Chemistry Chapter 4.2 - Crystal Field Theory

Question 1

What is the purpose of the CFT theory?

Answer 1

explain unique transition metal complex properties including paramagnetism and colour

Question 2

What are the principles of CFT?

Answer 2

1. Transition metal ion is a free metal ion by itself
2. Ligands are point charges
3. Bonds between metal & ligand is totally electrostatic (not covalent) creating an electrostatic-crystal-field

Question 3

What does the term “crystal field” refer to?

Answer 3

the electrostatic field of the ligands (treated as point charges)
–> similar to the electrostatic field of cations and anions in an ionic crystal

Question 4

How is energy change described by CFT?

Answer 4

energy of d electrons of a transition metal ion changes when the ion interacts with negative charges of the non-bonding (lone) electrons of the LIGANDS

Question 5

How do the five d orbitals compare in energy?

Answer 5

degenerate

Question 6

How does energy compare in an OCTAHEDRAL complex?

Answer 6

degeneracy is broken

Question 7

What are the 5 d orbitals?

Answer 7

dxy (z-axis intersects lobes perpendicularly and other lobes are between the axes)
dxz (y-axis intersects lobes perpendicularly and other lobes are between the axes)
dyz (x-axis intersects lobes perpendicularly and other lobes are between the axes)
dz^2 (lobes along z-axis with a ring)
dx^2-y^2 (z-axis intersects lobes perpendicularly and other lobes are along the x and y axes)

Question 8

Why does the orbital energy differ in octahedral arrangements?

Answer 8

dz^2 and dx^2-y^2 have lobes along the z-axes and the ligands (point charges) are also on the axes
–> the lobes of the metal ion point directly at the ligands causing repulsive interactions
–> repulsive interaction = destabilization = energy of orbitals increase relative to their values for an ion in a SPHERICAL crystal field

Question 9

What are the relative positions of the six ligands around a metal ion in a complex?

Answer 9

two on the end of each axis

Question 10

How does the energy compare for dxy, dxy, dyz orbitals?

Answer 10

these orbitals are oriented so they point between ligands = stabilized orbitals = energies decrease relative to their values for a free ion

Question 11

What are eg orbitals?

Answer 11

dz^2 and dx^2-dy^2 orbitals; degenerate and greater than relative values in a spherical crystal field for a free ion

Question 12

What are t2g orbitals?

Answer 12

dxy, dxz, dyz orbitals; degenerate and less than relative values in a spherical crystal field for a free ion

Question 13

What is Δo?

Answer 13

energy difference/seperation between eg and t2g

Question 14

How does the interesting properties of transition metals arise?

Answer 14

loss of degeneracy of all d orbitals

Question 15

How does the TOTAL energy of d orbitals in octahedral complex compare with d orbitals in a spherically symmetrical electric field?

Answer 15

total energy is the same

spherically symmetrical electric field = barycentre level of energy – all d orbitals are degenerate

octahedral complex = lower energy t2g orbitals and higher energy eg orbitals (compared with barycentre)

Question 16

How can you prove that the total energy of octahedral complex and spherically symmetrical electric fields are the same for d-orbitals?

Answer 16

eg orbitals have an increased energy by 0.6 Δo (from barycentre) and 0.4 Δo decrease in energy:

(2 x 0.6 Δo) + (3 x -0.4 Δo) = 0 – no net change in energy due to crystal field splitting

Question 17

What factors affect the amount of energy difference (Δo)?

Answer 17

1. oxidation state of the metal ion
2. identity of the metal
3. nature of the ligand

Question 18

How does oxidation state of the metal ion impact the Δo?

Answer 18

increasing oxidation state = greater Δo
–> increases the electrostatic metal-ligand interaction energy b/c:
more oxidized = nucleus more attracted to e- = smaller cation = ligands get closer since the size of the d-orbitals do not change= ligands repel more with e- = higher energy split

Question 19

How does the identity of the metal affect the Δo?

Answer 19

greater Δo going down a group
–> expansion of the metal’s d’orbitals = increase in metal-ligand interaction

Question 20

How does the nature of the ligand affect Δo?

Answer 20

Δo increases with the spectrochemical series
–> weak-field ligands have small Δo (prefer high spin) and strong field ligands have large Δo (prefer low spin config)

Question 21

How do the colour of visible light compare as it is absorbed vs you our eye see them?

Answer 21

light seen is complementary to the light that is absorbed
400nm = violet light absorbed = yellow light seen
750nm = red light absorbed = green light seen

Question 22

What colour is seen when an object absorbed across all visible wavelengths?

Answer 22

black/grey

Question 23

What colour is seen when an object absorbs weakly/not at all in the visible spectrum?

Answer 23

colourless

Question 24

Why do transition metal complexes absorbed visible light of different colours?

Answer 24

Δo between eg and t2g is relatively small = energy falls within the visible spectrum

Question 25

What is a d-d transition? What does it result in?

Answer 25

electron excited from t2g to eg orbital by absorbing visible light; causes compound to appear coloured

Question 26

How does frequency (v) compare with Δo?

Answer 26

directly proportional hv=∆o

Question 27

How does wavelength compare with Δo?

Answer 27

inversely proportional; as the wavelength of light ABSORBED increases, Δo decreases

hc/wavelength = ∆o

Question 28

How can the colour intensity absorbed by a chemical compound be measured?

Answer 28

UV-vis spectroscopy

Question 29

How is UV-vis spectroscopy used?

Answer 29

light is passed through a monochromator and a splitter
- reference beam and incident beam
- incident beam is passed through sample creating the transmitted beam (might by dimmer depending on how much light is absorbed)
- detector –> computer measures numerical absorbance

Question 30

What does a graph produced by UV-vis spectroscopy look like?

Answer 30

peaks at the wavelengths absorbed within the ultraviolet and visible regions
(absorbance vs. wavelength)

Question 31

What is the relationship between paramagnetism and number of unpaired e-? What does this have to do with transition metals?

Answer 31

more unpaired e- = higher paramagnetism
- most transition metal complexes contain unpaired e-

Question 32

How does magnetism and apparent weight relate?

Answer 32

increase in unpaired e- = increase in apparent weight proportional to # of unpaired e-
–> force exerted by magnetic field

Question 33

What does the number of unpaired e- depend on?

Answer 33

nature of ligands involved; compounds with the same oxidation number can have different numbers of unpaired e-

Question 34

What is high-spin configuration?

Answer 34

e- distributed to have more unpaired e- (eg orbitals are filled before pairing)

Question 35

What is low-spin configuration?

Answer 35

e- distributed so they pair in t2g before filling the eg orbitals

Question 36

When is high-spin configuration attained?

Answer 36

Δo (energy required to move e- to eg) < energy required to pair e-

Question 37

When is low-spin configuration attained?

Answer 37

energy required to pair e- < Δo (energy required to move e- to eg)

Question 38

Why are some transition metals in water colourless (ex. Zn2+)?

Answer 38

to produce colour, the electrons in the metal must be promoted to a higher energy level and its difference produces the colour; if there is a full 3d10 config., there isn’t any higher energy level for the electron to be promoted to