Metals and Main-Group Organometallic Chemistry. You

CHEMISTRY 3810 Problem Set #5
Topic: Metals and Main-Group Organometallic Chemistry. You are responsible for Ch. 9, sections 9.1-9.5; 9.12-9.18 and Ch. 15
of Shriver-Atkins
1. Without using reference material, sketch the s-block of the periodic table, including the chemical symbols for the elements,
and indicate the trends in (a) melting point, (b) radii for the common cations, (c) the tendency of peroxides to decompose
thermally to simple oxides.
2. Which of the following pairs are most likely to form the desired compound or undergo the process mentioned? Describe the
periodic trend and the physical basis for your answer in each case. (a) Cs + or Mg 2+ , form an acetate complex; (b) Be or Sr,
dissolve in liquid ammonia in the absence of air; (c) Li + or K + , form a complex with crypt-[2.2.2].
3. State the trend in the stability of the group oxidation number on descending a group of metallic elements in the p-block.
Illustrate the trend using standard potentials in acidic solution for Groups 13 (Appendices in Shriver-Atkins).
4. The acid-base chemistry of liquid (i.e. anhydrous) ammonia often parallels that of aqueous solutions. Assuming this to be
the case, write balanced chemical equations for the reaction of solid Zn(NH 2) 2 with (a) NH 4 + in liquid ammonia, and (b) with
KNH 2 in liquid ammonia.
5. Give balanced chemical equations (or NR for no reaction) for the following cases and give a rationale for the reaction or lack
of reaction: (a) Hg 2+ (aq) added to Cd(s); (b) [AlF 6] 3– (aq) with Tl 3+ (aq).
6. (a) Summarize the trends in relative stabilities of the oxidation states of the elements of Groups 13 and 14, and indicate the
elements that display the inert pair effect. (b) With this information in mind, write balanced chemical reactions or NR (for no
reaction) for the following combinations, and explain how the answer fits the trends.
(i) Sn 2+ (aq) + PbO 2(s) (excess) → (air excluded)
(ii) Tl + (aq) +Al(s) (excess) → (air excluded)
(iii) In + (aq) → (air excluded)
(iv) Sn 2+ + 0 2(aq) → (air)
(v) Tl + (aq) + 0 2(aq) → (air)
7. Use data from the Appendices in Shriver-Atkins to determine the standard potential for each of the reactions in Question 6.
In each case, comment on the agreement or disagreement with the qualitative assessment you gave for reactions (i) through
(v).
8. The elements of Group 11 generally mimic those of Group 1, but there are
important differences due to the much greater electonegativity of the former.
What is this greatly enhanced electronegativity due to? This question
compares the Period 6 elements cesium and gold. The enthalpy of electron
attachment of Cs is given in the notes (section 4.2.3) as –45.51 kJ/mol. The
value for Au is a staggering –223 kJ/mol. The first ionization energy of cesium
is 376 kJ/mol. Cesium reacts with gold to form a compound of stoichiometry
CsAu. Recently PDC Dietzel and M Jansen reported (Chem. Commun. 2001,
2208) that this compound undergoes ion exchange with tetramethylammonium
ions to form the new compound [Me 4N][Au], which crystallizes in with a
structure identical to that of [Me 4N][Br], and highly reminiscent of that of CsCl
(see Chemistry 2810 notes.) Rationalize these results.
9. Explain why certain of the following compounds qualify as organometallic
compounds whereas others do not: (a) B(CH 3) 3, (b) B(OCH 3) 3, (c) Na 4(CH 3) 4, (d) SiCl 3(CH 3), (e) N(CH 3) 3, (f) sodium acetate,
(g) Na[B(C 6H 5) 4.
10. Preferably without consulting reference material, construct the periodic table for the s- and p-block elements and the Group
12 elements. Head each group with the formula of a representative methyl compound and indicate (a) the positions of the
salt-like methyl compounds, (b) the positions of the electron-deficient, electron-precise, and electron-rich methyl
compounds, (c) trends in ΔH f° in the p-block.
11. Write formulas for each of the following compounds. For nonionic compounds give alternative names as derivatives of
their hydrogen compounds if they are named as organometallic compounds of the element, and vice versa: (a)
trimethylbismuth, (b) tetraphenylsilane, (c) tetraphenylarsonium bromide, (d) potassium tetraphenylborate.
12. Name each of the following compounds and classify them: (a) SiH(C 2H 5) 3, (b) BCl(C 6H 5) 2, (c) Al 2Cl 2(C 6H 5) 4, (d) Li 4(C 2H 5) 4, (e)
RbCH 3.
13. Sketch the structures of: (a) methyllithium, (b) trimethylboron, (c) hexamethyldialuminum, (d) tetramethylsilane, (e)
trimethylarsane, and (f) tetraphenylarsonium.
14. Describe the tendency toward association through methyl bridges for the trimethyl compounds of boron, aluminum,
gallium, and indium. Give a plausible exp lanation for the difference between boron and aluminum.
15. For each of the following compounds, indicate those that may serve as (1) a good carbanion nucleophile reagent, (2) a mild
Lewis acid, (3) a mild Lewis base at the central atom, (4) a strong reducing agent. (A compound may have more than one of
these properties.) (a) Li 4(CH 3) 4, (b) Zn(CH 3) 2, (C) (CH 3)MgBr, (d) B(CH 3) 3, (e) A1 2(CH 3) 6, (f) Si(CH 3) 4, (g) As(CH 3) 3.

CHEMISTRY 3810 Problem Set #5
16. For an appropriate compound from problem 15, give balanced chemical equations for: (a) the reactions of one of the
carbanion reagents with AsCl 3 and SiPh 2Cl 2, (b) the reaction of a Lewis acid with NH 3, (c) the reaction of a Lewis base with
the Lewis acid [HgCH 3][BF 4.
17. Give examples of the synthesis of organometallic compounds by each of the following reaction types and describe the
factors that favor reaction in each case: (a) reaction of a metal with an organic halide, (b) transmetallation, (e) double
replacement.
18. Determine which compound in each pair is likely to be the stronger reducing agent and explain the physical basis for your
answer: (a) Na[C 10H 8] and Na[C 14H 10], (b) Na 2[C 10H 8] and Na 2[C 14H 10] (where C 10H 8 is naphthalene and C 14H 10 is
anthracene).
19. Determine the likely reaction type, write a reasonable balanced chemical equation (or NR for no reaction), and explain the
systematics of the reaction type that led you to decide whether or not a reaction is likely.
(a) Calcium with dimethylmercury.
(b) Mercury with diethylzinc.
(c) Methyllithium with triphenylchlorosilane in ether.
(d) Tetramethylsilane with zinc chloride in ether.
(e) Trimethylsilane with. ethene in a solution of chloroplatinic acid in 2-propanol.
20. Give balanced chemical equations that illustrate: (a) direct synthesis Of SiCl 2(CH 3) 2; (b) redistribution. of SiCl 2(CH 3) 2.
21. Using silicon and a chloromethane as the primary starting materials, give equations and conditions for the synthesis of a
poly(dimethylsiloxane).
22. Disregarding compounds with metal-metal bonds, describe the trends in the oxidation states for the organometallic
compounds of Groups 13 through 15.
23. For the simple organometallic compounds, briefly describe the periodic trends in (a) metal-carbon bond enthalpies, (b)
Lewis acidity, (c) Lewis basicity for Groups 1, 2, 12, 13, and 14.
24. Summarize the trend in each process listed below, giving your reasoning:
(a) The relative ease of pyrolysis of Si(CH 3) 4 and Pb(CH 3) 4 at 300ºC.
(b) The relative Lewis acidity of Li 4(CH 3) 4, B(CH 3) 3, Si(CH 3) 4, and Si(CH 3)Cl 3.
(c) The relative Lewis basicity of Si(CH 3) 4 and As(CH 3) 3.
(d) The tendency of Li 4(CH 3) 4 and Hg(CH 3) 2 to displace halide from GeCl 4.
25. Taking into account sensitivity to oxygen and moisture and the volatility of the compound, indicate the general techniques
(vacuum line, inert atmosphere, Schlenk ware, or open flasks) that are most appropriate for handling the following
organometallic compounds: (a) solution of Li 4(CH 3) 4 in ether, (b) trimethylboron, (c) triisobutylaluminum, (d) AsPh 3 (solid),
(e) (CH 3) 3SiOSi(CH 3) 3 (liquid).
26. The 'saturated' cyclic compound Si 6(CH 3) 12, which has a near-UV absorption band, can be reduced by sodium to produce a
cyclic radical anion [Si 6(CH 3) 12] – . What types of molecular orbitals are thought to be involved in these processes? Model
this process by making the hydride analog [Si 6H 12] in HyperChem, using the Model Builder followed by a PM3 geometry
optimization of the neutral molecule. Then change the charge to –1, and multiplicity to 2, perform a single point calculation,
and plot the SOMO. What kind of orbital is this? Why does reduction of Si 6(CH 3) 12 not lead to cleaving of the Si–Si bond
in this molecule (Hint: consider the Si–Si bond order in the anion.)
27. Give equations and conditions for the synthesis of R 2Si=SiR 2 from R 2SiCl 2 and indicate the type of R group that is
necessary to yield a stable product (list specific structures of some suitable groups.)
28 The bulk price per mole of trimethylaluminum is an order of magnitude greater than that of triethylaluminum. Describe the
methods of synthesis available for these two compounds and rationalize the price difference.
29 Correct any errors and explain the nature of the errors in the following statements. (a) The methyl compounds of lithium,
zinc, and germanium are associated through multi-center M-C-M bonds because they each contain fewer methyl groups
than valence orbitals on the metal. (b) Substitution reactions of organometallic compounds of silicon such as
chloroethylmethylphenylsilane generally occur by an associative mechanism with complete loss of stereochemistry. (c)
The tendency toward radical reaction by ER 4 compounds increases on going down the group from E = Si to Pb, which
correlates with decreasing E–C bond enthalpies.
30 The compound (C 6H 5) 3Pb–Pb(C 6H 5) 3 survives up to about 300°C. Write plausible reactions for this compound with (a)
sodium in liquid ammonia, (b) HBr, and (c) [Pt(PPh 3) 4] (this compound readily loses up to two triphenylphosphine ligands).
Give your reasoning in each case.
31 Based on general trends in atomic energy levels, explain the probable order of the π-π* separation in R 2Si=SiR 2 and
R 2G=GeR 2. Give specific examples of how this energy order might be reflected in the reactions of digermene in comparison
with disilaethenes.
32 Use the AM1 method in HyperChem to calculate the MO diagram for the model disilaethene H 2Si=SiH 2. Now convert this
into the trans-pyramidalized form by breaking the Si-Si bond, rotating each fragment about 40° in opposite directions, and
re-forming the bond. This structure should be saved and single-point calculations peformed; it will not likely stay in the
bent form on minimization. Compare the energies and shapes of the frontier orbitals (in this case the two highest filled and

CHEMISTRY 3810 Problem Set #5
two lowest empty MO’s). Rationalize the observation of significant trans-pyramidalization in many solid-state structures
of disilaethene compounds.
33 The synthesis of the first stable germaethene was reported by C. Couret, J. Escudie, J. Satge, and M. Lazraq, J. Am. Chem.
Soc. 109, 4411 (1987). Summarize the reactions involved in this synthesis and speculate on why each reaction is favorable.
Describe the interaction of Lewis bases with this germaethene and propose reasons for the interaction of the germaethene
with a Lewis base and lack of similar interaction for a simple alkene. (Copy appended to the end of this problem set.)
34 Given Ge(CH 3) 3Cl, H 2C=CH(C 6H 5), and other reagents of your choice, and drawing on analogies with the chemical
properties of silicon, give balanced chemical equations and conditions for the synthesis of (CH 3) 3GeCH 2CH 2(C 6H 5).
35 Provide equations for two different synthetic routes to compounds possessing a stable Si=Si double bond.
36 Discuss the criteria used in deciding whether a pp-pp double bond exists in E=E compounds (E a group 14 or 15 element).
37 When an E=E compound is bent towards transpyramidalization: (a) which MO is most stabilized? Why?; (b) which MO is
most destabilized? Why?
38 Starting with (a) an organic halide of your choice, (b) elemental arsenic (c) elemental chlorine, and (d) other standard
reagents and solvents, describe with equations a synthetic route to a symmetrical compound with a stable As=As bond.
39 Predict the product(s) of the reaction of Mes 2Si=SiMes 2 with (a) bromine; (b) phenol; (c) sulfur; (d) (PPh 3) 2Pt (Hint: see
structure #15 on p. 277).
40 Write out arrow-pushing step-by-step reaction paths which rationalize the production of the following products from a
symmetrical diphosphene: # 3; #7; #8; #13 and 14; #12 (as listed on page 84 of the lecture notes).

CHEMISTRY 3810 Problem Set #5

CHEMISTRY 3810 Problem Set #5