Created by Eugene Chong, University of Michigan ([email protected]); Elon Ison, North Carolina State University
([email protected]); Stephanie Poland, Rose-Hulman Institute of Technology ([email protected]); Matthew Riehl, Bethany
Lutheran College ([email protected]); Murielle Watzky, University of Northern Colorado ([email protected]); Carmen
Works, Sonoma State University ([email protected]); posted on VIPEr (www.ionicviper.org) on June 30, 2016. Copyright Eugene
Chong, Elon Ison, Stephanie Poland, Matthew Riehl, Murielle Watzky, Carmen Works (2016). This work is licensed under the
Creative Commons Attribution-NonCommerical-ShareAlike 4.0 License. To view a copy of this license visit
http://creativecommons.org/about/license/.
Relating 1H NMR Data to the Electronic Structure of Re(V) Oxo Complexes - KEY
These questions are based upon the work from:
Robbins, L.K; Lilly, C.P.; Smeltz, J.L.; Boyle, P.D.; Ison, E.A. Organometallics 2015, 34, 31523158.
This problem set deals particularly with the 1H NMR of the rhenium-oxo compounds as well as
the d-orbital splitting. There are other learning objects taken from this paper that are referenced
on the Viper site.
All images were provided by Elon Ison and used with permission.
The orbital splitting diagrams used in this LO are not drawn to scale.
1. For the complexes shown in the above figure, choose a ligand set a, b, c, or d.
a. Characterize each ligand as L, X or Z. Determine the MLlXxZz classification,
valence number, ligand bond number, electron number, and dn count for example
compounds 3 and 5 for your chosen ligand set.
Here, the CBC method has been used. For all of the complexes:
i. The classification is ML2X5. This is because N3 is LX2, R is X and O is LX2
(since it is -2)
ii. Valence Number: (X + 2Z) is +5
iii. Ligand Bond Number (L + X + Z) is 7
iv. Electron number is 16 (there are 7 bonds to the metal plus 2 electrons
remaining on the rhenium.)
v. dn is 2
b. Why is the oxygen shown with a triple bond? How many electrons does it
contribute to the electron number?
Oxygen is a -2 ion, but it contributes six electrons; two are in a sigma-orbital
(donating to the z2 orbital) and four electrons in p-orbitals donating in a pi-fashion
(to the xz and yz orbitals).
c. Consider a different ligand set. Should your answers for (a) change? Explain.
No, the answers will be the same as the bonding to the metal is the same for
each ligand set.
Created by Eugene Chong, University of Michigan ([email protected]); Elon Ison, North Carolina State University
([email protected]); Stephanie Poland, Rose-Hulman Institute of Technology ([email protected]); Matthew Riehl, Bethany
Lutheran College ([email protected]); Murielle Watzky, University of Northern Colorado ([email protected]); Carmen
Works, Sonoma State University ([email protected]); posted on VIPEr (www.ionicviper.org) on June 30, 2016. Copyright Eugene
Chong, Elon Ison, Stephanie Poland, Matthew Riehl, Murielle Watzky, Carmen Works (2016). This work is licensed under the
Creative Commons Attribution-NonCommerical-ShareAlike 4.0 License. To view a copy of this license visit
http://creativecommons.org/about/license/.
2. In the 1H NMR spectrum for complex 3a shown above, there is a singlet peak at ~5.3
ppm that has a line drawn through it. Why has this peak been isolated in this way?
This is the residual CH2Cl2 solvent peak from CD2Cl2 solvent.
3. Which peak(s) in the 1H NMR spectra of the reactant 3a do you expect to shift the most
upon formation of the product 5a?
The peak corresponding to protons in the CH2R group should shift the most (delta = 4.55
pm).
4. Based only on this 1H NMR spectrum, is complex 3a diamagnetic or paramagnetic?
Explain.
The peaks are sharp and the multiplicities can be easily defined. Thus, the
complex is diamagnetic.
5. 3a is described by the authors as a pseudo-square pyramidal complex.
a. Sketch a 3-dimensional Lewis Structure a generic ML5 square pyramidal
complex.
Created by Eugene Chong, University of Michigan ([email protected]); Elon Ison, North Carolina State University
([email protected]); Stephanie Poland, Rose-Hulman Institute of Technology ([email protected]); Matthew Riehl, Bethany
Lutheran College ([email protected]); Murielle Watzky, University of Northern Colorado ([email protected]); Carmen
Works, Sonoma State University ([email protected]); posted on VIPEr (www.ionicviper.org) on June 30, 2016. Copyright Eugene
Chong, Elon Ison, Stephanie Poland, Matthew Riehl, Murielle Watzky, Carmen Works (2016). This work is licensed under the
Creative Commons Attribution-NonCommerical-ShareAlike 4.0 License. To view a copy of this license visit
http://creativecommons.org/about/license/.
b. A quick googling of “square pyramidal d-orbital splitting
diagram” yields the schematic to the right. How is this splitting
different than an octahedral complex? Why have the orbital
energies changed in this way?
For a square pyramidal complex, the z2’s energy has been
lowered relative to x2-y2 as the ligand is removed from the zaxis (for example). The loss of a ligand along the z-axis also causes a loss of
degeneracy between the xz, yz, and xy orbitals. The xy is contained within the
same plane as the x2-y2 orbitals and is slightly destabilized with respect to xz and
yz.
c. What is the dn count for complex 3a? Fill these electrons into the proper orbitals
from this splitting diagram. Is the complex predicted to be diamagnetic or
paramagnetic? Explain.
As determined in #1, 3a is Re(V) d2. Filling these two
electrons into the degenerate xz and yz orbitals (one each)
mean that this compound is paramagnetic.
d. Do your answers from parts (a) and (b) match? Should they?
Why or why not?
The answers are not the same. However, they should be consistent with one
another because the complex cannot be both paramagnetic and diamagnetic at
the same time under the same conditions.
6. A separate argument could be made that the d-orbital
splitting diagram for square pyramidal complexes should
look like the image on the right. In this case, only sigmainteractions between the ligand and the metal are
considered.
a. How is this representation different than the one from
Google?
In this case, the eg* orbitals are again split, but the t2g
orbitals are degenerate.
Created by Eugene Chong, University of Michigan ([email protected]); Elon Ison, North Carolina State University
([email protected]); Stephanie Poland, Rose-Hulman Institute of Technology ([email protected]); Matthew Riehl, Bethany
Lutheran College ([email protected]); Murielle Watzky, University of Northern Colorado ([email protected]); Carmen
Works, Sonoma State University ([email protected]); posted on VIPEr (www.ionicviper.org) on June 30, 2016. Copyright Eugene
Chong, Elon Ison, Stephanie Poland, Matthew Riehl, Murielle Watzky, Carmen Works (2016). This work is licensed under the
Creative Commons Attribution-NonCommerical-ShareAlike 4.0 License. To view a copy of this license visit
http://creativecommons.org/about/license/.
b. “…only sigma-interactions … are considered.” What does that mean? Why has
that changed the splitting diagram in this way?
Pi-interactions are not considered for this diagram. The removal of the 6th ligand
happens along the z-axis (for example) and so the z2 orbital lowers in energy
relative to x2-y2. The xy, xz, and yz orbitals do not point directly at the 6th
coordination site. They can only be involved in pi-interactions. Since no piinteractions are considered by this model, these three orbitals are degenerate.
c. If this model is correct, should complex 3a be diamagnetic or paramagnetic?
Explain.
Two electrons would fill into the three degenerate orbitals (possibly leading to
Jahn-Teller distortion). Complex 3a would be paramagnetic.
d. Is this your answer to (c) consistent with the observed 1H NMR data?
No.
7. Detailed DFT calculations by Ison and coworkers show that the splitting for complex 3a
has the xz and yz orbitals being slightly destabilized with respect
to the xy orbital.
a. Using this orbital splitting diagram, is complex 3a
diamagnetic or paramagnetic? Is this consistent with the
observed 1H NMR data?
According to this diagram, the two electrons from the
Re(V) metal center would be paired in the xy orbital. The
complex is thus predicted to be diamagnetic. This is
consistent with the observed 1H NMR data.
b. Why are the xz and yz orbitals destabilized compared to
the xy orbital? Which ligand is most responsible for this
change? (Hint: if only sigma-interactions are considered in
question 6, how might adding in pi-interactions change the
diagram?)
The oxo ligand is acting as a pi-donor. It will donate its pi electrons into the dorbitals with the appropriate symmetry and orientation which in this case are the
xz and yz. This pi-donation will cause a slight destabilization in both of these
orbitals.
8. What other experiments could be performed to confirm the electronic nature of complex
3a?
For example: magnetic susceptibility experiments.