Chemistry Chapter 4.2 - Crystal Field Theory Flashcards
What is the purpose of the CFT theory?
explain unique transition metal complex properties including paramagnetism and colour
What are the principles of CFT?
- Transition metal ion is a free metal ion by itself
- Ligands are point charges
- Bonds between metal & ligand is totally electrostatic (not covalent) creating an electrostatic-crystal-field
What does the term “crystal field” refer to?
the electrostatic field of the ligands (treated as point charges)
–> similar to the electrostatic field of cations and anions in an ionic crystal
How is energy change described by CFT?
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
How do the five d orbitals compare in energy?
degenerate
How does energy compare in an OCTAHEDRAL complex?
degeneracy is broken
What are the 5 d orbitals?
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)
Why does the orbital energy differ in octahedral arrangements?
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
What are the relative positions of the six ligands around a metal ion in a complex?
two on the end of each axis
How does the energy compare for dxy, dxy, dyz orbitals?
these orbitals are oriented so they point between ligands = stabilized orbitals = energies decrease relative to their values for a free ion
What are eg orbitals?
dz^2 and dx^2-dy^2 orbitals; degenerate and greater than relative values in a spherical crystal field for a free ion
What are t2g orbitals?
dxy, dxz, dyz orbitals; degenerate and less than relative values in a spherical crystal field for a free ion
What is Δo?
energy difference/seperation between eg and t2g
How does the interesting properties of transition metals arise?
loss of degeneracy of all d orbitals
How does the TOTAL energy of d orbitals in octahedral complex compare with d orbitals in a spherically symmetrical electric field?
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)
How can you prove that the total energy of octahedral complex and spherically symmetrical electric fields are the same for d-orbitals?
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
What factors affect the amount of energy difference (Δo)?
- oxidation state of the metal ion
- identity of the metal
- nature of the ligand
How does oxidation state of the metal ion impact the Δo?
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
How does the identity of the metal affect the Δo?
greater Δo going down a group
–> expansion of the metal’s d’orbitals = increase in metal-ligand interaction
How does the nature of the ligand affect Δo?
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)
How do the colour of visible light compare as it is absorbed vs you our eye see them?
light seen is complementary to the light that is absorbed
400nm = violet light absorbed = yellow light seen
750nm = red light absorbed = green light seen
What colour is seen when an object absorbed across all visible wavelengths?
black/grey
What colour is seen when an object absorbs weakly/not at all in the visible spectrum?
colourless
Why do transition metal complexes absorbed visible light of different colours?
Δo between eg and t2g is relatively small = energy falls within the visible spectrum
What is a d-d transition? What does it result in?
electron excited from t2g to eg orbital by absorbing visible light; causes compound to appear coloured
How does frequency (v) compare with Δo?
directly proportional hv=∆o
How does wavelength compare with Δo?
inversely proportional; as the wavelength of light ABSORBED increases, Δo decreases
hc/wavelength = ∆o
How can the colour intensity absorbed by a chemical compound be measured?
UV-vis spectroscopy
How is UV-vis spectroscopy used?
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
What does a graph produced by UV-vis spectroscopy look like?
peaks at the wavelengths absorbed within the ultraviolet and visible regions
(absorbance vs. wavelength)
What is the relationship between paramagnetism and number of unpaired e-? What does this have to do with transition metals?
more unpaired e- = higher paramagnetism
- most transition metal complexes contain unpaired e-
How does magnetism and apparent weight relate?
increase in unpaired e- = increase in apparent weight proportional to # of unpaired e-
–> force exerted by magnetic field
What does the number of unpaired e- depend on?
nature of ligands involved; compounds with the same oxidation number can have different numbers of unpaired e-
What is high-spin configuration?
e- distributed to have more unpaired e- (eg orbitals are filled before pairing)
What is low-spin configuration?
e- distributed so they pair in t2g before filling the eg orbitals
When is high-spin configuration attained?
Δo (energy required to move e- to eg) < energy required to pair e-
When is low-spin configuration attained?
energy required to pair e- < Δo (energy required to move e- to eg)
Why are some transition metals in water colourless (ex. Zn2+)?
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
