Inorganic Chemistry Cumulative Exam
5 December 2013
Neal P. Mankad
Part I: Bonding and Spectroscopy of M–H2 and M–CO complexes
The famous Kubas complexes are classics in the field of organometallic chemistry. These
complexes have the general formula M(CO)3(PR3)2(η2-H2) and are famous because they
provided the first evidence not only for molecular hydrogen binding to metals but also for σcomplexes in general. An example, where M = W and R = i-Pr, is shown below.
Reference: Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984,
106, 451-452
1) In the example shown above, what is the oxidation state of the metal and its
corresponding d-electron count?
2) The interactions between metal d-orbitals and the molecular orbitals of H2 can be
analyzed to understand M–H2 bonding. For the side-on η2-H2 ligand shown above, these
interactions can be classified as H2→M σ-donation and M→H2 π-backdonation. Draw
lobal representations that depict these two interactions, which work together to make H2
binding transition metals energetically favorable. (The full MO diagram need not be
derived; simply sketches of the M–H2 orbital interactions are sufficient.)
3) The energies of the CO vibrations in the Kubas complexes can be measured by IR
spectroscopy and serve as “reporters” for the electronic environment at the metal center.
In each series below, rank the derivatives in order from lowest to highest in terms of CO
vibrational energy.
a. R = i-Pr, Ph, 4-C6H4CF3
b. M = Cr, Mo, W
4) Imagine that 13CO had been used to synthesize W(13CO)3(Pi-Pr3)2(η2-H2). How would the
energies of the CO vibrations compare to those in natural-abundance W(CO)3(PiPr3)2(η2-H2)?
5) Kubas and coworkers were able to establish that the M–H2 complexes had not undergone
oxidative addition to yield metal dihydride, or M(H)2, complexes. In other words, the
H–H bonds had not been cleaved but instead were intact. In that time, this was a
landmark accomplishment. In modern times, several experiments could be used to
establish this fact. Name two of them.
Part II: Metal-metal bonding in heterobimetallic complexes
Recently there has been a renewed interest in the synthesis and characterization of
heterobimetallic complexes featuring direct metal-metal bonds. Among the leaders in this area is
the group of Connie Lu at the University of Minnesota. An example of a heterobimetallic
complex synthesized in their laboratory, in this case featuring a Ni–Cr pair, is shown below.
Reference: Clouston, L. J.; Siedschlag, R. B.; Rudd, P. A.; Planas, N.; Hu, S.; Miller, A. D.; Gagliardi, L.; Lu, C. C.
J. Am. Chem. Soc. 2013, 135, 13142-13148.
6) What is the oxidation state and corresponding d-electron count for each metal in the
above example?
7) Construct a frontier molecular orbital diagram describing the Ni–Cr bonding, and use it
to show that the Ni–Cr bond order is, indeed, equal to 1.
Using the same ligand scaffold, several other M–Cr pairs were synthesized and characterized
crystallographically by Lu and coworkers. The M–Cr bond distances vary as indicated in the
table below, consistent with M–Cr bond order increasing across the series from M = Ni to Mn.
M
Ni
Co
Fe
Mn
M–Cr distance (Å)
2.41
2.14
1.94
1.82
8) Show that the observed trend in M–Cr distances is expected based on the frontier MO
diagram you derived for the Ni–Cr case above. Calculated predicted M–Cr bond orders
for the cases where M = Co, Fe, and Mn.