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 pageas it will be used for scoring. This exam is closed-book and no calculators or electronic devicesmay be employed. There are five sections so please read over the exam before you begin and expectto take on average ten minutes per section. Read all questions carefully before writing down anyanswers. Enjoy the material!

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1 CBC Method and Metal Carbonyls

The questions for this section are worth 4 points each.

1. Draw the MO energy level diagram for η7-cycloheptatrienyl and populate it with electrons insuch a way as to correspond to an L2X3 classification.

2. Overwhelmingly, the majority of iron compounds are of the type ML4X2. Give a real exampleof a molecule of this type, and also give an example of a known iron organometallic compoundthat is not of the ML4X2 variety.

3. Calculate the Valence (VN), the electron count (EN), the dn count, and the ligand bondnumber (LBN) for the sandwich complex CpMo(η7-C7H7). Explain how LBN is superior to“coordination number” in such a case.

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4. How many νCO bands are expected in the infrared spectrum of fac MoL3(CO)3? What arethe symmetry species (Mulliken symbols) of these νCO bands?

5. How many νCO bands are expected in the infrared spectrum of mer MoL3(CO)3? What arethe symmetry species (Mulliken symbols) of these νCO bands? For this problem take the COligands to lie in the yz plane.

6. For mer MoL3(CO)3, sketch out a molecular orbital showing the simultaneous π-backbondinginteraction between molybdenum’s dyz orbital and the three CO LUMOs. Make sure to showthe atomic orbital contributions from all atoms involved (Mo, 3 C, 3 O) with proper shadingto indicate orbital phase properties. Omit the three unspecified ligands L from this sketch.Again, take the CO ligands to lie in the yz plane.

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2 The Isolobal Analogy

The questions for this section are worth 4 points each.

1. Is (η4-TMM)Fe(CO)3 isolobal with ferrocene? Explain. Note, TMM = C(CH2)3.

2. To what simple organic fragment is a C2v-symmetric Fe(CO)4 fragment isolobal?

3. A C2v symmetric d10 ML2 fragment such as Pt(PPh3)2 is perfectly prepared to bind a ligandsuch as C2H4 or O2 in an η2 fashion. Will the C2H4 or O2 ligand prefer to bind (a) in thePtP2 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.

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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) C2v, or (b) Td?

3. Explain in terms of partial charge/bond polarization what is the difference between a Fischercarbene on the one hand, and a Schrock alkylidene, on the other.

4. Briefly explain what was the significance of the compound W(CtBu)(CHtBu)(CH2tBu)(dmpe).

Note, dmpe is 1,2-bis-dimethylphosphino ethane, a common bidentate phosphine ligand.

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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 dn count andalso, what happens to the metal’s ligand bond number?

3. In olefin hydrogenation by Wilkinson’s catalyst, ClRh(PPh3)3, what is the species that bothresults from H2 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(CH2CH3)3.

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5. Sketch out the final step in the catalytic cycle for rhodium-catalyzed hydroformylation ofpropene, wherein a linear aldehyde product is released. Assume that HRh(CO)(PR3)2 is theformula of the catalyst.

6. When an incoming ligand L reacts with MeMn(13CO)(CO)4, where 13CO is cis to Me, amixture of products results in which one product is the specific signature that migratoryinsertion has taken place (i.e., that methyl moves, rather than CO). Draw this product, clearlyshowing the relative disposition of the ligands and the isotopic label.

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5 Metallocenes

The questions for this section are worth 4 points each.

1. Take the electron configuration of ferrocene to be as follows: (e1′′)4(e1′)4(e2′)4(a1′)2(e1′′)0. An

electronic transition (UV-vis spectroscopy) that promotes an electron from the (e1′)4 molecularorbital to the (e1′′)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 D5h (attached) to help you determine whether or not theelectronic transition of the previous problem is orbitally allowed.

3. On the graphic of the ferrocene photoelectron spectrum shown below, circle the bands thatcorrespond principally to ionization of metal-ring bonding electrons.

68101214

Ionization Energy (eV)

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4. How many metal-based lone pairs are there for the C2v-symmetric Cp2ReH molecule, and whatare their symmetries?

5. Draw the organometallic product obtained upon photolysis of Cp2WH2 in benzene.

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10

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Structure

Linear orlow symmetry

highly symmetric(octahedral,tetrahedral,icosahedral)

Normalsymmetry

Sure? More than oneCn axis of C3

or higher?

linear i

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Dnd

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Yes

Yes

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No

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