nmr 5 Flashcards
(46 cards)
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.
