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Chem 104A, UC, Berkeley
Main Group Chemistry
MT Ch. 8
Ref:Huheey, Keiter & Keiter: Ch 16-18
Chem 104A, UC, Berkeley
Periodic Trends
Generally, atoms with same outer-orbital structure appear in the same Column.
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Chem 104A, UC, Berkeley
Group 1: Alkali Metal
Li, Na, K, Rb, Cs, Fr
symbol electron configuration
lithium Li [He]2s1
sodium Na [Ne]3s1
potassium K [Ar]4s1
rubidium Rb [Kr]5s1
cesium Cs [Xe]6s1
francium Fr [Rn]7s1
Chem 104A, UC, Berkeley
Atomic Number
Relative Atomic Mass Melting Point/K Density/kg m-3
Li 3 6.94 453.7 534
Na 11 22.99 371.0 971
K 19 39.10 336.8 862
Rb 37 85.47 312.2 1532
Cs 55 132.91 301.6 1873
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Chem 104A, UC, Berkeley
Atomic Radius/nm Ionic Radius/nm
Li 0.152 0.068
Na 0.185 0.098
K 0.227 0.133
Rb 0.247 0.148
Cs 0.265 0.167
Chem 104A, UC, Berkeley
Ionization Energies/kJ mol-1
1st 2nd 3rd
Li 513.3 7298.0 11814.8
Na 495.8 4562.4 6912.0
K 418.8 3051.4 4411.0
Rb 403.0 2632.0 3900.0
Cs 375.7 2420.0 3400.0
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Chem 104A, UC, Berkeley
The Solvated Electron
)()()( 333 NHeNHANHA
Solvated electron in cavity of 3-3.4 Ǻ diameterDensity of Liquid decreases.
Chem 104A, UC, Berkeley
Charles PedersonDupont, 1960s1987, Nobel PrizeNew field: Host-guest chemistry
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Chem 104A, UC, Berkeley
0.31 nm
Chem 104A, UC, Berkeley
Cation Ionic diameter Crown Ether Hole size
Lithium 1.46 12-crown-4 1.5
Sodium 2.28 15-crown-5 2.3
Potassium 3.04 18-crown-6 3.1
Rubidium 3.4 21-crown-7 3.4
Cesium 3.9 24-crown-8 4.0
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Chem 104A, UC, Berkeley
Cryptand
2,2,2-crypt = c222
Chem 104A, UC, Berkeley
Electron-pair trapping centers and channels in K+(cryptand[2.2.2])e-
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
methyl
Li
MeLi is better called (CH3Li)4, as it is tetrameric.
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Chem 104A, UC, Berkeley
Group 2 Alkaline Earth MetalBe, Mg, Ca, Sr, Ba, Ra
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Completed d and f shells interveneLess Effective shieldingStronger Attraction
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Ih
Total: 50 e12 B-H bond, 24 e26 e for skeleton B-B bonds
Projection Operator Method: MOExact 13 B-B bonding MOs
Chem 104A, UC, Berkeley
36 e per B12:
26 for skeleton B-B10 e for linking B12 units
6 2c-2e bonds6 3c-2e bonds
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Chem 104A, UC, Berkeley
MO picture for 3c-2e bond
Chem 104A, UC, Berkeley
BoraneAlfred Stock
B2H6
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Chem 104A, UC, BerkeleyBoron Hydrides
These form one of the most structurally diverse series of compounds.
Simplest is diborane, B2H6.
Similar formula to ethane, but structurally very different because it is electron deficient.
Gets around the problem by forming delocalized bonds.
H C
H
H
C
H
H
H
HB
HHH B
H
H
C has 4 valence e, H has 1, so C2H6
has enough electrons(8+6) for 7 2c2ebonds.
B2H6 only has6+6=12 electrons.This makes anethane-like structure impossible
Chem 104A, UC, BerkeleyBH3
B2H6 is a dimer of boron trihydride.
This is a fugitive species, present in low concentration in diborane at high T.
Important in mechanisms of reactions of B2H6 at high T.
H
BH H
HB
HHH B
H
H
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Chem 104A, UC, BerkeleyBonding in Diborane
The B-H-B unit is held together by 2e.
This is called a 3 centre - 2 electron bond (3c2e).
The orbital basis can be made up of two sp3 hybrids of the B atoms and two H(1s) orbitals.
The remaining boron orbitals form normal 2c2e bonds to the terminal H’s.
HB
HHH B
H
H
B B
H
H
Chem 104A, UC, Berkeley3c2e Bonds in Diborane
The two electrons occupy the fully bonding combination, so that the overall bond order between the B and the bridging H is 1/2.
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Chem 104A, UC, Berkeley
3c2e Bonds
3c2e bonds are occasionally shown in structural diagrams like this:
Bond Energies:
BH 381 kJ/mol
BHB 441 kJ/mol
HB
H
H
B
H
H
H
HB
HHH B
H
H
1.19 Å
1.32 Å
Chem 104A, UC, BerkeleyElectron Deficiency
All boranes are electron deficient.
The need to form 3c2e bonds (BHB and BBB) causes the molecules to ‘curl-in’ on themselves.
The more electron deficient the more ‘spherical’ a molecule becomes.
For example [B6H6]2- is more electron deficient than B4H10
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Chem 104A, UC, BerkeleyElectron Counting
Just how electron deficient a borane is can be derived by counting the number of skeletal pairs of electrons.
Each HB has 4 valence electrons. One pairs used for a 2c2e bond (e.g a terminal BH).
The remaining 2e are used for delocalized cluster bonding.
Any remaining H contribute 1e to the cluster
[B6H6]2-
write as (BH)62-
Each BH unit contributes 2e
Plus the 2- charge gives 14 electrons
6 boron atoms in the cluster bonded with 7pairs (6+1).
Chem 104A, UC, Berkeley
Electron Counting
Just how electron deficient a borane is can be derived by counting the number of skeletal pairs of electrons.
Each HB has 4 valence electrons. One pairs used for a 2c2e bond (e.g a terminal BH).
The remaining 2e are used for delocalized cluster bonding.
Any remaining H contribute 1e to the cluster
B4H10: Write as (BH)4H6
Each BH => 2e (8e in all)
Each additional H gives 1e (6e in all)
Total number of electrons = 14
4 Borons in cluster bonded by 7 pairs of electrons (4+3).
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Chem 104A, UC, BerkeleyElectron Counting
Just how electron deficient a borane is can be derived by counting the number of skeletal pairs of electrons.
Each HB has 4 valence electrons. One pairs used for a 2c2e bond (e.g a terminal BH).
The remaining 2e are used for delocalized cluster bonding.
Any remaining H contribute 1e to the cluster
B5H9: (BH)5H4
10 + 4 = 14 electrons
5 Boron atoms bonded by 7 electron pairs (5+2).
In terms of electron deficiency
B6H62- > B5H9 > B4H10
All have 7 e pairs for skeletal bonding (ie cluster bonding).
Chem 104A, UC, BerkeleyWade’s Rules
6+1n+1
Closo
5+2n+2Nido
4+3n+3
Arachno
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Chem 104A, UC, Berkeley
Closo –[BnHn]2-
n=4-12Closed n-vertex Polyhedral
2n+2 B-B electrons
Nido –[BnHn]4-
n=4-11“nest” n+1 vertex Polyhedral
Missing one vertex
2n+4 B-B electrons
Arachno –[BnHn]6-
n=4-10“web” n+2 vertex Polyhedral
Missing 2 vertices
2n+6 B-B electrons
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
For a regular polyhedron having n vertices,
there will ben+1 bonding molecular orbitals.
Chem 104A, UC, Berkeley
7 bonding MOs
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Chem 104A, UC, BerkeleyMolecular Orbitals of closo-B6H62- (Oh)
Oh E 8C3 6C2 6C4 3C2 i 6S4 8S6 3h 6d
r() 6 0 0 2 2 0 0 0 4 2
r() = A1g + Eg + T1u; orbitals of these symmetries suitable for -bonding can be formed by six s or six pz atomic orbitals (two sets of six “radial” orbitals result)
S4, C4, C2
C2S6, C3
h
d
d
xy
z
basis set for -bonding
x1
y1
basis set for -bonding;vectors x and y are in h planes
BH
B
B B
B
B
H
H
H
H
H
2-
r() 12 0 0 0 -4 0 0 0 0 0
r() = T1g + T2g + T1u + T2u ; orbitals of these symmetries suitable for B-B -bondingcan be formed by six px and six py orbitals (twelve “tangential” orbitals)
Chem 104A, UC, Berkeley
Character table for Oh point group
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Chem 104A, UC, Berkeley
Molecular Orbitals of closo-B6H62-. “Radial” group orbitals
a1g
eg
t1u
6H and 6B 2s symmetry adapted atomic orbitals
a1g
eg
t1u
6B 2pz symmetry adapted atomic orbitals
a1g
(2pz) (1s)
a1g(2s)
1a1g
2a1g
3a1g
Note that only one of the six 2pz boron group orbitals, namely a1g, is bonding
Six 2s and six 2pz boron group orbitals will mix to form two sets of radial orbitals.
One of these two six-orbital sets will be used to combine with six 1s hydrogen group orbitalsto form six bonding and 6 antibonding MO’s (B-H bonds)
a1g(2s+2pz)2a1g(2s-2pz)
1a1g(2s+2pz+1s)3a1g(2s+2pz-1s)
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Chem 104A, UC, BerkeleyMolecular Orbitals of closo-B6H62-. “Tangential” group
orbitals• Remaining twelve 2px and 2py boron orbitals form four sets of triply degenerate“tangential” group orbitals of t1g, t2g, t1u and t2u symmetry.
• Only two of these sets , t2g and t1u, are suitable for B-B -bonding in closo-B6H62-. They
form six -bonding MO’s (B-B -bonds).
t1u
Bonding and antibonding 6B 2py and 2px symmetry adapted group orbitals
t2g
t2u
t1g
...
...
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Chem 104A, UC, BerkeleyB-B and B-H bonding MO’s of closo-B6H6
2-
closo-B6H62- has 7 core bonding orbitals, 6 of them are - (t1u & t2g) and one is -MO (a1g).
In boron cages of the formula closo-(BH)x (x = 5, … 12) the optimum number of the core electron pairs is x+1 (all bonding orbitals are filled). That explains enhanced stability of dianionic species closo-(BH)x
2-.
t2g1.9 eV
-1.1 eV 2t1u
eg
2a1g
1a1g
1t1u
-4.4 eV
-5.0 eV
-7.3 eV
-15.3 eV B6-core -orbital
B6-core -orbitals
BH bond orbitals
BH bond orbitals
BH bond orbital
B6-core -orbitals
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
t2g1.9 eV
-1.1 eV 2t1u
eg
2a1g
1a1g
1t1u
-4.4 eV
-5.0 eV
-7.3 eV
-15.3 eV B6-core -orbital
B6-core -orbitals
BH bond orbitals
BH bond orbitals
BH bond orbital
B6-core -orbitals
[B6H6]2-B6 H6
t1u
Bonding and antibonding 6B 2py and 2px symmetry adapted group orbitals
t2g
t2u
t1g
...
...
a1g
eg
t1u
6B 2pz symmetry adapted atomic orbitals
a1g
eg
t1u
6H and 6B 2s symmetry adapted atomic orbitals
a1g
eg
t1u
6H and 6B 2s symmetry adapted atomic orbitals
Energy not to scale
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
For a regular polyhedron having n vertices,
there will ben+1 bonding molecular orbitals.
Chem 104A, UC, Berkeley
Closo –[BnHn]2-
N=4-12Closed n-vertex Polyhedral
2n+2 B-B electrons
Nido –[BnHn]4-
N=4-11“nest” n+1 vertex Polyhedral
Missing one vertex
2n+4 B-B electrons
Arachno –[BnHn]6-
N=4-10“web” n+2 vertex Polyhedral
Missing 2 vertices
2n+6 B-B electrons
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
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Chem 104A, UC, BerkeleyWade’s Rules: Example 1
B6H10
(BH)6H4
12 + 4 = 16e = 8 pairs
8 pairs = 6B + 2
Nido cluster
Remove one vertex from 7-vertex polyhedron.
Chem 104A, UC, BerkeleyWade’s Rules: Example 2
B5H11
(BH)5H6
10 + 6 = 16 = 8 pairs
5 B atoms, 8 pairs
n+3 arachno cluster
based on seven vertex polyhedraon via removal of two vertices.
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Chem 104A, UC, Berkeley
Zintl ionsFirst in 1891
Pb(s) -------------- 4Na+ + [Pb9]4-
Many such ions were made in 1930s
Structures established after cryptand ligands enabled crsytallization (J. Corbett)
Na
NH3 (l)
[Pb9]4- +Pb -------- 2[Pb5]2-
in [Na(C222)]2[Pb5]
222-Crypt
NH3(l)
Chem 104A, UC, BerkeleyWade’s Rules: Example 3
[Sn9]4- Zintl ions
Each Sn has a lone pair and contributes 2e to cluster bonding, 18 + 4 = 22 e
9 atoms, 11 pairs
Nido cluster, remove 1 vertex from 10 vertex polyhedron.
Bi-cappedsquare anti-prism
Isolobal B-H & Sn, Pb
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Chem 104A, UC, BerkeleyWade’s Rules: Example 4
[Pb5]2-
Pb has 1 lone pair
2e/Pb for cluster bonding
10 + 2 = 12e
5 Pb, 6 pairs
Closo structure
Chem 104A, UC, BerkeleySynthesis of Boranes: Diborane
Hf = +80 kJ/mol, so direct combination of B and H is not possible.
2NaBH4 + I2 B2H6 + 2NaI + H2
2NaBH4 + 2H3PO4 B2H6 + 2NaH2PO4 + 2H2
4BF3 + 3LiAlH4 2B2H6 + 3LiAlF4
Air and moisture must be rigorously excluded: diborane is highly pyrophoric!
Boranes burn with a characteristic green flash (decay of excited state of BO)
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Chem 104A, UC, BerkeleyHigher Boranes
Made by controlled pyrolysis of B2H6
Highly specific and not at all predictable.
B2H6B4H10
B5H9B10H14
80°C/200 atm/5hr
H2/200-240°C/rapid hot tube pyrolysis160-200°Cslow hot tubepyrolysis
Chem 104A, UC, Berkeley
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Chem 104A, UC, BerkeleyTypical Reactions 1: Lewis Base Cleavage
Boranes are electron deficient.
Lewis bases add electrons
Small boranes may cleave:
BH
HB
H HHH
B N
H
Me
Me
HH
Me
NMe3
Chem 104A, UC, BerkeleyReactions of B2H6with Bases
B2H6
NMe3H3BNMe3
CO
H3BCO
H-
BH4-
NH3
[BH2(NH3)2]+[BH4]-
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Chem 104A, UC, BerkeleyWade’s Rules: Example 5
Heteroatoms:
B10C2H12
BH contribute 2e
CH contribute 3e (BH)10(CH)2
20 + 6 = 26 e
12 atoms in cluster
13 pairs
Closo 12-vertex polyhedron
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
1,2-dicarba-closo-dodecaborane ortho
Chem 104A, UC, Berkeley
1,7-dicarba-closo-dodecaborane meta
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Chem 104A, UC, Berkeley
1,12-dicarba-closo-dodecaboranepara
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
[C2B9H11]2-Cp
cyclopentadienide
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
MgB2, superconductor, Tc=39 K
Chem 104A, UC, Berkeley
CaB6
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
Noble Gas ChemistryHe, Ne, Ar, Kr, Xe
Inert, not discovered until 1800’s by Sir William Ramsay.
Prof. Neil Bartlett (Berkeley, Chemistry), 1962, chemistry of PtF6
][
/1169
/1175
][
66
22
6262
PtFXePtFXe
molkJIE
molkJH
eOO
PtFOPtFO
Xe
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Chem 104A, UC, BerkeleyHistory of Noble Gas Compounds
1962, Bartlett and Lohmann:
• demonstrated the great oxidizing strength of PtF6 in producing O2
+PtF6-
• IP(Xe) ≈ IP(O2)
Xe + PtF6 XePtF6 + Xe(PtF6)2
RT
- dependent on reactant ratio- red-tinged yellow solid
Graham, L.; Graudejus, O.; Jha, N. K.; Bartlett, N. Concerning the nature of XePtF6. Coord. Chem. Rev. 2000, 197, 321-334.
Chem 104A, UC, Berkeley
Molecular Orbital Theory
MO Theory does not involve outer orbitals 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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Chem 104A, UC, Berkeley
XeF2
Three AOs goes to three MOs. Xe 5px and 2 F 2px
Best overlap occurs when is centrosymmetric or D∞ h
symmetry (choose them to be on x-axis)
Xe contributes 2e- (1 to each), each F contributes 1e-
Chem 104A, UC, BerkeleyMO Diagram
Net bond order of 1
Bonding
Non-Bonding
Anti-Bonding
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Chem 104A, UC, Berkeley
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
Chem 104A, UC, BerkeleyVSEPR cont.
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Chem 104A, UC, BerkeleyVSEPR oxides
Chem 104A, UC, Berkeley
XeF2
• first prepared 1962
• colorless as solid, liquid, or gas
• homogeneous reaction
Xe + F2 XeF F
electric discharge, heat, UV light, sunlight
cat. HF
• thermal heterogeneous reaction using solid NiF2
• production favored with low F pressures and high temp
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Chem 104A, UC, BerkeleyXeF2
• large crystals at RT
• body-centered tetragonal• strong interactions between XeF2
molecules (high ∆Hsub)
• -0.5F-Xe+1-F-0.5
• packing structure distances F from equatorial nonbonding electrons on Xe
Xe
F
FUnit 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.
Chem 104A, UC, Berkeley
XeF4
• first noble gas binary fluoride synthesized
Xe + F2 XeF F
1 : 5 tot pressure 0.6 MPa
673 K
closed nickel can
F
F
• colorless as crystals, liquid, or vapor
• strong oxidative fluorinator, but has high kinetic inertness like XeF2
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Chem 104A, UC, Berkeley
Molecular packing, projection down b axis
XeF4
• square planar in gas phase
• nearly square planar as a solid
• strong electrostatic interactions between molecules in solid
Zemva, B. Noble Gases: Inorganic Chemistry. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons: New York, 1994; pp 2660-2680.
Chem 104A, UC, Berkeley
Xenon Oxides• XeO3
• colorless, hygroscopic, detonatable solid
• XeO4
• pale yellow solid• unstable• tetrahedral in gas phase• great oxidizing agent
• gas phase XeO
XeF6 (g) + 3 H2O (l) 6 HF (aq) + XeO3 (aq)low temp
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Chem 104A, UC, Berkeley
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
XeF FF
F
O
C4v
Chem 104A, UC, Berkeley
The Amazing [AuXe4]2+
Seidel and Seppelt: 2000, Goal: AuF AuF3 + HF/SbF5
dark red solution -78°C : AuXe4
2+ (Sb2F11-)2
Bond = 272.8 – 275.1 pm Stable up to -40°C Raman: 129 cm-1 Au-Xe
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Chem 104A, UC, BerkeleyKrypton 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
Chem 104A, UC, Berkeley
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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Chem 104A, UC, Berkeley
KrF2
Lowest average bond
energy of any fluoride
compound.
D∞h 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.
Chem 104A, UC, Berkeley
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.