CFT AND LFT

• Crystal Field Theory (CFT)

  Explains colors of solids
• Ligand Field Theory (LFT)


  Explains the complete version of CFT
• Octahedral Complexes


  • Ligands approach Mn+ along x,y,z axes
  • d electrons are repelled by charges of ligands, hence E increases
  • Degree of repulsion depends on orientation of d orbitals
• Ligand Field Splitting (Δo)


  The energy separation between the two sets of orbitals in an octahedral complex
• Ligand Field Splitting (Δt)


  The energy separation between the two sets of orbitals in a tetrahedral complex
• Strong Field and Low Spin


  • Splitting of d orbitals energies explains
  • The splitting produced by the ligands is very large, causing electrons to pair in the low energy t2g orbitals
  • Strong field case is also called low spin
  • The complex is diamagnetic with all electrons paired
  • Δo>P
• Weak Field and High Spin


  • The splitting produced by the ligands is small, causing electrons to occupy all five orbitals before pairing occurs
  • The complex is paramagnetic
  • Δo<P
• For octahedral d8, d9, and d10 complexes there is only one way to write satisfactory configurations
• Charge effect of metal ions

  As the metal ion charge increases the ligand are drawn closer to the metal ion because of its increased charge density

  The ligands are drawn closer to the metal ion, causing greater splitting of the d orbitals and a larger Δ value

  Magnitude Δ for a given ligand increases as the charge of the metal ion increases
• Spectrochemical Series


  • I- < Br- < SCN- ~ Cl- < F- < OH- ~ ONO- < C2O4²- < H2O < NCS < EDTA4- < NH3 ~ pyr ~ en < phen < CN- ~ CO
• Spectrochemical Series (metals)


  • Mn2+ < Ni2+ < Co2+ < Fe2+ < V2+ < Fe3+ < Co3+ < Mn3+ < Mo3+ < Rh3+ < Ru3+ < Pd4+ < Ir3+ < Pt4+
• Tetrahedral Complex


  • The magnitude of the splitting is smaller than in octahedral complexes
  • Δt = 4/9 Δo
• Square Planar and Linear Complexes

  • The d orbitals are split differently than in octahedral and tetrahedral complexes
• Magnetic Properties of Complexes


  • Strong-field ligands often lead to low-spin, weakly paramagnetic complexes
  • Weak-field ligands often lead to high-spin, strongly paramagnetic complexes
• Ligand Field Theory (LFT)


  An extension of crystal field theory that incorporates all levels of covalent interactions using molecular orbital theory
• Molecular Orbital Model of Complexes

  • The d2z, dx2-y2, 4s, 4px, 4py and 4pz orbitals are involved in the MOs
  • The dxz, dyz and dxy orbitals are nonbonding
• The e* orbitals have relatively little contribution from ligand orbitals due to the large energy difference between the ligand orbitals and the metal ion 3d orbitals
• Effect of Weak Field Ligands

  • Ligand lone pair orbitals have very low energy and do not mix thoroughly with metal ion orbitals, resulting in a small Δ
• Effect of Strong Field Ligands

  • Strong field ligands produce a large degree of mixing between ligand and metal ion orbitals, resulting in a large Δ and low spin
• Factors affecting Δ


  • Δ increases as the ligand field strength increases
  • Δ increases as the oxidation state of the metal increases
  • Δ increases as you go down a group in the periodic table
• Transition metal complexes are colored because the energy difference between d-orbital energy levels corresponds to the energy of visible light
• Color of complexes

  • common for the first transition series
  • energy corresponds that of visible light
  • d-d transitions are the cause of delicate colors of many complexes
• Energy splitting of tetrahedral complex

  • Because the tetrahedral complex has fewer ligands, the magnitude of the splitting is smaller
  • The difference between the energies of the t2g and eg orbitals in a tetrahedral complex (Δt) is slightly less than half as large as the splitting in analogous octahedral complexes (Δo)
  • Δt=4/9Δo
• Factor affecting Δ

  1. Δ depends on nature of ligand
  2. Δ depends on oxidation state of metal
  3. Δ depends on row which metal occurs
• Δ Depends on nature of ligand

  • some ligands produce larger splitting of d orbitals than other
  • Δo increases as ligand field strength increses
  • CN- always give large splitting
  • F- always gives small spltting
  • Consequence: changing ligand changes Δ; same metal ion can form variety of complexes with wide range of colors
• Δ depends on oxidation state of metal

  • given metal and ligand set; Δ increases as oxidation state of M increases
  • Why: electrons are removed from M
  • Charge on Mn+ becomes more positive
  • ion size becomes smaller
  • Result:
  • ligands attracted to metal more strongly
  • greater repulsion with electron in dx2-y2 and dz2
  • greater splitting of d orbitals and larger Δ
• Δ depends on row in which metal occurs

  • Δ increases as you go down a group