InorganicChemistryTutorials:ThirdYear DrAFRKilpatrick(2017)

ModernMainGroupChemistry

SuggestedBooklist*A,G.Massey,MainGroupChemistry,Ellis-Horwood,Chichester,1990*J.D.Woollins,Non-metalRings,Cages,ChainsandClusters,Wiley,Chichester,1988

C.E.Housecroft,ClusterMoleculesofthep-blockElements,OUP,Primerno.14,1994

*J.E.Huheey,E.A.KeiterandR.L.Keiter,InorganicChemistry,Harper-Collins,4thEd.1993(Ch16,17)B.E.Douglas,D.H.McDanielandJ.J.Alexander,InorganicChemistry,Wiley,3

rdEd,1994(Ch16,17)

D.F.Shriver,P.W.Atkinsetal.,InorganicChemistry,OUP,Oxford,4thEd,2005(Chs10-17)*C.ElschenbroichandA.Salzer,Organometallics,aConciseIntroduction,VCH,Weinheim,2

ndEd,1992(Chs3,6-9,11)

Ref.D.M.P.Mingos,EssentialTrendsinInorganicChemistry,OUP,1998Ref.N.N.GreenwoodandA.Earnshaw,ChemistryoftheElements,Butterworth-Heinmann,London,2

ndEd,1997

Ref.F.A.CottonandG.Wilkinson,AdvancedInorganicChemistry,Wiley,5thEd,1988

ResearchArticles*DaltonTrans.,(2001),2055-2067and(1991),565-573(perspectives)Angew.Chem.Int.Ed.(2002),41(3),454-456(AuXe42+)Chem.Commun.,(2001),21,2208-2209(Au−)andAngew.Chem.Int.Ed.(2003),42,4818-4821(Cs2Pt)*Chem.Soc.Rev.,(1998),27,225-232(p-blockmetallocenes)

Angew.Chem.Int.Ed.(1992),31,1364-1366(GaR)4andAngew.Chem.Int.Ed.(1991),30,564-565(Cp*Al)4*J.Am.Chem.Soc.,(2005),127,6954-6955(MII

radicals)

*Chem.Commun.(2003),2091-2101(Si,Ge,Sn,Pbanaloguesofacetylene)*Phys.Rev.Lett. (2006),96,157403(PbOlonepair?)and*Chem.Soc.Rev., (2011),40,4455-4463(revising lonepairmodel)

Chem.Commun.,(2007),927-929(GaI–Ca,Sr,Ba(η5-C5Me5)2*Science,(2016).354(6317),aaf7229.DOI:10.1126/science.aaf7229(frustratedLewispairs)

OrientationThistutorialbuildsuponthefoundational1

stand2

ndyearp-blockcourses.Itfocussesonthepost-transitionmetalsof

groups11-18-thebottomleftofthep-blockandsomemodernaspectsofthechemistryofgroups13,14and15.A

familiaritywiththetrendsinsimpleatomicandchemicalpropertiesofthep-blockisassumed.Whatisimportantinthe

post-transitionmetalsistoconsidertheminrelation;theirsimilaritiesanddifferences,toothermetals(thes-blockandd-block) and other elements within the p-block as a whole - rather than a collection of unrelated facts. Key areasnecessarily include thermodynamics, structure and bonding and how these are reflected in their reactivity. The

enclosedreactionschemeproblemsseektointegratebasicpost-transitionmetalconceptualunderstandingandfactual

knowledge with skills handling data and experimental evidence from physical analytical methods (e.g. NMR and

vibrational spectroscopy). Secondly, more contemporary aspects of p-block chemistry focus on carbenes, various

element-elementmultiple bonding, frustrated Lewis pairs, smallmolecule activation and catalysis as detailed in the

lecturecourse.AveryhelpfuloverviewisgiveninNature,(2010),463,p171-177showinghowrecentdiscoveriesofp-blockchemistryreflecttraditionald-blockchemistry!Pleasereadit.Researcharticleshighlightedinthelecturenotes

detailtheseadvances.

Inordertogainsomeperspectiveonp-blockchemicalresearchupto2001-highlightsandthemes-youmayfindthe

two accessible research articles written by N. N. Greenwood entitled orientating, interesting and helpful - DaltonTrans.,(1991),565-573andDaltonTrans.,(2001)2055-2066.

LearningObjectivesA.Tosurveyanddescribethediversechemistryofthep-block–metalsandnon-metals.

B.Toanalysethesedifferencesintermsofelectronicstructure,size,electronegativityandbondenergies.

C.Todescribethedifferentchemistriesofelementsnearthetopandbottomofthep-block.D.Toanalyseandinterpretthesedifferencesmakinguseofthermodynamicdataandcycles.

E.Toidentifyandexemplifythekindsofreactionswhicharecommontosimplemolecularcompoundsofthep-block.F.Tointerpretelementaryspectroscopic(e.g.NMR,infra-red,Raman)andanalyticaldata(e.g.conductivity,mass

spectrometry)inthecontextofcharacteristicpost-transitionmetalchemicalreactivitytodeduceand/orpredict

productsofreactions.

G.Tocompareandcontrastmetalchemistryacrossthes-,p-,d-andf-blocksofthePeriodicTable.H.Todiscussandexemplifyrecentadvancesinp-blockchemistrythatmimicd-blockchemistry.

Modern Main Group Chemistry

Example Tutorial Questions

1. (a) By making use of a second order Jahn-Teller approach, explain the differences between the geometries adopted by Group 14 compounds of the type R2E2 for E = C–Sn.

(b) Hence or otherwise rationalize the following E–E bond lengths (in Å):

ArGeGeAr 2.285 [ArGeGeAr]•– 2.309 [ArGeGeAr]2– 2.394 [ArGaGaAr]2– 2.347 [ArGeGeAr].CNtBu 2.343 [ArGeGeAr].2(CNtBu) 2.663 (RR'N)GeGe(NRR') 2.709

2. (a) What are the different electronic states possible for a carbene, CX2?

(b) Why is the electronic ground state for CH2 a triplet, yet those for SiH2, GeH2, SnH2 and PbH2 are singlets?

(c) By considering the effects of changing the angle at carbon, and of incorporating S-donor and/or V-withdrawing substituents at X, rationalize the singlet ground state adopted by the first long-lived (imidazolylidene) carbene reported by Arduengo and co-workers (below). Why are very sterically demanding adamantyl groups necessary at R?

(d) Explain why carbenes incorporating only a single amino group (such as cyclic amino alkyl carbenes) are more reactive than imidazolylidenes.

3. For each of the following sets of compounds rationalize the trends observed in the measured E–E distances (in Å):

(a) Si–Si distances in:

RSiSiR 2.062 Mes2SiSiMes2 2.160 LSiSiL 2.229 LSi(Cl)Si(Cl)L 2.393

(b) B–B (and C–C) distances in:

Mes2BB(Ph)Mes 1.706 [Mes2BB(Ph)Mes].- 1.649 [Mes2BB(Ph)Mes]2- 1.636 Ph2CCPh2 ca. 1.34

(c) P–P distances in:

Mes*PPMes* 2.034 LPPL 2.205 LP(O)2P(O)2L 2.310

4. Predict the outcome of the following reactions. In each case comment on whether the reactions are reversible or not.

5. Describe the role of the β-diketiminato complex [(HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(SiMe3)2}(THF)] in the intermolecular hydrophosphination of 1-phenylethene with diphenylphosphine.

6. Discuss the role of frustrated Lewis pairs (FLPs) in the catalytic hydrogenation of imines.

Post-TutorialQuestions(forrevisionpurposes)I.

II.

Compare and contrast the chemistry of K & Cu or Ca & Zn.

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Example : Compound of Interest, Me4NAu This compound is formulated as [Me4N]+Au and therefore named tetramethylammonium auride. It is interesting primarily because it contains a naked metal anion with a non-metal cation. Gold is very unusual in this respect among the transition metals. However, the phenomenon is not completely unexpected. Gold has the highest electron affinity of all metals reflecting its position at the end of the 5d period (Au: d10s1): the effective nuclear potential experienced by the 6s orbital is particularly stabilising on account of the effects of the lanthanide contraction and the not insignificant relativistic contraction of penetrating orbitals in heavy metals. Accordingly, in liquid ammonia cesium, a strong reducing agent, and gold react to form cesium auride (Cs+Au ) which is non-metallic (band gap ~ 2.6 eV) but exhibits ionic conduction in the molten state. The tetramethylammonium salt is isolated through ion-exchange in liquid ammonia and forms colourless, pyrophoric crystals which crystallise with a slightly distorted CsCl (8:8) structure (as CsAu). Indeed, the auride ion has been likened to the bromide ion. Their comparable size and charge are reflected in the isomorphous crystal structures of [Me4N]+Au and [Me4N]+Br which also reveal identical H….Br and H….Au distances suggesting some ‘hydrogen bonding’ interactions between hydrogen and gold!

Post-Tutorial Questions 1. Discuss points of interest in the following: [Ga2Cl6]2 , ( 5-C5Me5)Ga, [Ga(C(SiMe3)3]4, [( 5-C5Me5)Al]4, Pb3O4 PbO, PbO2, Sn{CH(SiMe3)2}2, Sn{CH(SiMe3)2}3, [(tBu2MeSi)3Ga ][K+(222crypt)] Hg3(AsF6)2, CsAu, Cs2Pt, SnR2 (R = Ph; CH(SiMe3)2), B(C6F5)3, 2. Comment on the fact that the solid-state crystal structures of many simple binary post-

transition metal compounds are different to stoichiometric analogues of the s-block and d-block. Include in your answer, the following: [Ref: A. F. Wells, Structural Inorganic Chemistry, OUP 3rd Ed 1975]

GaS vs NiS, CaS AgO, PbO vs CaO, CdO GaCl2, SnCl2 vs NiCl2, PdCl2, CrCl2, MoCl2 Pb3O4 vs Co3O4, Fe3O4 3. Compare and contrast post-transition and transition metal chemistry. Hints: Thermodynamics & bonding coordination complexes

mixed-valence compounds metal-metal bonding simple solid-state structures

4. Compare and contrast the chemistry of K & Cu or Ca & Zn.

III.

3

B. Discuss the ‘N 2’ oxidation state exhibited by post-transition metals making reference to the data given below. (use a separate sheet for your answer to this question.)

(a) Ionization energies / kJ mol 1 I1 I2 + I3 I1 + I2 I3 + I4 B 801 6090 C 3439 11130 Al 577 4555 Si 2363 7583 Ga 578 4919 Ge 2297 7711 In 558 4500 Sn 2119 6870 Tl 589 4820 Pb 2165 7162 (b) The enthalpies of dissociation, MCln(g) MCln 2(g) + Cl2(g) / kJ mol 1 B 301 Al 335 Ga 343 Ge 381 In 305 Sn 276 Tl 209 Pb 121 (c) Average bond energies / kJ mol 1 MF2 MF4 MI2 MI4 Ge 481 452 264 218 Sn 481 414 262 205 Pb 394 330 205 140 (d) Standard free energies of formation / kJ mol 1 M2O M2O3 MO MO2 Ga 315 998 In 280 831 Sn 286 581 Tl 147 312 Pb 218 277 (e) Standard Reduction Potentials / V M2+ / M M3+ / M+ MO2 / M2+ Zn 0.763 Ga ? Ge 0.2? Cd 0.403 In 0.44 Sn +0.125 Hg +0.854 Tl +1.25 Pb +1.46

IV.PartIB(Paper1)2016Q3

A10496W1 4

3. Answer BOTH part (a) AND part (b).

(a) Answer BOTH part (i) AND part (ii).

(i) The scheme and the observations below summarise some reactions of tin and its compounds. Identify the compounds A – F and suggest structures for each of the tin-containing compounds. Account for all the observations noted. [12]

Product A forms readily in Et2O solution. In the solid state, A dimerises to form a diamagnetic, red crystalline solid, B containing a centrosymmetric molecule with a Sn–Sn bond length of 2.76 Å.

Reaction of SnCl2 with one equivalent of LiAr in Et2O forms crystalline product C. Reaction of C with 1 equivalent of potassium yields D which is neutral and centrosymmetric, with a Sn–Sn bond length of 2.67 Å. Further reaction of D with potassium in the presence of 18-crown-6 forms paramagnetic, crystalline solid E with a Sn–Sn bond length of 2.81 Å.

D reacts with 2 equivalents of t-BuNC to give F which exhibits infrared stretching bands in the range 2160 to 2175 cm–1 (free t-BuNC has a stretching band at a lower wavenumber of 2134 cm–1). The Sn–Sn bond length in F is 2.91 Å. Question continues.

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(ii) A typical Sn–Sn single bond length is 2.81 Å. With reference to the Sn–Sn bond lengths provided for B, D, E, and F, discuss the nature of the Sn–Sn bonding in these species. Make reference to the second order Jahn-Teller effect in your discussion of bonding in B. Compare B and D to their analogues formed from the lightest element of the same group. [8]

(b) The reaction between PF3 and BF3 produces a 1:1 adduct. However there is no reaction between P(t-Bu)3 and B(C6F5)3. Neither P(t-Bu)3 nor B(C6F5)3 react separately with molecular hydrogen, but when dry H2 is bubbled though a mixture of these two molecules dissolved in toluene, a white solid product precipitates. This solid dissolves in the polar solvent C6D5Br enabling the solution 1H NMR spectrum shown in the diagram below to be obtained. (Note that B(C6F5)3 was enriched to 100 % 11B and an additional doublet in the 1H NMR spectrum centred at 1.01 ppm with an integrated intensity 13.5 times that of the total integrated intensity of the resonances depicted is not shown).

Rationalise all these observations as far as possible and comment on their significance. [5]

NMR nuclei: 1H: I = ½,100 % abundance; 31P: I = ½, 100 %; 11B: I = 3 2⁄ , 100 % in this experiment. Coupling to other magnetic nuclei may be neglected.

End of question, turn over.