화학2. CFT이론

CFT이론은 리간드와 금속의 결합에 의한 d 오비탈 분열로 배위화합물의 성질을 설명한다. 이 이론을 바탕으로 주어진 전이금속 착화합물의 d 오비탈 분열을 예측하여라. (d오비탈의 종류를 명확히 명시할 것)

(1) 사면체구조 (배위수 coordination number : 4)

 

(2) 사각평면구조 (배위수 coordination number : 4)

 

 

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CFT explains well the magnetic properties and in some degree the electronic spectra of the complexes.

 

However, there is no explanation of the bondings In other words, the purely electrostatic approach does not allow for the lower (bonding) molecular orbitals, and thus fail to provide a complete picture of the electron structures of complexes. (It cannot explain the spectrochemical series.)

 

CFT + MO theory > Complte theroy

Ligand Field Theory (Bonding Theory of Transition-Metal Complexes)

 

Constructing MOs to explain the electronic structure, magnetic properties, and bondings.

 

2.1. Ligand Field Theory

History

• Crystal Field Theory only includes ionic interactions in the solid state
• MO Theory developed and applied only to non-metal compounds
• Ligand Field Theory combines both for transition metal coordination compounds

 

MO’s for O_h complexes

• Donor atom = atom in the ligand with a p-orbital or hybrid orbital directly approaching the metal ion to form a sigma-bond
• The d_xy, d_xz, d_yz orbitals are not of correct symmetry to sigma-bond with ligands
• The d_x2-y2, d_z2, p_x, p_y, p_z, and s orbitals all have correct symmetry for interaction with ligands

 

3 things to consider to form MOs

N atomic orbitals > N molecular orbitals

Relative energy of atomic orbitals Symmetry match of atomic orbitals

Orbital Interactions in Octahedral Complexes.

Use the Group Theory Approach to find Molecular Orbitals.

• The six ligand orbitals generate the group orbitals to combine with metal Atomic Orbitals.
• Nondbonding metal orbitals: d_xy, d_xz, d_yz orbitals have T_2g symmetry
• Bonding metal orbitals: s orbital has A_1g symmetry; p_x, p y, p_z have T_1u symmetry, and d_x2-y2, d_z2, have E_g symmetry

• The 6 metal AO’s of proper symmetry combine with the six ligand group orbitals
• 6 bonding MO’s are filled by ligand electron pairs
• The metal t_2g Atomic Orbitals are nonbonding (d_xy, d_xz, d_yz )
• 6 antibonding orbitals are formed with the same symmetries as the bonding orbitals
• The 2 e_g* antibonding orbitals are the lowest energy antibonding orbitals available
• The d-electrons from the metal ion will fill in the t_2g and e_g* MO’s

• All octahedral metal complexes will have the exact same MO diagram, only the number of d-electrons will change
• The 6 bonding MO’s, with lowered energy for their electron pairs is what holds the metal complex together
• The d-electrons in the t_2g and e_g* MO’s( Determine the “Ligand Field”, Determine the geometry and many characteristics of the metal complex)

Orbital Splitting and Electron Spin

• The energy difference between the t_2g and e_g* MO’s = dleta_o = “delta octahedral”
• Strong-Field Ligands = ligands whose orbitals interact strongly with metal ion
• Weak-Field Ligands = ligands whose orbitals interact weakly with metal ion
• Electron Spin(Low Spin = least number of unpaired electrons; favored by strong field ligands with large delta_o, High Spin = maximum number of unpaired electrons; favored by weak field ligands with small dleta_o)
• Explanation for low/high spin complexes(Pairing Energy = PI = energy it costs to pair 2 e- in an orbital, Delta Octahedral = delta_o = energy gained by having e- in t_2g not e_g*, Strong-Field ligands have large delta_o favors pairing up in t_2g MO ( delta_o > PI ),  Weak-Field ligands have small delta_o favor keeping e- unpaired ( delta o < PI ))

 

Ligand Field Stabilization Energy = LFSE

• LFSE = energetic stabilization of the d-electrons due to orbital splitting caused by metal-ligand environments (measured in units of delta_o)
• Essentially equivalent to CFSE, although the theoretical approach is different
• Treat electrons in t_2g orbitals as stabilized by –2/5 delta_o and electrons in e_g* orbitals as destabilized by +3/5 delta_o
• Importance of LFSE

Importance of LFSE

• Hydration of M2+ first row ions
• Predict a smooth change as nuclear charge increases and size decreases
• The observed pattern has a “double hump” that parallels LFSE

 

Uses of LFSE

• Prediction of high spin or low spin based on ligand type
• Explanation of electronic spectra (UV-Vis spectra)
• Explanation of magnetic behavio