5.03 In-Class Exam 3

Transcription

1 5.03 In-Class Exam 3 Christopher C. Cummins April 9, 2010 Instructions Clearly write your name at the top of this front page, but otherwise do not write on this front page as it will be used for scoring. This exam is closed-book and no calculators or electronic devices may be employed. There are five sections so please read over the exam before you begin and expect to take on average ten minutes per section. Read all questions carefully before writing down any answers. Enjoy the material! 1

2 1 CBC Method and Metal Carbonyls The questions for this section are worth 4 points each. 1. Draw the MO energy level diagram for η 7 -cycloheptatrienyl and populate it with electrons in such a way as to correspond to an L 2 X 3 classification. 2. Overwhelmingly, the majority of iron compounds are of the type ML 4 X 2. Give a real example of a molecule of this type, and also give an example of a known iron organometallic compound that is not of the ML 4 X 2 variety. 3. Calculate the Valence (VN), the electron count (EN), the d n count, and the ligand bond number (LBN) for the sandwich complex CpMo(η 7 -C 7 H 7 ). Explain how LBN is superior to coordination number in such a case. 2

3 4. How many ν CO bands are expected in the infrared spectrum of fac MoL 3 (CO) 3? What are the symmetry species (Mulliken symbols) of these ν CO bands? 5. How many ν CO bands are expected in the infrared spectrum of mer MoL 3 (CO) 3? What are the symmetry species (Mulliken symbols) of these ν CO bands? For this problem take the CO ligands to lie in the yz plane. 6. For mer MoL 3 (CO) 3, sketch out a molecular orbital showing the simultaneous π-backbonding interaction between molybdenum s d yz orbital and the three CO LUMOs. Make sure to show the atomic orbital contributions from all atoms involved (Mo, 3 C, 3 O) with proper shading to indicate orbital phase properties. Omit the three unspecified ligands L from this sketch. Again, take the CO ligands to lie in the yz plane. 3

4 2 The Isolobal Analogy The questions for this section are worth 4 points each. 1. Is (η 4 -TMM)Fe(CO) 3 isolobal with ferrocene? Explain. te, TMM = C(CH 2 ) To what simple organic fragment is a C 2v -symmetric Fe(CO) 4 fragment isolobal? 3. A C 2v symmetric d 10 ML 2 fragment such as Pt(PPh 3 ) 2 is perfectly prepared to bind a ligand such as C 2 H 4 or O 2 in an η 2 fashion. Will the C 2 H 4 or O 2 ligand prefer to bind (a) in the PtP 2 plane, (b) perpendicular to that plane, or (c) will there be no preference. 4. Draw the symbol that signifies an isolobal relationship between two molecules or fragments. 4

5 3 Classic Organometallics The questions for this section are worth 4 points each. 1. Draw either Vaska s complex or Zeise s salt and indicate which one it is that you have drawn. 2. Draw the structure of the first d-metal σ-alkyl complex. To which point group does it belong, (a) C 2v, or (b) T d? 3. Explain in terms of partial charge/bond polarization what is the difference between a Fischer carbene on the one hand, and a Schrock alkylidene, on the other. 4. Briefly explain what was the significance of the compound W(C t Bu)(CH t Bu)(CH 2 t Bu)(dmpe). te, dmpe is 1,2-bis-dimethylphosphino ethane, a common bidentate phosphine ligand. 5

6 4 Hydrogenation and Hydroformylation The questions for this section are worth 4 points each. 1. What species serves as the reducing agent in the synthesis of Wilkinson s catalyst? 2. In the reaction type known as reductive elimination, what happens to the metal d n count and also, what happens to the metal s ligand bond number? 3. In olefin hydrogenation by Wilkinson s catalyst, ClRh(PPh 3 ) 3, what is the species that both results from H 2 oxidative addition and resides in the catalytic cycle where turnover is rapid. Draw this species. 4. Give an example of a phosphine having a larger cone angle than P(CH 2 CH 3 ) 3. 6

7 5. Sketch out the final step in the catalytic cycle for rhodium-catalyzed hydroformylation of propene, wherein a linear aldehyde product is released. Assume that HRh(CO)(PR 3 ) 2 is the formula of the catalyst. 6. When an incoming ligand L reacts with MeMn( 13 CO)(CO) 4, where 13 CO is cis to Me, a mixture of products results in which one product is the specific signature that migratory insertion has taken place (i.e., that methyl moves, rather than CO). Draw this product, clearly showing the relative disposition of the ligands and the isotopic label. 7

8 5 Metallocenes The questions for this section are worth 4 points each. 1. Take the electron configuration of ferrocene to be as follows: (e 1 ) 4 (e 1 ) 4 (e 2 ) 4 (a 1 ) 2 (e 1 ) 0. An electronic transition (UV-vis spectroscopy) that promotes an electron from the (e 1 ) 4 molecular orbital to the (e 1 ) 0 orbital can be regarded as a ligand-to-metal charge transfer (LMCT) band. Explain this interpretation while indicating which of the metal d orbital(s) is/are involved. 2. Use the direct product table for D 5h (attached) to help you determine whether or not the electronic transition of the previous problem is orbitally allowed. 3. On the graphic of the ferrocene photoelectron spectrum shown below, circle the bands that correspond principally to ionization of metal-ring bonding electrons Ionization Energy (ev) 8 6 8

9 4. How many metal-based lone pairs are there for the C 2v -symmetric Cp 2 ReH molecule, and what are their symmetries? 5. Draw the organometallic product obtained upon photolysis of Cp 2 WH 2 in benzene. 9

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13 Structure Linear or low symmetry linear i i D h 8 C v 8 Cs Ci C1 single element linear Special Group Elements highly symmetric (octahedral, tetrahedral, icosahedral) Sure? More than one Cn axis of C3 or higher? 6C 5 3C 4 3S 4 i i i Ih I Oh O T d Th T icosahedral octahedral Tetrahedral High Symmetry Elements rmal symmetry C C 2 n v S 2n D nh D nd D n C nh C nv S 2n C n Dihedral S i n g l e A x i s rmal Symmetry Elements (most common) 13

14 A1' * A2'' X A1' * E1'' A1' * E2'' E1' * A1' - - X E1' * A2' - - X E1' * E1' X X - X - - E1' * E2' - - X X - - E1' * A1'' E1' * A2'' E1' * E1'' X X E1' * E2'' E2' * A1' X - - E2' * A2' X - - E2' * E1' - - X X - - E2' * E2' X X X E2' * A1'' E2' * A2'' E2' * E1'' E2' * E2'' X X A2'' * A1' X A2'' * A2' X - A2'' * E1' A2'' * E2' A2'' * A1'' - X A2'' * A2'' X A2'' * E1'' - - X A2'' * E2'' X - - E1'' * A1' E1'' * A2' E1'' * E1' X X E1'' * E2' E1'' * A1'' - - X E1'' * A2'' - - X E1'' * E1'' X X - X - - E1'' * E2'' - - X X - - E2'' * A1' E2'' * A2' E2'' * E1' E2'' * E2' X X E2'' * A1'' X - - E2'' * A2'' X - - E2'' * E1'' - - X X - - E2'' * E2'' X X X