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Crystal Field Theory

1. A purely ionic model for transition metal complexes. In its simplest treatment CFT does not consider covalent bonding in complexes.
2. Ligands are considered as point charge.
3. The CFT does not provide for electrons to enter the metal orbitals, i.e. it does not consider any orbital overlap.
4. Predicts the pattern of splitting of d-orbitals.
5. Used to rationalize spectroscopic and magnetic properties. Octahedral complex (Oh) Tetrahedral complex (Td) Square planer complex (Sp)

• (^) We assume an octahedral array of negative charges placed around the metal ion (which is positive).
• (^) The ligand and orbitals lie on the same axes as negative charges. - (^) Therefore, there is a large, unfavorable interaction between ligand (-) and these orbitals. - (^) These orbitals form the degenerate high energy pair of energy levels.
• (^) The dxy, dyz, and dxz orbitals bisect the negative charges.
  • (^) Therefore, there is a smaller repulsion between ligand and metal for these orbitals.
  • (^) These orbitals form the degenerate low energy set of energy levels.

Octahedral Field

d

x 2 -y 2

d

z 2

d

xy

d

xz

d

yz In Octahedral Field

Distortions of Octahedral Complexes: Jahn-TellerDistortions of Octahedral Complexes: Jahn-Teller

‘ Any non-linear molecule having an orbitally degenerate electronic effect^ effect

configuration is unstable, and the system undergo distortion to remove the degeneracy .’ Examples: [Cu(NH 3

6

]

2+

• d 9 , d 7 (LS), d 4 (HS), d 1 , d 2 , d 5 (LS), d 6 (HS)

t

o  t

o For the same metal and ligands and the same internuclear distances Crystal field splitting favors the formation of octahedral complexes Since

Strong and weak ligands: Spectrochemical SeriesStrong and weak ligands: Spectrochemical Series

I-^  Br- S2- SCN- Cl- NO 3 -  F-^  C 2 O 4 2- H 2 O NCS- CH 3 CN NH 3  en  bipy phen NO 2 -  PPh 3  CN- CO Weak Field Strong Field

Crystal Field SplittingCrystal Field Splitting Energy (CFSE)Energy (CFSE)

• (^) In Octahedral field, configuration is: t2gx^ egy
• (^) Net energy of the configuration relative to the average energy of the orbitals is: = (-0.4x + 0.6y)O O = 10 Dq BEYOND d^3
• (^) In weak field: O  P, => t2g^3 eg^1
• (^) In strong field O  P, => t2g^4
• (^) P - pairing energy

d 4 Strong field = Low spin (2 unpaired) Weak field = High spin (4 unpaired) P <  o

P > 

o When the 4 th electron is assigned it will either go into the higher energy eg orbital at an energy cost of  o or be paired at an energy cost of P , the pairing energy. Coulombic repulsion energy and exchange energy

[Mn(CN) 6 ]3- Strong field Complex total spin (S) is 2  ½ = 1 Low Spin Complex Ground-state Electronic Configuration, Magnetic Properties [Mn(H 2 O) 6 ]3+ Weak Field Complex the total spin (S) is 4  ½ = 2 High Spin Complex Weak field d 4 Strong field d 4