Main Group Chemistry MT Ch. 8 Ref: Huheey, Keiter & Keiter: Ch 16 ...

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Chem 104A, UC, Berkeley

Main Group Chemistry

MT Ch. 8

Ref:Huheey, Keiter & Keiter: Ch 16-18


Periodic Trends

Generally, atoms with same outer-orbital structure appear in the same Column.

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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


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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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


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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The Solvated Electron

)()()( 333 NHeNHANHA

Solvated electron in cavity of 3-3.4 Ǻ diameterDensity of Liquid decreases.


Charles PedersonDupont, 1960s1987, Nobel PrizeNew field: Host-guest chemistry

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0.31 nm


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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Cryptand

2,2,2-crypt = c222


Electron-pair trapping centers and channels in K+(cryptand[2.2.2])e-

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methyl

Li

MeLi is better called (CH3Li)4, as it is tetrameric.

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Group 2 Alkaline Earth MetalBe, Mg, Ca, Sr, Ba, Ra


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Completed d and f shells interveneLess Effective shieldingStronger Attraction


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Ih

Total: 50 e12 B-H bond, 24 e26 e for skeleton B-B bonds

Projection Operator Method: MOExact 13 B-B bonding MOs


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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MO picture for 3c-2e bond


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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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).


Electron Counting





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





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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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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For a regular polyhedron having n vertices,

there will ben+1 bonding molecular orbitals.


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)


Character table for Oh point group

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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


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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


a1g

eg

t1u


Energy not to scale


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For a regular polyhedron having n vertices,

there will ben+1 bonding molecular orbitals.


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, 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.


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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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)


[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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[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


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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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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


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1,2-dicarba-closo-dodecaborane ortho


1,7-dicarba-closo-dodecaborane meta

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1,12-dicarba-closo-dodecaboranepara


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[C2B9H11]2-Cp

cyclopentadienide

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MgB2, superconductor, Tc=39 K


CaB6

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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.


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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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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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


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.


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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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



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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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


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


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.

D∞h symmetry

Unit Cell Molecules aligned perp.

Places negatively charged

F atoms close to positively

charged krypton atoms.



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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Main Group Chemistry MT Ch. 8 Ref: Huheey, Keiter & Keiter: Ch 16 ...

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