5c Flashcards
whats cool about the Td and Oh geometries
theyre idolised!!
theres no distortions or anything,, all bonds are equal length etc
do distortions in the Oh and Td geometries affecct the dd abs bands in abs spec
yesss ofc girl
changing the geometry of a Td or Oh complex due to distortions does what
it alters the energy of the d orbitals. things can be reduced in energy leading to a different order in their energies.
going from Oh to square planar,, what geometry is between the two
u have octahrdral,, then square pyramid,, then square planar.
when theres no ligand field,, whats up with the 5 d orbitals
they will be degenerate.
what diff between the square based pyramid and octahedral geometries
the square pyramid is missing a ligand field on the z axis,, meaning there is less ligand interaction with the orbitals.
meaning the orbitals with z character will be lower in energy + therefore more stabilised compared to the orbitals with x and y character.
diff between square planar and square based pyramid geometris and the order of their individual d orbitals
square based pyramid = one less z ligand interaction so the orbitals with z character are lower in energy however the eg and t2g dont overlap.
however with the square planar geometry there are 2 less Z ligand interactions,, meaning the orbitals with z character are even lower in energy,, allowing the orbitals to make up the eg and t2g things to overlap
in terms of octahedral, square based pyramid and square planar,, which ones have a geometry that alters the d orbitals sm that the eg and t2g orbitals overlap,, and wich ones would overlap
square planar ones would overlap!!
bc youve removed 2 Z axis incoming igand fields.
meaning z orbitals interact with them less
meaning z orbitals are stabilised better and also lower in energy
meaning dz^2 will be mixed with the t2g dxy orbitals
and the dxz and dyz will also be lower in energy
which geometry has the most symmetry
Oh
diff geometries have what
diff splitting patters
by diff geometries we mean Oh, Td, square planar etccc
diff geometry symmetry means what
means there will be diff energy terms for transitions
whiich can be predicted using groups theory
what does a jahn teller distortion do
- reduces the symmetry of a system to reduce // stabilise the energy of that system
- it creates a distortion which is energetically favourable
what orbitals // what systems is jahn teller distortion seen in
ones where there is an asymmetrical // uneven filling of degenerate orbitals
aka 3 e- in the eg orbital will lead to uneven filling as one would get 2 e- and the other will have just 1.
jahn teller would occur to alter their energies and separate them,, so one e- in in the one orbital. which is the most stable configuration.
2 e- configurations we focus jahn teller on in these lectures
we look at d1 and d9 configurations
2 types of jahn teller
theres the Z axis elongation and the Z axis compression
what happens in d9
in d9,, if we have the t2g and eg,, its clear that we will have an uneven distribution of e- in the eg ,, higher energy orbital.
to overcome this the sustem will elongate the Z axis. this will reduce the energy and stabilise any orbitals with the Z component.. this lowers the dz^2 orbital but keeps the dx2y2 orbital high energy. this also lowers all orbitald with a z component in the t2g orbital too.
this removes the degeneracy of the t2g and eg orbitals.
thid allows the 1 e- in the dx2y2 orbital to feel as if its in one orbital,, not 2 degenerate ones,, making it energetically favourable.
what type of distortion is a jahn teller distortion
its a tetragonal distortion
bc ur going from Oh to D4h
how do u read an orgel diagram
we look at the compound we have + we use the orgel diagram to tell us how many transitions we expect and what transition this will be
aka for a d9 e- config with Oh symmetry we’re gonna want to look at the x axis and see which side of the orgel diagram to look at.
and then this side will tell us the energy levels the transition occurs betweennnnn
whats the limitation about using the orgel diagram to predict what transition // how many transitions we will have
the limitation is that it assumes ur complex is perfect // ideal Oh // Td geometry.
it assumes there are no distortions in its geometry and that all the bond are the same length.
this obvs is not true for every molecule.
once we find the transition we should predict our compound would have if it had ideal Oh // Td character,, what do we do
we look to see how the distortion it has effects and splits the energy levels that the orgel diagram predicted.
okay so u have ur free ion ground term symbol,, then what happens
u apply an Oh field to it and this splits it and u see the transitions to expect using ur orgel diagram.
the distortion it has then adds more splitting
so the prediction are then split into more levels,, and transitions can occur between these.
these would be in the Dh4 field,, the tetragonal field.
these are predicted using group theory’!!! dont need to memorise them .
how can we tell if a transition is spin allowed if were given the enrgy levels it occurs between
we look at the top left as this is the multiplicity.
and if the multiplicity is equal aka if the multiplicity is the same!! that means that 🔺S=0
meanign there was no change in multiplicity. meaning no change in spin,, meaning its spin allowed.
value of E,, molar absorption coefficient when a transition is spin allowed
single valued // up to 10.
what does distortion do thats good
- stabilised a complex
- allows energy levels that correspond to transitions split,, allowing more different transitions // a range of transitions to occur.
more vertical transtitions can occur // additional transitions are possible.
if distortion allows more transitions to occur what does this do to the uvvis spec
as distortions split transition levels allowing more transitions to occur,, the more transitions are possible,, the broader the uvvis abs band gets. bc ur absorbing diff amounts of energy for each transition.
each of the dd spin allowed transitions can contribute to the absorption band.
d9,, what jahn teller distortion occurs
the Z axis elongation,, reducing energy of orbitals with z character
d1,, what jahn teller distortion occurs
the z axis compression one
eg or t2g : which one is more important for d9
eg
eg or t2g: which is more important for d1
t2g
why does d1 lead to a jahn teller .// tetragonal distortion by compression of the z axis
bc u only have 1 d electron,, meaning ur three degenerate t2g orbitals will be asymmetrically filled
meaning a distortion where orbitals with a z contribution will increase in energy and be less stabilised,, making orbitals with an xy character appear to be higher in energy,
so d x2y2 and dxy will be lower in energy and dz2 and dxz dyz will appear higher in energy. than the other orbitals theyre normally degenerate with.
so the 1e- in now in one non degenerate dxy orbital which is most stable
in both jahn teller cases,,where does the 1 e- want to be
it wants to be in one orbital that is non degenerate.
distortion splits the t2g and eg stuff found by orgnel diagrams into what
into group theory points
which we app dont need to learn.
so distortion splits orgnel diagram orediction us,, bc orgnel is based on ideal symmetries but these obvs arent always the case.
when distortion splits the orgnel diagram predictions,, can transitions between those of similar energy occur
yes but theyre so small in energy that they normally dont show up in uvvis spec.
that means sometimes 3 transition s can be predicted between the point group split levels,, but not all of them can be seen in uvvis spec.
2 transitions seen between the point group levels means whattttt
normally 2 absorbance bands will be seen in the uvvis spec.
there can be a little dip between them
shower wavelength =
higher energy
larger transition gap
the abs wavelength of the band and therefore the transitions energy deoends on what
it depends on the ligands field strength!!
strong field ligands = high energy gap = lower wavelength of abs
weak field ligand = small split // separation = lower energy transition = larger wavelength of absssss
