Crystal Field Stabilisation Energies in Weak and Strong Fields

Question 1

What is the Crystal field stabilisation energy

Answer 1

1. In an octahedral complex it is the net energy of a t2g^x eg^y electronic configuration relative to the barycentre
2. CFSE= (-2/5x + 3/5y)Do
3. x= No e- in t2g
4. y= No e- in eg

Question 2

What happens to CFSE across a period and why

Answer 2

1. Increases across a period

2. As Mn+ radii decrease and zeff increases

Question 3

How is the ground state electronic configuration determined in octahedral complexes

Answer 3

1. Looking for the lowest energy configuration of the [t2g^x eg^y] set
2. Subject to Pauli Exclusion principle and Hund’s rule of maximum multiplicity

Question 4

Where would electrons be placed with 3d1, 3d2 and 3d3

Answer 4

1. Only one choice

2. Each e- in t2g

Question 5

Where would 3d4 electrons be placed

Answer 5

1. May pair with an electron in the t2g set

2. Or may remain unpaired and enter 3g set

Question 6

What are the names given to the arrangements where there is more than one possibility

Answer 6

1. Highest number of parallel spins- high-spin

2. Less parallel spins- low spin

Question 7

What happens when electrons pair up in an orbital

Answer 7

1. It will experience a strong coulombic repulsion- Pairing energy P

Question 8

Is P larger than Do for high or low spin

Answer 8

1. High spin Do<p>P</p>

Question 9

How do you calculate low-spin CFSE

Answer 9

1. CFSE=[(-2/5x + 3/5y)Do] + (1*P)= -1.6Do + P

Question 10

How do you calculate No of P in CFSE

Answer 10

1. (e- pairs in CF) - (e- pairs in spherical field)

2. P in CFSE is for e- pairs that aren’t present in spherical field

Question 11

When do you have a weak or strong field

Answer 11

1. Do<p>P = strong field= low spin</p>

Question 12

What does a weak field mean

Answer 12

1. Small gap between orbital sets

2. Lower energy is achieved if the upper eg orbital is occupied

Question 13

What does a strong field mean

Answer 13

1. large gap between orbital sets

2. Lower energy is achieved by occupying only the lower t2g orbitals despite the cost of the pairing energy (P)

Question 14

What does the strength of the crystal field and spin-pairing energy P depend on

Answer 14

1. identity of metal
2. Its oxidation state
3. The ligand

Question 15

Which octahedral complexes are generally low-spin

Answer 15

1. 4d and 5d

2. Do is usually higher than for 3d metals

Question 16

Which ligands are generally weak field

Answer 16

1. I-
2. Br-
3. S2-

Question 17

Which ligands are normally strong field

Answer 17

1. CN-

2. CO

Question 18

What is CFSE for tetrahedral complex of electronic configuration [e^x t2^y]

Answer 18

1. (-0.6x + 0.4y) Dt

Question 19

Which spin is usually observed for tetrahedral complexes and why

Answer 19

1. High-spin are usually observed

2. Dt is approx 4/9Do and therefore small compared to pairing energies (P)

Question 20

When do square planar complexes arise

Answer 20

1. Usually with d8 configurations

2. 3d8, 4d8, 5d8

Question 21

What spin do complexes containing 4d8 and 5d8 usually have

Answer 21

1. Have low spin
2. High crystal field splitting parameters due to the metals
3. So advantageous to have square planar over tetrahedral

Question 22

What are the general rules for 3d8 complexes

Answer 22

1. Only when the ligand is high in the spectrochemical series that the CFSE is large enough to result in a square planar complex for a 3d8 metal (Ni2+)
2. Otherwise CFSE is small and high spin and tetrahedral

Question 23

Which ligands are cause the largest crystal field splitting patterns

Answer 23

1. Those that are high in the SCS

Question 24

As the CFSE increases what happens to the preference for geometries

Answer 24

1. Greater preference for square-planar over tetrahedral geometry

Question 25

Why is square planar favoured over tetrahedral geometry for d8 ML4 complexes

Answer 25

1. Maximises CFSE