful in predicting the color and magnetic properties of coordination compounds.

Weak Field Ligands

  • Low crystal field splitting energy (Δ).
  • Examples: H2O, NH3, Cl-, F-, SCN-, OH-, NO2-
  • Do not cause significant splitting of d orbitals.
  • Result in small energy difference between d orbitals.
  • Increased probability of pairing electrons in low energy orbitals.

Strong Field Ligands

  • High crystal field splitting energy (Δ).
  • Examples: CN-, CO, en, ox2-, NH2-, NO, NO3-
  • Cause significant splitting of d orbitals.
  • Result in large energy difference between d orbitals.
  • Decreased probability of pairing electrons in low energy orbitals.

Pairing in Low Field Ligands

  • Low field ligands have small crystal field splitting energy (Δ).
  • Weak splitting of d orbitals.
  • Eg orbitals become higher in energy than t2g orbitals.
  • Pairing occurs in t2g orbitals when filling d orbitals.
  • Example: [Ni(CN)4]2-, [Co(NH3)6]2+

Pairing in Strong Field Ligands

  • Strong field ligands have large crystal field splitting energy (Δ).
  • Strong splitting of d orbitals.
  • T2g orbitals become higher in energy than eg orbitals.
  • Pairing occurs in eg orbitals when filling d orbitals.
  • Example: [Fe(CN)6]3-, [Co(en)3]3+

Example: [Ni(CN)4]2-

  • Percentage of pairing = (2 * Δ / Δoct) * 100%
  • Δoct = Crystal field splitting energy for octahedral complex.
  • For Ni2+ in [Ni(CN)4]2-, Δoct = 4/9 * Δoct
  • Percentage of pairing = (2 * 4/9) * 100% = 88.9%

Example: [Fe(CN)6]3-

  • Percentage of pairing = (3 * 4/9) * 100% = 133.3%
  • Impossible to have 133.3% pairing.
  • Electrons pair in lower energy t2g orbitals and occupy eg orbitals.

Example: [Co(NH3)6]2+

  • Cobalt ion in +2 oxidation state.
  • Number of d electrons = 7.
  • High spin complex.
  • All orbitals are singly occupied before any pairing occurs.

Example: [Co(en)3]3+

  • Cobalt ion in +3 oxidation state.
  • Number of d electrons = 6.
  • Low spin complex.
  • Pairing occurs in the lower energy eg orbitals.

Summary

  • Spectrochemical series helps in determining ligand strength.
  • Weak field ligands have low crystal field splitting energy (Δ).
  • Strong field ligands have high crystal field splitting energy (Δ).
  • Pairing occurs in low energy t2g orbitals for weak field ligands.
  • Pairing occurs in high energy eg orbitals for strong field ligands.