Crystal Field Theory Flashcards
Assumptions in crystal field theory
- Central metal ion is a point positive charge
- Ligands are point negative charges
- Not worried about orbital overlap, electron sharing, size or shape.
Describe the octahedral crystal field
- When metal is infinitely far from the ligands all 5 orbitals are degenerate and lowest energy
- In a spherical electric field, all six ligands bound so electrons in these ligands repel, increasing energy of d orbitals by the same amount, so still degenerate but higher energy
- Two d orbitals point directly to ligands whereas other three point between ligands (d orbital theory)
- Two of these orbitals are thus slightly more destabilised and 3 slightly more stabilised, creating a double degenerate set and a triple degenerate set. 5 set is lost
- The difference in energy between eg and t2g is called delta(o) = octahedral crystal field splitting energy
- Eg increases by 3/5 delta(o) and t2g decrease by -2/5 delta(o)
Describe a square planar crystal field
- Z axis is lost
- Orbitals with z component are stabilised
- Dx2-y2 has highest energy
Tetrahedral crystal field
- None of the orbitals point directly at ligands (using cube geometry)
- D orbitals pointing along the axis do not point towards ligands therefore dz2 and dx2-y2 are most stable
- Upside down octahedral crystal field diagram
- Difference in energy between ligands is called delta(t) = tetrahedral crystal field splitting energy
What is the d^n configuration
The d^n configuration is the number of electrons in a crystal field diagram
d^n configuration = group no. - oxidation state
E.g. [Cr(OH)2]3+ —> d^n = 6-3 = 3
3 electrons into a crystal field diagram^
Pauli exclusion
Hund’s
Pairing energy, P
- No more than two electrons in a single orbital, if if paired they must have opposite spins
- If there is more than one degenerate orbital electrons occupy separate orbitals with parallel spins
- The energy required to pair two electrons in a single orbital
High spin vs low spin configurations in octahedral complexes
- High spin configurations occur in octahedral complexes for d4-d7 when electron filling favours single electron filling since the delta(o) is less than the pairing energy
- Low spin favour pairing
Low spin complexes are favoured in 4d and 5d metals due to lower pairing energies
Why is the hexaaquacopper(II) ion not octahedral
- Cu oxidation state = +2
- Dn configuration = d9
- Coordination number is 6 (6 x monodentate ligands) so expecting octahedral geometry
- Jahn-Teller Distortion occurs when the ground electronic configuration of a non-linear complex is orbitally degenerate
- Occurs in order to achieve a lower energy
- Tetragonal distortion and lower energy
Electron filling for tetrahedral complexes explained
The splitting energy in tetrahedral d-orbitals is 4/9 of the splitting energy for octahedral d-orbitals
So filling of t2 orbitals is more favourable than pairing electrons in e orbitals
Since pairing energy is greater than splitting energy (deltat)
So high spin complexes formed
What affects the splitting energy in octahedral complex
- Charge on metal ion splitting energy increases since stronger metal-ligand attractions lead to ligands closer together = more orbital interaction and electron repulsion
- Metal ions in the same group, with the same charge and ligands, will exhibit higher splitting energy as principle quantum number increases
- The type of ligand. The more negative the higher the splitting energy
Give one of the properties which can distinguish between high spin and low spin complexes
- Magnetic properties
- Diamagnetic complexes are likely occurring in low spin complexes with larger splitting energies since there are mainly paired electrons
- Paramagnetic complexes occur more likely in high spin complexes where filling of empty t2 orbitals
Explain what the eg and t2g terms mean
These are the two sets of d orbitals of a metal ion
The sets are produced by the interaction with ligands, causing destabilisation in some and stabilisation in others
The eg set is doubly degenerate and consists of dz2 and dx2-y2
The t2g set is triply degenerate and consists of dxy, dxz and dyz orbitals
The energy difference between both sets of orbitals is called deltaoct
