Download CHEM 462 Inorganic/Organometallic Chemistry Fall 2016 Midterm

CHEM 462
Inorganic/Organometallic Chemistry
Fall 2016
Midterm Exam: Part I (20%)
Part II (take home, 15%)
October 25, 2016
Question
Points available
I
30
II
16
III
8
IV
8
V
20
VI
33
VII
35
TOTAL
150
Points received
Name: _____________________________
I.
Consider the following two structures and answer the questions relating to them:
a) Label them according to the type of divalent carbon atom ligands: Fischer Carbene or
Schrock Alkylidene (place answers on blanks below)
b) Choose the correct label for each type of the divalent carbons:
neutral, 2-electron donor; anionic, 2-electron donor; dianionic, 4-electron donor;
neutral, 4-electron donor.
c) According to your assignment for the divalent carbon ligand, give the oxidation state
of the metal and the d-electron count in each case.
d) Which divalent carbon ligand is most likely to react with PhLi? What is the product?
e) Which divalent carbon ligand is most likely to have resulted from an α-H elimination?
f) Which complex is more likely to react with BX 3 to yield a carbide?
a)
b)
c)
d)
e)
f)
__________________________
_______________________
__________________________
_______________________
__________________________
_______________________
__________________________
_______________________
__________________________
_______________________
__________________________
g) Exchange of a CO by PMe 3 in the
Cr(PhC(OCH 3 ))(CO) 5 complex is expected to
proceed by a (dissociative or associate)
mechanism and generate the (cis or trans)
isomer? At right, sketch the expected
reaction coordinate and the structures of the
transition state and any intermediate.
_______________________
E
rxn coord
Page 1 of 6
II.
Consider Kubas’ η2-dihydrogen complex shown right.
A) Give the oxidation state and electron count about tungsten.
B) Two types of π-backbonding are in the tungsten complex, to CO
and to the η2-H 2 ligand. Give drawings to illustrate the orbital
overlap between W and CO and between W and H 2 .
C) Discuss the geometrical position of ligands in this complex. That is, which
orientation of the P(i-Pr) 3 ligand, cis or trans to the W- η2-H 2 , should give the
“better” interaction? Explain your reasoning and why the solid state structure is the
same or different from your conclusion.
D) The following list of physical or spectroscopic techniques were used to
characterize the Kubas complex. Choose 3 and describe what could be or was
learned from the results: i) Infrared spectroscopy in diatomic (CO) ligand region.
ii) 1H NMR spectroscopy iii) 31P NMR spectroscopy iv) X-ray diffraction v)
Neutron diffraction (use the back of the page for your descriptions).
III.
Give the electron count for each Mn derivative in the following equation. Show how
you derived each overall count (by giving oxidation state of Mn and d-count).
Page 2 of 6
IV. Attempts to prepare a Kubas-type dihydrogen complex with the iridium complex
shown below resulted in a 6-coordinate complex. A) Sketch the structure of the
product. B) Give the oxidation states and electron counts about Ir in both reactant
and product. C) Discuss why should this result be different from the Kubas study?
Ir
Cl
Ph3P
V.
PPh3
CO
+
H2
Select and circle the answers for the following: (Partial credit can be given for those
with rationale given for your answer.)
A) The description/parameters that best fit the forward reaction that describes CO/L
(L = PR 3 ) exchange in the Ni0L4 complex:
Ni0L4 + CO  Ni0L 3 (CO) +
L
i) Dissociative, large positive ΔH≠; large neg. ΔS≠; a 16-electron, 3-coordinate
intermediate.
ii) Associative, small ΔH≠; large neg. ΔS≠ , 18-electron, 5-coordinate intermediate.
iii)Associative, large ΔH≠; large positive ΔS≠ , 16-electron, 3-coordinate intermediate.
iv)Dissociative, large positive ΔH≠; large positive ΔS≠ , 16-electron, 3-coordinate
intermediate.
B)
The experiment that will yield the ΔG for the reaction above, i.e.,
Ni0L4 + CO  Ni0L 3 (CO) +
L
i) Measure rates of forward reaction at different temperatures, determine rate
constants, plot ln k vs. 1/T.
ii) Measure equilibrium product/reactant ratio; calculate: ΔG = −RT lnK.
C)
The experiment that will yield ΔG≠ for the reaction above.
i) Measure rates of forward reaction at different temperatures, determine rate
constants, plot ln k vs. 1/T.
ii) Measure equilibrium product/reactant ratio; calculate: ΔG = −RT lnK.
D)
The phosphine most likely to yield the largest value for K eq and ΔG for the
reaction above:
PMe 3 ,
P(OEt) 3 ,
PPh 3 . (Is this a steric or electronic
effect?)
Page 3 of 6
VI.
Short answer questions:
a) The species with highest ν(CO) stretching frequency:
CO
or
H 3 BCO
or
Ni(CO) 4
b) The species with lowest ν(CO) stretching frequency:
Ni(CO) 4 ,
Ni(CO) 3 PMe 3 , Ni(CO) 3 P(OMe) 3 ,
or
Ni(CO) 3 PF 3
c) The species with highest ν(NO) stretching frequency:
NO
or
NO−
or
NO+
or
NO+
d) The species with longest N - O distance:
NO
or
NO−
e) The molecule with highest average v(CO):
Fe(CO) 4 =; Fe(CO) 4 (η2-CH 2 =CH 2 ); Fe(CO) 4 (η2-CF 2 =CF 2 );
or Fe(CO) 4 Cl 2
f) The orientation of ethylene in Fe(CO) 4 (η2-CH 2 =CH 2 ):
i) face on and parallel to the x/y plane;
ii) face on and perpendicular to the x/y plane;
iii) side on and parallel to the z axis; iv) side on and perpendicular to the z-axis.
(if you don’t understand “face on” or “side on”, ask.)
g) The MCO anion with fastest rate of reaction with MeI :
(η5-C 5 H 5 )Fe(CO) 2 –
Fe(CO) 4 =
(η5-C 5 H 5 )Cr(CO) 3 –
V(CO) 6 –
h) The metal carbonyl most likely to have a multiple metal-metal bond.
[(η5-C 5 H 5 )Mo(CO) 3 ] 2
[(η5-C 5 H 5 )Mo(CO) 2 ] 2
Co 2 (CO) 8
[(η5-
C 5 H 5 )Fe(CO) 2 ] 2
i) The molecule with greatest number of v(CO)IR absorption bands.
Mo(CO) 6 ; trans-(Ph 3 P) 2 Mo(CO) 4 ; cis-(Me 3 P) 2 Mo(CO) 4 ; transMo((diphos) 2 (CO) 2 .
j) The molecule with only one v(CO)IR absorption band:
Page 4 of 6
Mo(CO) 6 ; trans-(Ph 3 P) 2 Mo(CO) 4 ; cis-(Me 3 P) 2 Mo(CO) 4 ; transMo((diphos) 2 (CO) 2 .
k) The ligand least likely to be a pi-acceptor:
SR-
CO
(η-5C 5 H 5 )-
PF 3 -
VII. The following diagrams are energy levels in the frontier molecular orbitals (FMO’s)
of organometallic complexes. They correspond mainly to d-orbitals (or are largely of
d-orbital character).
e 1μ
e* g
a 1g
t 2g
t2
e
e 2g
i) _____________
ii) _____________
iii) _____________
iv) ____________
A. Underneath each diagram above (i, ii, iii, iv), place geometry labels: Oh, Td, Sq.
Planar, metallocene
B. Assign the following molecules to the appropriate energy level diagrams, i - iv,
give oxidation state of the M and the number of d electrons. Assign metal d
electrons to the FMOs using the splitting diagrams above as guides:
RhCl(PPh 3 ) 3 _________________________
Cr(CO) 6
_________________________
Ni(CO) 4
_________________________
(η5-C 5 H 5 ) 2 Fe _________________________
(η5-C 5 H 5 ) 2 V _________________________
Page 5 of 6
PtCl 4 =
_________________________
C. Consider the conversion of metallocene to a bent metallocene. Which d-orbital
labels goes on the highest FMO in the diagram at left (the e 1g *) and why is it seen
to split and lower (stabilized) in the diagram at right, i.e., as the bend occurs?
(Note: this question merely tests your understanding of Crystal Field splitting.)
D. Short answer questions about metallocenes:
a) The paramagnetic metallocene(s).
[(η5-C 5 H 5 ) 2 Fe]
2 Ni]
[(η5-C 5 H 5 ) 2 Fe]+
[(η5-C 5 H 5 ) 2 Co
[(η5-C 5 H 5 ) 2 Co]+ [(η5-C 5 H 5 )
b) The metallocene with largest average M-C bond distance.
[(η5-C 5 H 5 ) 2 Fe]
[(η5-C 5 H 5 ) 2 Co]
(η5-C 5 H 5 ) 2 Ni
(η5-C 5 H 5 ) 2 V
c) The oxidation state of the nickels Ni in the triple decker [(η5-C 5 H 5 ) 3 Ni 2 ]+
0
+1
+1.5
+2
(none of these)
Page 6 of 6
d) Circle the molecule that is not a bent metallocene:
Cp 2 Ti(CO) 2
Cp 2 TiCl 2
[Cp 2 Ti] 2
Cp 2 FeH+
Cp 2 Fe(CO) 2
Page 7 of 6