nmr 5 Flashcards
what is used to analyse complexes // metal complexes
nmr specccc
what are the 2 general methods in which nmr spectra be employed
spectra can be used on the supporting ligands
spectra can be used on the metal ion
what type of molecules are the supporting ligands
normally organic molecules
are most metal ions nuclei nmr active
yess most metal ion nucs are nmr active
what is spectra complicated by
its complicated by paramagnetic metal ions which broaden the chemical shift ranges
the metal ions we use nmr spectra on in this course areeeee
dimagneticcccc
1H spectra of complexes tend to resemble what
tend to resemble the chem shift values of free ligands
the 1H nmr spectra of complexes tend to resemble what!!!
those of free ligands
maybe a resonance shift of 0.5ppm
what causes a large change in ppm in metal complexes
if the ligand is bonded to a metal ion (large + charge)
if a flexible ligand becomes less flexible due to coordination
when a ligand bonds to a metal ion and forms a complex: why does the chem shift change
bc the metal has a large (+) charge,, this attracts e- from the ligand,, makes precession frequency larger,, gives a larger chemical shift.
what is a common ligand in transition metal complexes
M-H
hydride
hydride bc M is (+) which means H is (-)
H- nuc issss
highly shielded
due to being (-)
what is the chemical shift of a hydride,, H-
from 0 -> -20ppm
H- contain what type of nucccc
the same nuc as a 1H,, oxidation state doesnt effect the nuc
can the hydride couple,, can H- couple
yes coupling can still occur bc it has a 1H nuc.
it can couple to other nmr active nuclei in the COMPLEX.
in a free ligand,, things may be XXXX due to flexibility
but once its bonded to a complex,, XXXX can occur which causes them to be XXX
equivalent!!!
Ch2 may be the same as another Ch2
axial // equitorial arrangements can occur which makes them inequivalent.
due to the complex being rigid.
H bonded to metal will have a chemical shift where
0 ppm
to - 50ppm
what are chemical shifts influenced by
electron density around the nuc.
electron density of a ligand can be affected byyyy
neighbouring ligands
not just the electronegativity of what its bonded tooo
the stronger the pi accepting ability of a ligand,,, the
higher the chemical shift of the other ligandsssss
the chem shift its just affected by electronegativity but also the pi accepting abilities of neighbouring ligands.
explain back bonding
when electron density from the metals p orbitals is donated towards the p orbitals of the liganddd
increase in electron density = more shielding = lower precession frequency = lower chemical shift.
goes from pi to pi*
change in backbonding leads to a change innnn
chemical shiftttttt
okay explain the whole synergic bonding in carbonyl complexesss
basically u have ur lovely metal ion in the middle
u then have 5 CO ligands
and 1 PR3 ligand
the pi accepting P ligand competes for the same d orbitals as the CO ligand.
if the P is a stonger pi acceptor than the CO,, there will be less e- density going to the CO,, making it less shielded!!!
what ligand is a stronger pi acceptor:::
- The more strongly electron-withdrawing the group, the more the anti-bonding e-type LUMO orbitals are located at the P atom. This allows for better overlap with the ligand
- The energy difference between the vacant metal orbitals and filled ligand orbitals should not be very high
- Pi acceptors have empty, low energy π symmetry orbitals that accept electrons from the metal center. These orbitals are usually a π* antibonding molecular orbital or an empty p orbital.
when PPh3 is a ligand and is bonded to a metal describe what happensss
PPh3 has a lone pair which donates e- density to the positive metal ionnnn
this makes it less shielded
spectrum range of 31P
+200 —- -150ppm
In PF6 the p is coupled to how many nucs
P is coupled to 6 different F’s
P = 1/2
2 x 6 x 1/2 + 1
7 peakssss
in the nmr spectra of coordination complexes what must we consider
the coupling between different ligands
coupling to the metal nucleus
when tho R groups are the same,, they might not be in the same environment,, why is this
bc they could be ciss // trans to other groups,, and this will alter their environment and give a different amount of signals
if an octahedral complex has a fac configuration,, how many exvironments are there
if theres 3:3 ratio,, there’s only 1 environment!!
aka the 3 groups are in the same environment
if an octahedral complex has a mer configuration,, how many environments are there
there would be 2 environments if it’s a 3:3 ratio
2/3 will be in the same env (the ones trans to eachother)
and 1/3 will be in a diff environment (the one sticking out)
if each substituent is trans to the same thing and cis to the same things,, what does that mean
if means that they’re in rhe same environment.
if they’re all trans and cis to the same TYPE of molecule!!!
if each substituent is trans to the same thing and cis to the same things,, what does that mean
if means that they’re in rhe same environment.
if they’re all trans and cis to the same TYPE of molecule!!!
in an octahedral complex,, an equitorial group is cis to ehat
it’s cis to the axial!!!!
and to the other equitorial bonds
it’s trans to one thing and one thing only!!
in an octahedral complex,, an equitorial group is cis to ehat
it’s cis to the axial!!!!
and to the other equitorial bonds
it’s trans to one thing and one thing only!!
what does cis mean
next to ( both axial and equitorial groups)
what does cis mean
next to ( both axial and equitorial groups)
what does trans mean
trans can only occur to 1 thing.
aka 1 thing can only be trans to the thing it’s opposite
what does trans mean
trans can only occur to 1 thing.
aka 1 thing can only be trans to the thing it’s opposite
if groups are cis// trans to diff things what does that probs mean
probs means that they’re in different environments
means they will give different signals
if groups are cis// trans to diff things what does that probs mean
probs means that they’re in different environments
means they will give different signals
a larger coupling constant meansssss
the groups were probably trans to eachother
their interligand angle is more that 180*
quantum number for the coupling constant : i’ve an example of how to use it
J ab
is the coupling constant beteen a and b
a smaller interligand distance means
smaller interligand distance =
most likely cis to eachother
= smaller coupling constant
= signals are closer together
a large interligand distance
they’re most likely trans to eachother
the coupling constant will be larger
in a tree splitting signal,, the first split is usuallyyy
it’s usually the largest
larger amount of Hz
larger J value
usually between ligands that have the largest interligand angle
