Chemistry 324 Final Examination

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1 Chem 324 Final Examination 2008 December 11, 2008 Page 1 of 8 Chemistry 324 Final Examination Thursday, December 11, 2008 Instructor: Dave Berg Answer all questions in the booklet provided; additional booklets are available if necessary. There are 180 minutes and 150 marks so ration your time accordingly. A periodic table and an Orgel correlation diagram are included with this examination. There are 8 pages including additional resource materials. Question Total Total 150

2 Chem 324 Final Examination 2008 December 11, 2008 Page 2 of 8 1. Draw the structures of the following complexes and indicate the metal d n count. [15 pts] (a) hexa(ammine)cobalt(ii) sulphate (b) Δ-Cr(acac) 3 (c) [Ni(en)Cl] 2 (μ-cl) 2 (d) tris(acetato)iron(iii) (e) [(κ 2 -(O,O)-NO 3 )Pt(NH 3 )Cl] 2. Indicate whether each of the following statements is true or false and justify your answer with a brief explanation. [24 pts] (a) The pk a of NH 3 decreases by 10 units (orders of magnitude) compared to free NH 3 when it is complexed in [Cr(NH 3 ) 6 ] 3+. (b) There are three isomers possible for Fe(CO) 4 (PMe 3 ) but only two can be isolated. (c) Although high in energy, fluxional processes can interconvert octahedral isomers. (d) Ionization isomers can often be distinguished by wet chemical methods (that is by carrying out a simple reaction in the lab). (e) [Cr(H 2 O) 4 Cl 2 ]Cl 2 H 2 O and [Cr(H 2 O) 4 Cl 2 ]Cl are hydration isomers. (f) cis-cocl 2 (en) 2 forms left and right handed isomers (it exists as a pair of optical isomers) while trans-cocl 2 (en) 2 does not. (g) Octahedral [CrL 6 ] 3+ (L = neutral donor) forms high and low spin complexes depending on the crystal field strength of L. (h) Substitution of two waters in [V(H 2 O) 6 ] 2+ with ammonias to give trans- [V(H 2 O) 4 (NH 3 ) 2 ] 2+ has no effect on the energy or splitting of the t 2g set of orbitals.

3 Chem 324 Final Examination 2008 December 11, 2008 Page 3 of 8 3. Crystal field and molecular orbital theory offer two very different approaches to bonding in metal complexes. Answer the following questions about these two treatments and their relationship to one another. (a) Place the following ligands in order of increasing oct : Br -, PPh 3, H 2 O, bipy. [2 pts] (b) Explain how and why oct varies with oxidation state for a given metal. [2 pts] (c) Using a molecular orbital approach, show how and why CO increases the HOMO-LUMO gap relative to a pure σ-donor like NH 3. Construct an MO energy level diagram as part of your approach. [6 pts] (d) Show what oct from crystal field theory corresponds to on the MO diagram in (c). [2 pts] (e) Although the complex [V(CO) 6 ] + is unstable it can be observed spectroscopically. What predictions can you make about its structure? Explain your reasoning. [3 pts] 4. Answer the following questions related to electronic structure and spectroscopy. (a) Show that the ground state, free ion, Russell-Saunders term for any half-filled orbital set must be an S term. [2 pts] (b) What must the Russell-Saunders ground state term be for an f 3 ion? [3 pts] (c) Explain why [CoCl 4 ] 2- is a regular tetrahedron while [CuCl 4 ] 2- is a flattened tetrahedron. [3 pts] (d) The spectra of green [Ni(H 2 O) 6 ] 2+ and purple [Ni(en) 3 ] 2+ are shown on the next page. Explain why there are three bands for each compound and explain why the bands shift to higher energy for the en complex. [5 pts]

4 Chem 324 Final Examination 2008 December 11, 2008 Page 4 of 8 (e) Dissolving most iridium complexes in 12 M HCl results in an intense redbrown (almost black) soluble complex ion but the colour rapidly fades to pale yellow-green on neutralization in the presence of brine (excess aqueous NaCl). Suggest an identity for the complexes present in (i) strong HCl solution and (ii) neutral aqueous NaCl solution, based on these observations. Be specific about the types of transitions that led you to your assignment. (HINT: Ir(IV) complexes are generally unstable with respect to reduction to Ir(III)) [7 pts] 5. Answer the following questions about magnetism. (a) Define molar magnetic susceptibility, χ mol. [3 pts] (b) How is the magnetic moment, μ, obtained from a Curie Law (χ mol vs. 1/T) plot? [2 pts] (c) Calculate the spin-only magnetic moment, μ s, for [V(H 2 O) 6 ] 2+. Is this value likely to agree with the observed moment? Explain why or why not in detail. [5 pts]

5 Chem 324 Final Examination 2008 December 11, 2008 Page 5 of 8 (d) All atoms display diamagnetism to varying extents. Comment on each of the following aspects of diamagnetic behaviour : [5 pts total] (i) its atomic origin (ii) sign (iii) temperature dependence (iv) factors affecting its magnitude 6. (a) Write the expression for the overall stability constant β 3 in terms of the concentrations of the species shown in the following equilibrium. [3 pts] PMe 2 [M(H 2 O) 6 ] [M(dmpe) 3 ] H 2 O PMe 2 (dmpe) (b) Which side of this equilibrium do you think will predominate if M = Cr? Consider both the enthalpy and entropy terms carefully in your answer. [4 pts] (c) Why does the lattice enthalpy become less favourable going from VCl 2 to MnCl 2, despite the fact that Mn 2+ is smaller than V 2+? [3 pts] 7. (a) What is the fundamental difference between an I D and a D substitution mechanism? [3 pts] (b) Explain why high spin [Fe(H 2 O) 6 ] 3+ and [Ga(H 2 O) 6 ] 3+ display water exchange rate constants of 10 2 s -1 but [Cr(H 2 O) 6 ] 3+ exchanges at a much slower rate (10-6 s -1 ). [6 pts] (c) List at least three different experimental facts that can be used to distinguish an associative from a dissociative substitution mechanism. [6 pts]

6 Chem 324 Final Examination 2008 December 11, 2008 Page 6 of 8 8. (a) Explain clearly the difference between the trans influence and the trans effect using examples to illustrate each as required. [9 pts] (b) Draw the structures of the products for the following substitution reactions: [3 pts each] (i) [RhBr 3 (PPh 3 )] CN - A (ii) [RhI 3 (NH 3 )] PMe 3 B 9. IrCl(PPh 3 ) 3 reacts rapidly with CH 3 I to produce a 6-coordinate Ir(III) complex. (a) Draw the structure of the product. [3 pts] (b) Draw the mechanism for this reaction. [4 pts] (c) Would the rate increase or decrease if PMe 3 was substituted for PPh 3? Explain your answer. [3 pts] (d) Is this reaction likely to be very sensitive to solvent polarity? Explain your answer. [3 pts] 10. Answer the following questions about electron transfer. (a) Explain why large changes in M-L bond lengths between the reduced and oxidized form of the metal generally results in very slow outer sphere electron transfer in self-exchange. [5 pts] [ML 6 ] 2+ + [ML 6 ] 3+ [ML 6 ] 3+ + [ML 6 ] 2+ (b) What metal and ligand properties favour an inner sphere electron transfer mechanism? [3 pts] [END: Orgel Diagram and Periodic Table Follow]

7 Chem 324 Final Examination 2008 December 11, 2008 Page 7 of 8 General Orgel correlation diagram for Oh and Td geometries

8 Chem 324 Final Examination 2008 December 11, 2008 Page 8 of 8

I. Multiple Choice Questions (Type-I) ] 2+, logk = [Cu(NH 3 ) 4 O) 4. ] 2+, logk = 8.9

Unit 9 COORDINATION COORDINA COMPOUNDS I. Multiple Choice Questions (Type-I) 1. Which of the following complexes formed by Cu 2+ ions is most stable? (i) Cu 2+ + 4NH 3 [Cu(NH 3 ] 2+, logk = 11.6 (ii) Cu