REDEEMER’S UNIVERSITY
Off Gbongan-Osogbo road, Akoda-Ede, Osun State.
COLLEGE OF NATURAL SCIENCES
DEPARTMENT OF CHEMICAL SCIENCES
B. Sc. INDUSTRIAL CHEMISTRY DEGREE
CHE 425: ORGANOMETALLIC CHEMISTRY
FIRST SEMESTER EXAMINATION
2014/2015 SESSION
UNITS: 2
Time Allowed: 2 Hours
Instructions: Answer any THREE (3) questions in all. Each question carries 25
marks
Question 1
(a)
Several concepts of coordination chemistry are fundamental to organometallic chemistry.
Define the following terms: (i) Electron rich atom, (ii) Electron deficient atom, (iii) β-hydride
elimination, (iv) Chelate effect, (v) Early transition metals
[8 marks]
(b)
(i) Which of the following ligands will coordinate the most strongly to a generic metal center
that is not too electron-rich or deficient and with enough open coordination sites?
(ii) In which of the following complexes should the η6-benzene ligand coordinate the
strongest? (iii) What are the two key reasons?
[4 marks]
(c)
(i) Arrange the following phosphene ligands in order of increasing donor strength.
(ii) Which of the phosphine ligands above possess the least steric demands?
(iii) What are the factors on which position and number of spectral peaks of coordinated
carbonyl depends on?
(iv) Explain the trend in acidity of the compounds tabulated below.
[7 marks]
CpCrH(CO)3
CpMoH(CO)3
CpWH(CO)3
pKa = 5.4
pKa = 6.2
pKa = 8.0
(d)
With a reason for your choice in each case, which of the following pairs of metal hydrides is
the most acidic (lowest pKa )?
[6 marks]
(i)
HRh(CO)(PEt3)2
-orHCo(CO)(PPh3)2
(ii)
HMn(CO)5
-orHRe(CO)5
(iii) Cp2V(H)(NCMe)
-or[V(C6H6)(CO)2(H)]2+
Question 2
(a)
(i) Which of the following classes of organometallic ligands can benefit from metal-to-ligand
backbonding? Hydrides, Phosphines, Carbonyls, Carbenes, Alkyls, Aryls, Arenes,
(ii) What is ‘hapticity’ and mention the significance of even or odd values for x in ηx.
(iii) What is the significance of the ‘18 electron’ rule and in which region of the d-block is the
rule most kept?
[7 marks]
(b)
Justify the following facts:
(i) Early transition metal groups like Sc and Ti groups do not have carbonyl complexes.
(ii) The Zn and Cu groups do not have any appreciable carbonyl compounds.
(iii) There is rich carbonyl chemistry from Cr group to Co group.
[3 marks]
(c)
(i) Which of the complexes below will have the largest νCO vibrational frequencies?
(ii) Which of the metal centers possess the least electron density?
(iii) Provide a brief explanation for your choices in both (i) and (ii).
[4 marks]
(d)
(i) List the relevant orbitals on the carbonyl ligand that are used for bonding and (ii) describe
the function of each of these orbitals in the bonding?
Consider the following table and answer questions (iii) below:
Complex
W(CO)3(PF3)3
W(CO)3(PCl3)3
W(CO)3[P(OMe)3]3
W(CO)3(PPh3)3
νCO; cm-1
2090, 2055
2040, 1991
1977, 1888
1934, 1835
(iii) What is accountable for the reduction in vibrational frequencies of carbonyl ligands down
the table?
(iv) Provide a schematic sketch of a semi-bridging bonding of carbonyl ligand to two identical
metals M.
(v) Why doesn’t a 16e- Metal-phenyl undergo β-hydride elimination?
(vi) Pick the complex possessing the best alkyl donor property and the one possessing the
largest steric hindrance potential.
(vii)
Would a 16 e- M-(t-butyl) complex be stable or not? Why?
[11 marks]
Question 3
(a)
(i) What characteristics of phosphine ligands make them very important in organometallic
chemistry?
(ii) Compare and contrast the bonding mode of carbonyls with phosphines in terms of orbitals
concerned and how they are used.
(iii) Under what condition will the phosphine coordination be exactly like CO?
(iv) Draw the structures of the following phosphine ligands and order them from smallest to
largest in terms of Tolman’s cone angle.
P(OMe)3
PPh3
P(i-Pr)3
PCl3
PPhCy2
(iv) Order the phosphines above from best to worst π-acceptor.
[6 marks]
(b)
Which of the following should have the higher carbonyl IR frequency?
(i)
CpFeCl(CO)2
or
CpFeBr(CO)(PMe3)
(ii)
MnCl4(CO)2
or
CrCl4(CO)2
(iii) (η3-allyl)Co(PPh3)(CO)2
or
(η3-allyl)Co(PMe3)(CO)2
(iv)
Mo(CO)4{P(OMe)3}2
or
W(CO)4(PMe3)2
(v)
CpRe(CO)2{P(OMe)3}
or
Hf(O)Cl2(CO)(PEt3)2
[5 marks]
(c)
Sketch possible structures for the following organometallic complexes and write the
appropriate oxidation states, d-configurations and coordination numbers of the central metal
atoms under the respective sketches: (i) CpRuCl(=CHCO2Et)(PPh3), (ii) Co2(μ-CO)2(CO)6,
(iii) trans-HRh(CO)(PPh3)2, (iv) Ir2(μ-Cl)2(CO)4, (v) TiCl2CN2
[9 marks]
(d)
(i) What are the key differences between the mode of bonding for CO and alkyl organometallic
compounds?
(ii) Metal carbenes are said to be stable when having weak/longer metal to carbene carbon
bond and vice versa. Explain this contradiction.
[5 marks]
Question 4
(a)
Do the 18-electron assessment of the following and predict stabilities.
(i)
(b)
[4 marks]
(ii) TiCl2(CO)2(PMe3)2
(i) Copy and complete the following table with the appropriate information about metal and
carbene ligand characters that agree with weak or strong metal-carbene bond character.
Weak M=C
Strong M=C
Metal Character
Carbene ligand character
(ii) Cp2ReCH2CH3 is very stable under inert atmosphere, but Cp2ScCH2CH3 readily
“decomposes.” Why?
(iii) List the conditions that must be satisfied for β-hydride elimination to occur.
(iv) Complete the reaction mechanisms for the process of β-hydride elimination starting from
the figure below.
(v). Suggest the chemical structure of three alkyl ligands that should not be capable of
undergoing β-hydride elimination reaction.
[12 marks]
(c)
(i) Using suitable examples, state the two methods of preparing metal alkyls.
(ii) Clearly explain the structures presented below in which metal-alkene bond adopts three
different modes.
[5 marks]
(d)
Choose the complex that has the stronger M=C bond. Is there a large or small difference in
bond strengths? Explain.
[4 marks]
(i) [Cp(CO)2(PPh3)Mo=CH2]+
-or[Cp(CO)2(PPh3)W=CH2]+
(ii) [Cp(CO)2(PCl3)W=CH2]+
-or[Cp(CO)2(PEt3)W=CH2]+
Question 5
(a) Given the drug molecule below:
(i) Attempt to group the structure into 4 precursors.
[2 marks]
(ii) Use organometallic concept to synthesize the drug.
[6 marks]
(iii) What is the relevance of 18 electron rule in the course of the synthesis?
[2 marks]
+
(iv) State the importance of the following in the reaction: (α) Et3N; (β) Ph3C BF4 (γ) Me3NO.
[3 marks]
(b) Given the following organometallic metal cluster: Fe3(CO)12, Ni4(NR3)7 and Rh6(CO)16
(i) How would each of the clusters transform / catalyze a reactant to product. Give examples.
[8 marks]
(ii) In each case above, state the catalyst support and the optimum condition for a smooth
transformation of reactant to product.
[4 marks]