Applications of Spectroscopy to Organometallic Chemistry

Question 1

Considerations to make in NMR application relevant to organomettalics:

Answer 1

• inductive effect of the metal
• disruption of the p-system
  Ligands: carbonyl ligands
  cyclopentadienyl ligands
  hydride ligands
  alkylidene and alkylidyne ligands
  allyl ligands

Question 2

What are potential inductive effects of the metal in NMR?

Answer 2

• related to the electron releasing or withdrawing properties of the metal
• remember many of the metals are less electronegative than either C or H and therefore
  an alkyl group bound to a metal will tend to draw electron density from the metal and
  the C and H nuclei will be more shielded (lower d) than in a typical organic compound
• however if the metal is in a high oxidation state (or if you are observing a cationic complex)
  the trend may well be in the opposite direction (i.e. to higher d)
• thus the chemical shift range in the 13C spectrum in which you can find a methyl group,
  for example, is very large:

For neutral complexes in moderate to low oxidation states, the dMe generally falls in the range of d = 0 ± 20 ppm for 13C and d = 0 ± 2 ppm for 1H

Question 3

How do organomettalic complexes disrupt the π-system?

Answer 3

Background:
* The 1H chemical shifts of p-molecules (e.g. ethene, benzene) dominated by two factors: - sp2 and sp hybridized C atoms are more electronegative than an sp3 C atom and
therefore move signals of attached protons to higher d
- the electrons in the p-bond generate a diamagnetic field which influences the
magnetic environment of the attached protons
- both factors contribute positively to the high d value for ethene and benzene
TM Relevance:
- when a p-ligand coordinates to the transition metal the p-bond interacts with the orbitals of the metal and is partly ‘destroyed’
- the sp2 hybridization of the C atoms changes towards sp3
- both effects lead to a decrease in the chemical shift of the 1H signal for protons bound to “sp2” carbons which interact directly with the metal
- effect is mirrored in the 13C NMR spectrum although the origins of the coordination shift are different
- in general the extent of the coordination shift is very dependent on the coordination environment of the organic ligand and values can vary considerably:

Question 4

How do carbonyl ligands act in organometallic NMR?

Answer 4

• give very weak signals in the 13C spectrum because
  (a) they have no attached protons
  (b) they have very long natural relaxation time
  both often difficult to detect
• 13C chemical shift for a terminal CO are in the region d 150 - 220 ppm
• 13C chemical shift for a bridging CO are in the region d 230 – 280 ppm

Question 5

How do Alkylidene and Alkylidyne ligands act in organometallic NMR?

Answer 5

• ligands form multiple bonds to the transition metal
• highly deshielded with 13C chemical shifts in the region d 200 - 400 ppm

Question 6

How do Cyclopentadiene ligands act in organometallic NMR?

Answer 6

• if η5-bound they appear in both the 1H and 13C NMR spectrum as singlets
• usually found between:
  δ 3.5 and 5.5 ppm (proton spectrum)
  δ 50 and 80 ppm (carbon spectrum)

Question 7

How do Allyl ligands act in organometallic NMR?

Answer 7

• even the simplest allyl ligand C3H5 has a complicated spectrum because there are two different carbon nuclei and three different protons
• the signals for each nucleus may be in very different parts of the spectrum
• in particular the chemical shift of the central carbon and proton is usually much larger
  than for the other nuclei
  typically:
  carbon signals in the range 30 - 100 ppm proton signals in the range 2 - 5 ppm

Question 8

How do Hydride ligands act in organometallic NMR?

Answer 8

• most hydride ligands resonate to low frequency of TMS (i.e. at negative d values)
• typically in the range 0 to –30 ppm
• reasonably unique and provides a safe test for the presence of hydride ligands
• exceptions are found for hydride complexes of the early transition metals like zirconium
  for which positive values are often found

Question 9

What types of intramolecular rearrangement occurs and how would it be studied in NMR?

Answer 9

• Me groups of ethane rotate with respect to each other about the C–C bond
• a related but more complex case is the process that exchanges axial and equatorial
  hydrogens in cyclohexane
• intramolecular rearrangement processes may occur at a rate (k) which is comparable with the frequency separation (s–1) of signals in the NMR spectrum
• if this is the case, the process may be studied by variable temperature NMR spectroscopy
  i.e. measure NMR spectrum at several different temperatures and compare results

Question 10

What is Berry Pseudo roationa nd show PF5 example?

Answer 10

• 19F NMR spectrum measured at room temperature shows only one type of F nucleus, whereas other means of structure determination establish the true tbp structure which has two types of fluorine atom: axial and equatorial
• PF5 and other tbp molecules often undergo a rapid intramolecular rearrangement which exchanges axial and equatorial positions, known as the Berry pseudo-rotation :
• the intermediate has a square pyramidal structure which is close in energy to the tbp, and the energy barriers to interconversion are low
• thus the rate of the dynamic equilibrium is high

Question 11

What influence does the dynamic process (k) and the seperated frequencies involved (Δω) have on the reaction?

Answer 11

if k &laquo_space;Δω the NMR will reflect the static structure of the molecule
if k&raquo_space; Δω an averaged spectrum will be observed
* where k ~ Δω is known as the coalescence region and broad peaks are often observed
* clearly as the temperature of the NMR measurement is lowered, k will become smaller
and you are more likely to observe the spectrum corresponding to the static structure of the molecule - the low-temperature limiting spectrum
* more detailed study of the spectra as they change with temperature can often reveal activation parameters for the process
* can give important insights into energy barriers within the molecule
* the use of NMR spectroscopy for the study of reaction rates in not confined to
intramolecular processes: it may also be applied to intermolecular equilibria

Question 12

Draw the IR and NMR spectra of this example

Example : dicobalt octacarbonyl, Co2(CO)8

Answer 12

In the solid-state, Co2(CO)8 adopts a structure with both bridging and terminal CO ligands However, the solution spectrum is very complicated in the terminal n(CO) region around 2000 cm–1 and has been interpreted in terms of a mixture of isomers
By contrast, the 13C NMR spectrum consists of a single peak.