Literature Discussion
Spring 2007
The following refers to: J. Am. Chem. Soc., 128 (45), 14676 -14684, 2006.
1. What are the authors trying to accomplish here? In your answer, include:
a) What is a carbene? What is the difference between a Schrock and Fischer
carbene in structure, binding, and reactivity?
b) What types of catalysis are carbenes used for (it might help to look up the
references under footnote 2)? Describe in detail a typical mechanism for
carbene catalysis.
c) Why are transition metal carbenes much more common than alkali or alkaline
earth carbenes?
d) How are transition metal carbenes typically synthesized? How are alkaline
earth metal carbene complexes synthesized? How are main group metal (i.e.
Sn or Pb) carbene complexes synthesized? How was the title compound
synthesized in this paper?
e) Fundamentally, why is this new compound interesting and exciting to the
authors (and to all inorganic chemists!)?
2. Consider the related complex H2CCa:
a) The bonding in this complex can be broken down into two parts: the bonding
of the H’s to the C and then the bonding of the :CH2 to the Ca. Derive the two
SALCS of the H2 fragment, showing clearly how you determine their
symmetries. Determine the symmetries of the valence orbitals of the C under
the appropriate symmetry. Construct an MO diagram for the formation of
[:CH2] from the M and the H2-fragment, showing which MOs are occupied.
Do the best you can estimating the orbital energies. The end result should look
something like the MO diagram for H2O. Then, take these orbitals and
interact them appropriately with Ca (first, determine the symmetries of the
valence 3d, 4s, and 4p orbitals of the Ca under the appropriate symmetry then
interact them with the appropriate symmetries of the :CH2 fragment). When
you finish, you will have the MO diagram for H2CCa.
b) Using symmetry and point group tables, how many vibrations will be IR
active? Raman active?
c) Determine the expected bond order for H2CCa.
3. Trace the Ca-C, Ca-N and C-P bond lengths from 4-H2 (the ligand), (4-H)2Ca (the
monodeprotonated ligand-Ca complex), to (4-Ca)2 the carbene dimer. Are the trends
in the experimentally determined bond lengths the same as the calculated ones? What
do the changes in bond lengths tell you about the bonding both within the ligand and
the ligand to Ca? How are the Ca-C and Ca-N bond lengths related? Why? You
might want to think about the carbene as a possible π acid and what this might do to
bond lengths.
4. Why do you think complex [4-Ca]2 forms as a dimer and not a monomer? What is
the 13C NMR shift for the C in (4-Ca)2? How does this compare with transition metal
carbenes? Why? Why are the analogous Li, Cr, and Pb dimmers NMR silent? (Hint:
look at the numbers of unpaired electrons). Describe what happened by NMR when
the investigators dissolved their complex in THF. Draw the likely resulting product.
Do you think the mechanism for the ligand exchange is likely associative or
dissassociative? Why? Does the complex’s reactivity with THF tell you anything
about the binding of the ligand to the Ca?
5. Discuss the bonding of the carbene to Ca. Drawing on your discussion in question 3,
what would you expect the charge distribution on each atom to be? What did the
authors find when they did the calculations on a simplified model? Discuss the
conclusions you can draw from the results in Table 2 and figure 4 (calculations on the
original complexes) and relate them back to what you expected the charges to show.
It is not enough here to simply list what the authors have concluded. You need to put
these conclusions in your own words and show how the authors got there and why it
is important. Was there any difference between calculations on the simplified and
non-simplified model? How do the calculations of partial charges help you
understand the geometric features of the molecules?
6. Discuss the initial reactivity studies the authors have completed on [4-Ca]2 (Scheme
3). What general trends in reactivity do you see? How might you expect the
reactivity of this compound to be different than the reactivity of a transition metal
analogue? Propose one additional reaction (NOT discussed in the paper) that you
would like to see the authors perform. What do you expect as a product (and why)?
7. Take a look at the crystal structures summarized in Figure 1-2, and the corresponding
experimentals. Why were these crystal structure determinations done at low
temperature? What was the R factor of each of the structures? What does this mean?
Some of the hyrdrogens were “found” and refined, the others were estimated. Why is
“finding” hydrogens difficult by X-ray diffraction?