You are advised to spend an equal amount of time on each question. All questions carry an equal number of marks.

Transcription

1 UNIVERSITY OF EAST ANGLIA School of Chemistry Main Series UG Examination INORGANIC CHEMISTRY CHE-2C32/5301B Time allowed: 2 hours Answer THREE questions. You are advised to spend an equal amount of time on each question. All questions carry an equal number of marks. Answer EACH question in a SEPARATE answer book. The breakdown of marks within each question is indicated by the percentage figures in brackets on the right. The following are provided: Periodic Table. Do not remove this question paper from the examinations room. Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. (CHE-2C32/5301B) Module co-ordinator: Simon Lancaster (CHE) Copyright of the University of East Anglia Version 2

2 2 1. Answer BOTH parts. (a) The formulae and magnetic moments (µ) for four transition metal compounds are shown below. Na3[Fe(C2O4)3] (A) µ = 5.88 µb [Ni(NH3)6]Cl2 (B) µ = 3.30 µb Na4[Cr(CN)6] (C) µ = 3.20 µb Na2[CoCl4] (D) µ = 3.97 µb Using a simple ligand field approach, give for A, B, C, and D: (i) The oxidation states and the numbers of d-electrons; (ii) Orbital splitting diagrams showing the electronic configurations, stating the number of unpaired electrons in each case; (iii) The ligand field stabilisation energy; (iv) An explanation for the magnetic moments observed. [80%] (b) The table below illustrates trends in ionic radii, ionisation potentials and electrode potentials across the fourth row of the Periodic Table. Ca 2+ Mn 2+ Zn 2+ Ionic radius (Å) Sum of 1 st and 2 nd ionisation potentials (ev) E 0 M 2+ /M 0 (V) Give an explanation for these variations. [20%] (CHE-2C32/5301B) Version 2

3 3 2. Answer ALL parts (a) to (e). (a) Assign electron counts to complexes E to I, demonstrating how these counts were arrived at. [30%] (b) Predict and explain which of complexes G to I would exhibit the lowest ν(co) stretching frequency. [20%] (c) Name compound E and show how it can be prepared from FeCl2, using appropriate chemical reagents. Draw balanced chemical equations for two of these reactions. [20%] (d) Show how E and F can be interconverted. Give the appropriate chemical equations. [10%] (e) Describe, with a balanced chemical equation, how H can be converted into I in a single-step reaction. [20%] (CHE-2C32/5301B) TURN OVER Version 2

4 4 3. Answer ALL parts (a) to (c). (a) The shapes of borane clusters are described by Wade s rules, but these rules also apply to many other clusters of main group elements. Apply Wade s rules to first classify the following clusters and then to predict and draw their structures: (i) B5H5 2 (ii) B5H9 (iii) CB4H5 (iv) C2B9H12 (v) Pb5 2 [50%] (b) Why does increasing the number of electrons available for cluster bonding in boranes result in more open structures? [10%] (c) (i) Sketch the structures of the sulfur oxy-anions: sulfite (SO3 2 ), sulfate (SO4 2 ), thiosulfate (S2O3 2 ), dithionite (S2O4 2 ), dithionate (S2O6 2 ) and peroxodisulfate (S2O8 2 ), including any lone pairs located on sulfur. Calculate the oxidation states of sulfur and oxygen in each oxy-anion. [30%] (ii) Which three of these anions are used as reducing agents, and which is a strong oxidizing agent? Why? [10%] (CHE-2C32/5301B) Version 2

5 5 4. Answer ALL parts (a) to (c). (a) Explain, with an example of each taken from coordination chemistry, the following forms of isomerism: constitutional, co-ordination, linkage and ionisation. Describe how visible spectroscopy could be used to distinguish between the cis and trans forms of [CoCl2(NH3)4]Cl. [30%] (b) (i) In the presence of metal salts PF6 can hydrolyse stepwise to PO4 3, via fluorophosphate intermediates. A PF6 salt is refluxed with molybdate in the presence of water. An X-ray crystal structure of the product indicates tetrahedral geometry around phosphorus, and it gives rise to the following NMR spectra: 31 P: a doublet with a coupling constant of 880 Hz 19 F: a doublet with a coupling constant of 880 Hz. Sketch the structure around the phosphorus atom and support your assignment using the NMR data. [20%] (ii) Describe or sketch the shape of the 31 P NMR and 19 F NMR spectra you would expect for PF2O2 and PF6. ( 31 P, 100%, I = ½; 19 F, 100%, I = ½) [20%] (c) (i) Give chemical equations for the preparation of methyl lithium and of tetramethylsilane. Comment on the differences in the metal-carbon bonds in these compounds and draw their structures. (ii) Give a balanced chemical equation for the reaction of methyl lithium with dimethylamine. [30%] END OF PAPER (CHE-2C32/5301B) TURN OVER Version 2

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