Compounds of Noble Gases

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Compounds of Noble Gases
Cole Witham Hanhan Li Whitney
Duim Organometallics March 21, 2005
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• Outline
• History
• Bonding
• MO theory, VSEPR
• Compounds
• Applications
• Future Work

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• History of Noble Gas Compounds
• Before 1962
• all noble gases are inert
• weakly bonded species
• gaseous cationic species diatomic molecules
  between noble gas atoms or other atoms (H, O, N,
  Hg)
• very short lifetimes

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• History of Noble Gas Compounds
• weakly bonded species
• clathrates (ex 8Ng46H2O) noble gas atoms held
  in molecular cages by weak London exchange
  interactions

Vertex O atom site Edge O-H-O hydrogen
bond Noble gas atoms occupy the interior of
these cages
Bartlett, N. Sladky, F. O. The Chemistry of
Krypton, Xenon, and Radon. In Comprehensive
Organic Chemistry Bailar, J. C., Emeleus, H.
J., Nyholm, R., Trotman-Dickenson, A. F., Eds.
Pergamon Press Oxford, 1973 pp 213-330.
5

• History of Noble Gas Compounds
• 1962, Bartlett and Lohmann
• demonstrated the great oxidizing strength of
  PtF6 in producing O2PtF6-
• IP(Xe) IP(O2)

Graham, L. Graudejus, O. Jha, N. K. Bartlett,
N. Concerning the nature of XePtF6. Coord. Chem.
Rev. 2000, 197, 321-334.
6
Bonding in Noble Gas Compounds

• After Noble Gas compounds discovered, there was a
  concern that bonding theories were no longer
  valid
• But the theories are still applicable
• NG systems are similar to interhalogen or
  halogen-oxy species

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Bonding in NG Compounds

• All theories must (and do) account for the
  following
• Only heavier more readily ionizable NG form
  compounds
• Only the most electronegative atoms or groups are
  satisfactory ligands for NG
• The NG atom should bear a net positive charge,
  with ligands having a net negative charge

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Molecular Orbital Theory

• MO Theory does not involve outer orbitals
• Wont affect bond energy
• Too much energy is required to excite e- to these
  orbitals to fill them so bonding can occur
• Example Xe uses 5p (5s less important)
• F uses 2p
• So for XeF2 have three three-center MOs

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XeF2

• Three AOs goes to three MOs.
• Xe 5px and 2 F 2px
• Best overlap occurs when is centrosymmetric or D8
  h symmetry (choose them to be on x-axis)
• Xe contributes 2e- (1 to each), each F
  contributes 1e-
• Only have 2e- for bonding of 3 atoms, so get
  delocalization with net charge dist of
• F-0.5-Xe1-F-0.5 (semi-ionic)

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MO Diagram

• Net bond order of 1
• So have single bonds from Xe to each F
• Bonding e- from Xe, NB e- from F (restores ligand
  e- density

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An alternative look

• Each F atom gets share in Xe valence shell e-
• Thus, each Xe-F bond is of order 1
• F approaches Ne configuration
• Xe stays with an octet (slightly weaker)
• Single electron in Xe-F internuclear region is
  equally shared between the 2 atoms, you get a net
  charge distribution of F-0.5-Xe1-F-0.5

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Further Proof for 1e- bonds

• If remove fluoride ion
• XeF2 ? XeF F-
• Get
• If single e- bond idea is true, then will have
  shorter bond in this cation
• You do, by nearly 2 times the length in XeF2-
  from vibrational spectroscopic and crystal
  structure

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Xenon (NG) oxides

• Xe-O bond much stronger than Xe-F
• Xe-O linkages are electron-pair bonds
• Ex. XeO4 looks like
• Get a large positive charge on xenon with -1
  charge on each O ligand
• Greater polarity and bond strength shown from
  X-ray photoelectron spectroscopy as well as NMR
  chemical shift data

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Energy Considerations

• Why Fluorine?
• Noble gas will remain bound to ligand if it is so
  electronegative that energy required to remove
  electron from NG is compensated for by the
  electron acquisition of the ligand and the
  electrostatic attraction energy of Nd and Ld-
• The atoms of chlorine and other halogens are much
  larger than fluorine, so attraction energy for
  (LN)L- ion pair is much less.
• Thus only fluorine is stable ligand

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Energy Considerations

• In oxides, ionization energy is mostly
  compensated for by the electrostatic attraction
  term (though oxygen atom electron affinity also
  contributes)

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VSEPR

• This theory implies outer orbital involvement in
  the bonding
• Each bond between ligand and central atom
  involves an electron pair
• All non-bonding valence electrons have a steric
  effect
• MO theory proves to be just as effective as VSEPR
  for less than 6 coordinate complexes
• VSEPR correctly predicts XeF6 as non-octahedral

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VSEPR cont.
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VSEPR oxides
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Compounds of Xenon

• Xenon Fluorides
• readily preparable from the elements
• thermodynamically stable
• XeF2
• XeF4
• XeF6
• not known XeF8

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• XeF2
• first prepared 1962
• colorless as solid, liquid, or gas
• homogeneous reaction

• thermal heterogeneous reaction using solid NiF2
• production favored with low F pressures and high
  temp

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• XeF2
• large crystals at RT
• body-centered tetragonal

• strong interactions between XeF2 molecules (high
  ?Hsub)
• -0.5F-Xe1-F-0.5
• packing structure distances F from equatorial
  nonbonding electrons on Xe

Unit cell
Zemva, B. Noble Gases Inorganic Chemistry. In
Encyclopedia of Inorganic Chemistry King, R. B.,
Ed. John Wiley Sons New York, 1994 pp
2660-2680.
22

• XeF2
• soluble in BrF3, BrF5, IF5 sometimes forms
  complexes
• soluble in organic solvents such as MeCN
• strong oxidizer, but kinetics often slow
• stable in aqueous solution

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• XeF4
• first noble gas binary fluoride synthesized

• colorless as crystals, liquid, or vapor
• strong oxidative fluorinator, but has high
  kinetic inertness like XeF2

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• XeF4
• square planar in gas phase
• nearly square planar as a solid
• strong electrostatic interactions between
  molecules in solid

Molecular packing, projection down b axis
Zemva, B. Noble Gases Inorganic Chemistry. In
Encyclopedia of Inorganic Chemistry King, R. B.,
Ed. John Wiley Sons New York, 1994 pp
2660-2680.
25

• XeF6
• production favored with high F pressures and low
  temp
• solid is colorless liquid and vapor are
  yellow-green
• more volatile than XeF2 or XeF4
• controversy over crystal structure of solid
• more powerful oxidizer and fluorinator than XeF2
  or XeF4

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• Xenon Oxides
• XeO3
• colorless, hygroscopic, detonatable solid
• XeO4
• pale yellow solid
• unstable
• tetrahedral in gas phase
• great oxidizing agent
• gas phase XeO

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• Xenon Oxyfluorides
• all possible Xe(IV), Xe(VI) oxyfluorides are
  known
• XeOF2 (light-yellow solid)
• XeOF4 (colorless, liquid at RT, most thermally
  stable compound with a Xe-O bond)
• almost all possible Xe(VIII) oxyfluorides are
  known
• XeO2F4

C4v
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• Dixenon Cation
• Xe2
• green
• formed by oxidation of Xe by O2Sb2F11- salt or
  by reduction of XeF salt (with water)

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• Xenon(II) Compounds
• preparation
• mix stoichiometric quantities of XeF2 and MFn in
  an inert atmosphere or in a nonoxidizing or
  reducing solvent
• Lewis acid properties of MFn determine degree of
  ionic character of compound
• hexafluorides XeF2-MF6 (XeF2MoF6)
• pentafluorides 2XeF2MF5 (2XeF2 RhF5),
  XeF2MF5 (XeF2VF5), XeF22MF5 (XeF2 2RuF5)

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Xenon(II) Compounds
F
(a) XeFSb2F11- (XeF22SbF5)
(XeF2 donates F- to M)
(b) XeFAsF6- (XeF2AsF5)
Zemva, B. Noble Gases Inorganic Chemistry. In
Encyclopedia of Inorganic Chemistry King, R. B.,
Ed. John Wiley Sons New York, 1994 pp
2660-2680.
31

• Other Xe Compounds
• fluorosulfates
• Xe(IV)
• Xe(VI)
• Xe(VIII)
• Xe compounds with bonds to N and C
• FXeN(SO2F)2
• C6F5XeC6F5BF3-
• xenates (salts of xenic acid, hypothetically
  Xe(OH)6)
• xenon chlorides and bromides

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Krypton Compounds

• Krypton Difluoride
• First synthesized by Turner and Pimentel in 1963.
• Krypton Oxide
• KrF2 hydrolized by moist air to KrO.
• Unstable and decomposes explosively.
• Krypton (II) Compounds
• Cationic salts, KrF / Kr2F3
• Molecular adducts of KrF2
• Other Krypton Compounds

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KrF2

• Characteristics
• Thermodynamically unstable
• Colorless as solid or gas
• Decomposes at above 250 K
• Methods of synthesis
• Electric discharge, near-UV light, frequency
  discharge, thermal decomposition, or sunlight
• Low temperature synthesis (77 K)
• Most efficient method yields 1 g/h

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KrF2

• Lowest average bond
• energy of any fluoride
• compound.
• D8h symmetry
• Unit Cell
• Molecules aligned perp.
• Places negatively charged
• F atoms close to positively
• charged krypton atoms.
• Zemva, B. Noble Gases Inorganic Chemistry. In
  Encyclopedia of Inorganic Chemistry King, R. B.,
  Ed. John Wiley Sons New York, 1994 pp
  2660-2680.

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Kr(II)

• Preparation
• React KrF2 with appropriate binary fluoride,
  neat/in solution.
• Low thermal stability
• Usually characterized by Raman and IR
  Spectroscopy.
• Kr2F3 is asymmetric
• Two different KrF bonds

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Other Krypton Compounds

• Krypton bound to O/N
• Kr(OTeF5)2 RCNKrFAsF6-
• Räsänen and coworkers using low-temperature
  matrix-isolation spectroscopy
• Kr-H, Kr-C, Kr-Cl bonds

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Radon Compounds

• Experimental difficulties
• Radiation hazard
• 222Rns half life is 3.83 days
• Decomposition caused by radiation
• Clathrates
• RnF2 and RnF salts
• Radon ionization energy similar to Xe
• Fluorides, oxides, oxyfluorides, and chlorides
  are anticipated.

38
HArF

• Räsänen and co-workers, 2000.
• Neutral covalent molecule (ArH)(F-)
• Stable at low temperatures in a matrix
• Elimination of HF calculated to be a 8 kcal/mol
  barrier.
• Possibility of ArF salt complexes existing
• Anions need to have high ionization potentials
  and be poor fluoride donors.

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Lighter Noble Gases

• High 1st ionization potentials
• Xe 12.13 eV
• Ar 15.76 eV
• Ne 21.56 eV
• He 24.59 eV
• Continue to be inert

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More Compounds of Noble Gases
Cole Witham Hanhan Li Whitney
Duim Organometallics March 23, 2005
41
As promised

• Noble gases as ligands and more!
• More
• Noble gases bound to carbon compounds
• Fun applications
• Realistic applications

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L Noble Gas

• Turner 1974, 1975
• Flash photolysis at 12-20 K in Ng matrix
• M(CO)x Ng(s) ? Fe(CO)x-1(Ng)
• M Fe, Cr, Mo, W
• Ng Xe, Kr, Ar
• Simpson 1983
• FTIR at cryogenic T following UV photolysis, they
  obtain Cr(CO)5(Xe) in solution

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More L Ng

• Bergman 2000
• Time-resolved infrared spectroscopy (TRIR)
• Detect intermediates of CH activation at room
  temperature
• Xenon solution
• Less rigid than matrix
• None reactive but will stabilize metal center
• CpRh(CO)(Xe) decays slower than CpRh(CO)(Kr)
  ?HRh-Xe 11.7 kJ

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Effects of L Ng

• George 2000
• Reactivity of CpM(CO)2(Ng) at room temperature
  with CO.
• Ng XeltKrltAr
• M ReWltMnltMoCrNbTa
• M-Xe complex and corresponding M-alkane have
  similar reactivity.

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Early vs. Late

• George 2003
• Early metals Re, Mn, Ng Xe, Kr
• CpM(CO)(Ng) CpM(CO)(Ng) have similar
  reactivity towards CO.
• Late metals Rh, Ng Xe, Kr
• CpRh(CO)(Ng) is 20x less reactive than
  CpRh(CO)(Ng)
• Suggests different mechanisms
• But then again, Rh is group 9
• Re is group 7 gtgt early???
• Bergman 1995

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The Amazing AuXe42

• Seidel and Seppelt 2000, Goal AuF
• AuF3 HF/SbF5
• ? dark red solution
• -78C AuXe42 (Sb2F11-)2
• Bond 272.8 275.1 pm
• Stable up to -40C
• Raman 129 cm-1 Au-Xe
• X ligand

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Theoretical Calculations

• Hu and Huang 2001
• Support experimental findings AuXe42
• Also predict stability of various complexes
• trivalent, tetravalent, and hexavalent in Xe/Kr
• Metals such as Pt, Mo, W.

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Xenon-Carbon Compounds Recent Developments

• Xe(II) compounds with C-Xe-C
• Frohn and Theißen 2000
• -C6F5 and CN are highly electronegative groups
• Xe(C6F5)2 and Xe(CN)2 should be favored
• C(1) atom part of a polarizable p system
• strong electron withdrawing groups make the
  C-ligand electron poor

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Synthesis of Xe(C6F5)2

• Xe-F distance is longer in C6F5XeF than in XeF2
  F- a good leaving group
• The permanent dipole moment of C6F5XeF
  facilitates attack of nucleophiles at the
  electrophilic Xe center
• Xe(C6F5)2 is unstable and decomposes

50
Synthesis of Xe(CN)2

• Xe(CN)2 is unstable and decomposes
• heteronuclear NMR spectroscopy used to confirm
  products (19F, 129Xe, 13C, 15N)

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Synthesis of Xe(C6F5)2

• Naumann 2000 a more direct synthesis using XeF2
  and (CH3)3SiC6F5

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Novel Compounds

• Frohn, Schroer, Henkel 1999
• First two xenon(II) chlorine compounds reported
• Have both Xe-C and Xe-Cl bonds
• Have 3-center-4-electron bonds as in XeF2 these
  bonds are ½ as strong as the mostly covalent Xe-C
  and Xe-Cl bonds in XeC6F5 and XeCl

53
C6F5XeClXeC6F5AsF6
Frohn, H. J. Schroer, T. Henkel, G. Angew.
Chem. Int. Ed. 1999, 38, 2554.
54
Novel Compounds

• First Xe(IV)-C compounds in 2000
• No Xe(VI)-C bonds yet
• First Xe(VI)-N and Xe(VIII)-N bonds in 2000
• O3Xe-NCCH3

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Explosions

• Multiple Noble gas compounds are explosive
• Most dangerous is XeO3-made from reaction of
  XeF4, XeF6 or any XeF3 or XeF5 salts and water
• XeO3 is as sensitive as nitrogen triiodide and
  has the explosive force of TNT
• Many Xenon Oxyfluorides/perchlorates/
  trifluoroacetates also thermally unstable

56
Legitimate Explosions

• Noble-gas fluorides could be used as oxidants,
  for instance in rocket propellant systems
• When Xe oxides explode, they leave no solid
  residues.

57
Applications

• Gives us better grasp of bonding theories
• Unexpected complexes but theories still valid
• Oxidizing Power
• Bonds are weak, reduction products are stable
• Very electrophilic
• Lighter NG better oxidizing power
• Ex. Oxidation of XeF2 to XeF

58
More Applications

• (NG-F) is capable of supplying F
• XeF2 is not only a good oxidizing agent, it also
  can be fluorinating agent
• Source of F- ligands in the substitution of a F-
  ligand into aromatic hydrocarbons
• Radon will react with strong oxidizers (O2
  salts)
• Scrub radon from air (useful in uranium mines)

59
More on Fluoride sources

• 2 possible reactions
• XeFx ? XeFx-1 F- or 2XeFx ? Xe2F2x-1 F-
• Assisted by formation of one electron-pair bond
  in the cation
• Ease of fluoride donation decreases in the
  following series XeF6 gt XeF2 gtgt XeF4
• Due to a combination of steric and electronic
  factors-seriously!

60
Light!

• RnF2 glows with a yellow light in the solid
  state.
• Light-emitting component in lasers.
• Mixtures of 10 Xe, 89 Ar, and 1 F2 can be
  excited with high-energy electrons to form
  excited XeF molecules, which emit a photon with a
  wavelength of 354 nm.

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More Light!

• Discharge lamps which using Xe halides (e.g.,
  XeCl) with a cold fill pressure of 500-1500 mbar
• Radiation has wavelengths in the ranges 300-315
  nm or 315-350 nm
• Used for phototherapy or for tanning

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Applications

• XeF2 converts uracil into 5-fluorouracil, which
  was one of the first anti-tumor agents
• A solution of XeF4 in anhydrous HF causes C6H6 to
  form a dark, rubbery polymer of unknown structure.

63
Radioactive application

• Large amounts of radioactive Xe and Kr are
  liberated during the processing of used
  reactor-fuel elements and are difficult to trap
  completely
• Maybe trap them as fluorides or as one of the
  other less volatile compounds?

64
Challenges for the Future

• Extension of NG chemistry to other elements other
  than Kr, Xe and Rn is possible only with argon in
  ArF salts.
• Explore new oxidation states in radon and
  krypton.
• Investigate proton affinities of heavier noble
  gases.
• Possibility of (XeH) and (KrH) salts with value
  as protonating agents
• Continue work in fluorinating and oxidizing agents

65
References

• Bartlett, N. Sladky, F. O. The Chemistry of
  Krypton, Xenon, and Radon. In Comprehensive
  Organic Chemistry Bailar, J. C., Emeleus, H. J.,
  Nyholm, R., Trotman-Dickenson, A. F., Eds.
  Pergamon Press Oxford, 1973 pp 213-330.
• Christe, Karl O. A Renaissance in Noble Gas
  Chemistry. Angew. Chem. Int. Ed. 2001, 40,
  1419-1421.
• Frohn, H. J. Theißen, M. Angew. Chem. Int. Ed.
  2000, 39, 4591.
• Frohn, H. J. Schroer, T. Henkel, G. Angew.
  Chem. Int. Ed. 1999, 38, 2554.
• Graham, L. Graudejus, O. Jha, N. K. Bartlett,
  N. Concerning the nature of XePtF6. Coord. Chem.
  Rev. 2000, 197, 321-334.
• Grills, D.C., Sun, X.Z., Childs, G.I., George,
  M.W. J. Phys. Chem. A 2000, 104, 4300-4307.
• Hu, W.P., Huang, C.H. J. Am. Chem. Soc. 2001,
  123, 2340-2343.
• Jina, O.S., Sun, X.Z., George, M.W. Dalton Trans.
  2003, 1773.
• Maggiarosa, N. Naumann, D. Tyrra, W. Angew.
  Chem. Int. Ed. 2000, 39, 4588.
• Seidel, S., Seppelt, K. Science 2000, 290, 117.
• Yeston, J.S., McNamara, B.K., Bergman, R.G.,
  Moore, C.B. Organometallics 2000, 19, 3442-3446.
• Zemva, B. Noble Gases Inorganic Chemistry. In
  Encyclopedia of Inorganic Chemistry King, R. B.,
  Ed. John Wiley Sons New York, 1994 pp
  2660-2680.

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