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CHE3063 Organometallic Chemistry, Molecular Symmetry and Inorganic Electronics

Coursework Questions 2021-22

Deadline: 7th December 2021 submit to SurreyLearn assignments folder

 

Note that some of these questions are formative (F), meaning that you may complete them and ask for quick feedback from Dr Turner or Dr Riddlestone. The summative (S) questions will be used to determine your mark for this coursework.

The following abbreviations are used

Cy = cyclohexyl, Et = ethyl, Ph = phenyl, Me = methyl, bipy = 2,2'-bipyridine, Cp = cyclopentadienyl

 

Formative Questions

F1. Determine the metal valence electron count for each of the following compounds.

(i) [(5-Cp)Rh(2-C2H4)(PMe3)]

(ii) [TiCl2(5- Cp)]

(iii) [(3-C3H5)2Rh(2-Cl)2Rh(3-C3H5)2]

(iv) [Rh(Cl)(H)2(2-C2H4)(PPh3)2]

(v) [Ir(H2)2(H)2(PCy3)]+

(vi) [(5- Cp)Co(Me)(PMe3)2]+

[Formative, Dr Turner]

F2. Using symmetry arguments, show if and how vibrational spectroscopy can be used to distinguish between pure geometric isomers of [Cr(PMe3)4(CO)2]. Would you be able to recognise a mixture of isomers from vibrational spectroscopy?

[Formative, Dr Turner]

F3. The hydroformylation of 1-butene can be catalysed by [RhH(CO)(PPh3)3] and results in the

formation of major and minor products. Draw the structures of both products and construct

a catalytic cycle for the formation of the major product. Why are both linear and branched

products both formed in this reaction?

[Formative, Dr Riddlestone]

 

Summative Questions

S1. At 30°C the 1H-NMR spectrum of [Fe(CO)2(Cp)2] shows two peaks, one of which is at 5.6 ppm. At -55°C the spectrum shows 3 peaks at 4, 5.6 and 6.5 ppm with relative intensity 4:5:1, respectively. Further cooling results in the broad peak at 4 ppm splitting into two equally intense multiplets. Fully explain this data and sketch the structure of the compound.

[6]

 

 

S2. Explain all the factors that could affect the carbonyl stretching frequency in the generalized compound [MwLx(CO)y]z. M is a metal atom or ion; L represents non-carbonyl ligands; y is at least 1, w and x are positive integers, z is a positive or negative integer.

[8] S3. The following table lists the vibrational bands for an isomer of N2F2.

Band position (cm-1) infra-red spectrum

Band position (cm-1) Raman spectrum

360 592

421 1010

989 1636

(i) Show how a simple calculation can determine the total number of vibrational bands. Fully explain the origin of the method that you use.

[3]

(ii) Which isomer of N2F2 is characterized by the data above? Explain a method to determine your answer without having to do any calculations.

[3]

(iii) For the isomer, chosen in part (ii), determine the irreducible representations for the normal vibrational modes. Determine which of the irreducible representations are observable in Raman and IR spectroscopies. Explain your reasoning.

[9]

(iv) One of the vibrations, listed in the table, is the symmetric stretching of the bond between two nitrogen atoms. Which one of the frequencies can be assigned to this mode, and why?

[2]

S4. A simulated IR spectrum of [Rh(CO)3] is shown below. Use this data to deduce the geometry of the Rh complex that was used in the simulation. Fully justify your answer.

 

[6]

 

 

 

S5. The IR spectra (below) are of two complexes, each with stoichiometry [M2(CO)x] (M = a first- row transition metal, x = an integer). Both compounds retain the same structures in solution and the solid state. Determine the possible identity of M and x for samples A and B. Fully explain your reasoning.

 

 

 

[4]

S6. Explain / rationalize the following statements:

(i) A researcher experimentally determines the Tolman cone angles for PPh3 and P(MeC6H4)3 to be identical. However, a different researcher determines the angles to be different.

[3] (ii) [Fe(CO)5] has bands in its IR spectrum at 2025 and 2000 cm-1 but has only one peak

in the 13C-NMR. When [Fe(CO)5] is reacted with PPh3 under UV radiation the IR peaks are replaced by a single IR band at 1885 cm-1.

[5] (iii) The reaction of PtCl2 and two equivalents of CO at high pressure and temperature

gives a mixture of isomers. These isomers have different Pt-C and C-O bond lengths. Which of the isomers has the longer or shorter of each of these bond lengths. Explain your answer.

[6] S7. The reaction between [Fe(CO)5] and Na2[Fe(CO)4] in THF evolves CO gas and gives a salt (A)

with a 2:1 ratio of Na to anion. The Raman spectrum of the product has a low frequency absorption at 160 cm-1. Looking at both Raman and IR spectra, the bands attributed to CO stretching are all are around 2100 cm-1. Suggest a structure for the anion of salt A and show how it agrees with the spectroscopic evidence.

[4]

S8. The cationic rhodium complex [Rh(NBD)(PPh3)2]+ (NBD = norbornadiene) reacts with excess

dihydrogen in the coordinating solvent EtOH to give a metal complex A and an organic

molecule B.

 

 

(i) Draw the structures of [Rh(NBD)(PPh3)2]+, complex A and the organic product B.

[5]

[Rh(NBD)(PPh3)2] +

excess H2 EtOH

A B+

 

 

(ii) It is possible for complex A to exist as different isomers. How could the specific

isomer formed in the reaction be identified and characterised?

[3]

(iii) A THF solution of complex A is an efficient catalyst for the hydrogenation of the

terminal alkene 1-hexene. However, if PPh3 is added to the reaction mixture or if the

reaction solvent is changed to acetonitrile (MeCN) no catalytic hydrogenation is

observed. How can these observations be rationalised?

[5]

 

S9. State whether you would expect each of the four following transition metal alkyl complexes

to be stable? Explain your reasoning.

[CpFe(PPh3)(CO)Et] [Pd(PCy3)2(Et)Br] [Ir(PMe3)3(Et)Br2] [Fe(bipy)2(Et)2]

[4]

S10. The complexes [Ir(PPh3)2(CO)Cl] and [Ru(dppe)2H]+ both react with dihydrogen to form

complexes A and B respectively, shown below.

 

 

(i) Draw the structures of A and B and indicate how the oxidation states of Ir and Ru

change upon reaction with dihydrogen.

[4]

(ii) Explain how the products A and B differ with regards to their progression along the

oxidative addition pathway and rationalise this difference in reactivity between the

two complexes.

[8]

S11. Homogeneous catalysis can be understood at the molecular level. Using relevant examples

explain how this understanding can be used to improve and optimise a catalyst. (max 1 page

font size 12 single spaced, references must be included but do not count in the page limit).

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