nmr 5

Question 1

what is used to analyse complexes // metal complexes

Answer 1

nmr specccc

Question 2

what are the 2 general methods in which nmr spectra be employed

Answer 2

spectra can be used on the supporting ligands

spectra can be used on the metal ion

Question 3

what type of molecules are the supporting ligands

Answer 3

normally organic molecules

Question 4

are most metal ions nuclei nmr active

Answer 4

yess most metal ion nucs are nmr active

Question 5

what is spectra complicated by

Answer 5

its complicated by paramagnetic metal ions which broaden the chemical shift ranges

Question 6

the metal ions we use nmr spectra on in this course areeeee

Answer 6

dimagneticcccc

Question 7

1H spectra of complexes tend to resemble what

Answer 7

tend to resemble the chem shift values of free ligands

Question 8

the 1H nmr spectra of complexes tend to resemble what!!!

Answer 8

those of free ligands

maybe a resonance shift of 0.5ppm

Question 9

what causes a large change in ppm in metal complexes

Answer 9

if the ligand is bonded to a metal ion (large + charge)

if a flexible ligand becomes less flexible due to coordination

Question 10

when a ligand bonds to a metal ion and forms a complex: why does the chem shift change

Answer 10

bc the metal has a large (+) charge,, this attracts e- from the ligand,, makes precession frequency larger,, gives a larger chemical shift.

Question 11

what is a common ligand in transition metal complexes

Answer 11

M-H

hydride

hydride bc M is (+) which means H is (-)

Question 12

H- nuc issss

Answer 12

highly shielded
due to being (-)

Question 13

what is the chemical shift of a hydride,, H-

Answer 13

from 0 -> -20ppm

Question 14

H- contain what type of nucccc

Answer 14

the same nuc as a 1H,, oxidation state doesnt effect the nuc

Question 15

can the hydride couple,, can H- couple

Answer 15

yes coupling can still occur bc it has a 1H nuc.

it can couple to other nmr active nuclei in the COMPLEX.

Question 16

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

Answer 16

equivalent!!!

Ch2 may be the same as another Ch2

axial // equitorial arrangements can occur which makes them inequivalent.

due to the complex being rigid.

Question 17

H bonded to metal will have a chemical shift where

Answer 17

0 ppm
to - 50ppm

Question 18

what are chemical shifts influenced by

Answer 18

electron density around the nuc.

Question 19

electron density of a ligand can be affected byyyy

Answer 19

neighbouring ligands

not just the electronegativity of what its bonded tooo

Question 20

the stronger the pi accepting ability of a ligand,,, the

Answer 20

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.

Question 21

explain back bonding

Answer 21

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*

Question 22

change in backbonding leads to a change innnn

Answer 22

chemical shiftttttt

Question 23

okay explain the whole synergic bonding in carbonyl complexesss

Answer 23

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!!!

Question 24

what ligand is a stronger pi acceptor:::

Answer 24

• 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.

Question 25

when PPh3 is a ligand and is bonded to a metal describe what happensss

Answer 25

PPh3 has a lone pair which donates e- density to the positive metal ionnnn

this makes it less shielded

Question 26

spectrum range of 31P

Answer 26

+200 —- -150ppm

Question 27

In PF6 the p is coupled to how many nucs

Answer 27

P is coupled to 6 different F’s
P = 1/2

2 x 6 x 1/2 + 1
7 peakssss

Question 28

in the nmr spectra of coordination complexes what must we consider

Answer 28

the coupling between different ligands

coupling to the metal nucleus

Question 29

when tho R groups are the same,, they might not be in the same environment,, why is this

Answer 29

bc they could be ciss // trans to other groups,, and this will alter their environment and give a different amount of signals

Question 30

if an octahedral complex has a fac configuration,, how many exvironments are there

Answer 30

if theres 3:3 ratio,, there’s only 1 environment!!

aka the 3 groups are in the same environment

Question 31

if an octahedral complex has a mer configuration,, how many environments are there

Answer 31

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)

Question 32

if each substituent is trans to the same thing and cis to the same things,, what does that mean

Answer 32

if means that they’re in rhe same environment.

if they’re all trans and cis to the same TYPE of molecule!!!

Question 33

if each substituent is trans to the same thing and cis to the same things,, what does that mean

Answer 33

if means that they’re in rhe same environment.

if they’re all trans and cis to the same TYPE of molecule!!!

Question 34

in an octahedral complex,, an equitorial group is cis to ehat

Answer 34

it’s cis to the axial!!!!
and to the other equitorial bonds

it’s trans to one thing and one thing only!!

Question 35

in an octahedral complex,, an equitorial group is cis to ehat

Answer 35

it’s cis to the axial!!!!
and to the other equitorial bonds

it’s trans to one thing and one thing only!!

Question 36

what does cis mean

Answer 36

next to ( both axial and equitorial groups)

Question 37

what does cis mean

Answer 37

next to ( both axial and equitorial groups)

Question 38

what does trans mean

Answer 38

trans can only occur to 1 thing.

aka 1 thing can only be trans to the thing it’s opposite

Question 39

what does trans mean

Answer 39

trans can only occur to 1 thing.

aka 1 thing can only be trans to the thing it’s opposite

Question 40

if groups are cis// trans to diff things what does that probs mean

Answer 40

probs means that they’re in different environments

means they will give different signals

Question 41

if groups are cis// trans to diff things what does that probs mean

Answer 41

probs means that they’re in different environments

means they will give different signals

Question 42

a larger coupling constant meansssss

Answer 42

the groups were probably trans to eachother

their interligand angle is more that 180*

Question 43

quantum number for the coupling constant : i’ve an example of how to use it

Answer 43

J ab

is the coupling constant beteen a and b

Question 44

a smaller interligand distance means

Answer 44

smaller interligand distance =
most likely cis to eachother
= smaller coupling constant
= signals are closer together

Question 45

a large interligand distance

Answer 45

they’re most likely trans to eachother
the coupling constant will be larger

Question 46

in a tree splitting signal,, the first split is usuallyyy

Answer 46

it’s usually the largest
larger amount of Hz
larger J value
usually between ligands that have the largest interligand angle