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Boron (p102). Introduction (p102). Valence electron configuration, n s 2 n p 1 – so to complete octet, the chemistry of boron involves one of the following means (you can suggest): +3 oxidation state is expected to be most stable, yes or no ?

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introduction p102
Introduction (p102)
  • Valence electron configuration, ns2 np1 – so to complete octet, the chemistry of boron involves one of the following means (you can suggest):
  • +3 oxidation state is expected to be most stable, yes or no ?

The ionization enthalpies are so high that the lattice energies or hydration enthalpies cannot offset the energy,

Note: boron is a nonmetal element

introduction p1021
Introduction (p102)

2. –5 oxidation state, yes or no ?

Comparing the electronegativities of carbon and boron: 2.5 and 2.0, you can understand that there are no borides similar as carbides formed with the most electropositive metals.

3. Form three covalent by sp2 hybrid orbitals, which are not octet. These compounds such as BX3 act as strong Lewis acids.

introduction p1022
Introduction (p102)
  • Boron can form many stable compounds, of which there are the problem of electronic deficiency, according to the conventional 2c–2e bonds.
  • Note: the special chemical bonding in some compounds of boron.
boron resembles silicon more than aluminum in some ways
Boron resembles silicon more than aluminum in some ways:
  • B2O3 and B(OH)3 are acidic rather like SiO2 and Si(OH)4 whereas the Al compounds are weakly amphoteric.
  • The borates have some features in common with the silicates.
  • The halide compounds of B and Si are readily hydrolyzed (except BF3) whereas the halide compounds of Al are only partly hydrolyzed.
  • The B and silicon hydrides are volatile molecular compounds, which inflame in air, while AlH3 is an involatile solid.
introduction
Introduction
  • To understand the chemistry of boron, you should note three aspects:
  • Form normal covalent bond, such as BF3 and BF4–
  • Resemblances to silicon
  • Distinguished bonding and structures
sources
Sources
  • Ulexite硼钠钙石: NaCa[B5O6(OH)6].5H2O
  • Borax硼砂: Na2[B4O5(OH)4].8H2O
  • Colmanite: Ca2[B3O4(OH)3].2H2O
  • Kenite: Na2[B4O5(OH)4].2H2O
  • Preparation: reduction of the oxide, halides
slide8

Structures of Elements

  • Boron would like to be covalent, but it ‘wants’ to make “5” bonds to achieve a Lewis octet.
  • It only has 4 orbitals though (1x2s+3x2p)
  • B gets around its problem by forming delocalised polyhedra.
  • Very important in B chemistry.
slide11

Triborate

No BO4, hydrate rapidly; stable in solid

slide13

Tetraborate

Both BO3 and BO4 are present, N(BO4) = Z

boric acid and borate ions in solution p106
Boric Acid and Borate ions in solution (p106)
  • B(OH)3:
  • moderately soluble in water
  • Act as a weak Lewis acid to OH– giving B(OH)4– and H+
  • B(OH)4– occurs in minerals. If formed by fusion, borates have more complex structure.
  • Form very stable complexes with 1,2-diols
halides p107
Halides (p107)
  • the boron halides are highly reactive
  • They are volatile, covalent compounds

2B + 3X2 2BX3 X = F, Cl, Br, I

  • BF3 and BCl3 (gases); BBr3 (liquid); BI3 (low
  • melting point solid)
  • BX3 acts as Lewis acids
slide17

Boron halides are highly reactive, volatile compounds. The gas BF3 acts as a Lewis acid with ammonia.

Vacant P orbital

general properties
General Properties
  • The compounds which are coordinatively unsaturated (e.g.BCl3) are very strong Lewis acides.
  • Tetrahedral adducts and anions are common, for example:

BF3:O(C2H5)2      BF4-      B(C6H5)4-

boron subhalides
Boron Subhalides
  • BX3 are the most common halides.
  • Extensive sub-halide chemistry though, e.g. B4Cl4 and B8F12.
  • B10F12 structurally characterised for the first time last year!
metal borides
10B absorbs neutrons strongly. 40 tonnes of boron carbide were dropped on Chernobyl.

Boron carbides also used in light-weight protective armour and bullet-proof clothing.

Metal borides can be prepared by direct combination of elements:

Ca + 6B CaB6

Metal-rich borides (e.g. TiB2) can have higher conductivities than parent metals.

Also much more inert and harder.

B-Rich borides contain B-clusters

Metal Borides
hydrides p113
Hydrides (p113)
  • boron compounds of hydrogen are
  • called boranes
  • Molecular, volatile compounds; BnHm
  • Simplest borane – diborane, B2H6
boron hydrides

C has 4 valence e,

H has 1, so C2H6

has enough electrons

(8+6) for 7 2c2e

bonds.

B2H6 only has

6+6=12 electrons.

This makes an

ethane-like

structure impossible

Boron 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 delocalised bonds.
slide23
BH3
  • 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.
slide24

Boranes have unusual structures and bonding

  • Notice that there are two types of bonds in
  • boranes
  • :- the regular two-center 2e- bond
  • :- a three-center 2e- bond
bonding in diborane
Bonding 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.

H

B

B

H

3c2e bonds in diborane
3c2e 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.
3c2e bonds
3c2e Bonds
  • 3c2e bonds are occasionally shown in structural diagrams like this:
  • Bond Energies:

BH 381 kJ/mol

BHB 441 kJ/mol

1.19 Å

1.32 Å

electron deficiency
Electron 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
structure bonding elements p116
Structure-bonding elements (p116)
  • Terminal (2c–2e) boron-hydrogen bond

B––H

  • (3c–2e) hydrogen bridge bond B B
  • (2c–2e) boron-boron bond B––B
  • Open (3c–2e) B-B-B bond B B
  • Closed (3c–2e) boron bond B B

B

H

B

electron counting wade s rules p129
Electron Counting: Wade’s Rules (p129)
  • Framework electrons: the number of electrons that are available in the compound for bonding within the polyhedral framework
  • Framework electrons F

F = 3b + 4c + h + x -2n

  • Suitable for triangulated, regular polyhedron
electron counting
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
electron counting1
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 7 pairs (6+1).

Electron Counting
electron counting2
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).

Electron Counting
electron counting3
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 delocalised. 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).

Electron Counting
slide37

closo B7H72-

nido B6H10

cage

nest

闭式

巢式

arachno B5H11

网式

spider’s web

slide38

carborane 碳硼烷

Carboranes or carbaboranes are compounds having as the basic structural unit a number of C and B atoms arranged on the vertices of a triangulated polyhedron. Their structure are closely related to those of the isoelectronic boranes.

wade s rules
Wade’s Rules

5+2

n+2

Nido(鸟)巢状

4+3

n+3

Arachno蛛(网)状

6+1

n+1

Closo闭合型(笼型)

nest

cage

spider’s web

wade s rules example 1
Wade’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.
wade s rules example 2
Wade’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.
wade s rules example 3
Wade’s Rules: Example 3
  • 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
wade s rules example 4
Wade’s Rules: Example 4
  • [Sn9]4-
  • 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-capped

square anti-

prism

wade s rules example 5
Wade’s Rules: Example 5
  • [Pb5]2-
    • Pb has 1 lone pair
    • 2e/Pb for cluster bonding
    • 10 + 2 = 12e
    • 5 Pb, 6 pairs
    • Closo structure
  • Mingos developed Wade’s rules for use in transition metal clusters
synthesis of boranes diborane
Synthesis 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)
higher boranes
Higher Boranes
  • Made by controlled pyrolysis of B2H6
  • Highly specific and not at all predictable (cf disconnection approach in organic chemistry).

80°C/200 atm/5hr

B4H10

B2H6

160-200°C

slow hot tube

pyrolysis

H2/200-240°C/rapid hot tube pyrolysis

B10H14

B5H9

mechanism of pyrolysis
Mechanism of Pyrolysis
  • Key step is dissociation of B2H6 into highly reactive BH3:
  • For example, synthesis of B4H10:

B2H6 2BH3

B2H6 + BH3  B3H7 + H2

BH3 + B3H7  B4H10

handling boranes
Handling Boranes
  • Boranes are TOXIC and combust EXPLOSIVELY in air.
  • Lots of shielding, gloves & face protection are necessary.
  • Only use small quantities, so if there is an accident it can be contained.
  • Use vacuum techniques for handling and transferring.
typical reactions 1 lewis base cleavage
Typical Reactions 1: Lewis Base Cleavage
  • Boranes are electron deficient.
  • Lewis bases add electrons
  • Small boranes may cleave:

NMe3

reactions of b 2 h 6 with bases
Reactions of B2H6with Bases

[BH2(NH3)2]+[BH4]-

NH3

NMe3

B2H6

H3BNMe3

CO

H-

H3BCO

BH4-

reactions of higher boranes with bases

-

Reactions of Higher Boranes with Bases
  • Reactions may lead to simple abstraction of BHn
  • This can be used to interconvert boranes and their anions.
  • Note: Cleavage way;

Charge

NH3

[BH2(NH3)2]+

reactions of b 2 h 6 with acides
Reactions of B2H6with Acides

O2

B2H6

B2O3 + H2O

H2O

B(OH)3 + H2

ROH

Cl2

HCl

B(OR)3 + H2

B2H5Cl + H2

BCl3 + HCl

typical reactions 2 deprotonation
Typical Reactions 2: Deprotonation
  • Very large boranes tend to deprotonate with bases.
  • This is because larger clusters can delocalise the -ve charge better than smaller clusters
  • (cf acidity of CH3OH versus PhOH)

B10H14 + NMe3

[NMe3H]+[B10H13]-

B4H10 < B5H9 < B10H14

typical reactions 3 cluster building reactions

5K[B9H14] + 2B5H9

5K[B11H14] + 9H2

2BH4- + 5B2H6

B12H122- + 13H2

Typical Reactions 3: Cluster Building Reactions
  • Boranes may react with with anionic borohydrides to build larger clusters.
  • This is a useful route to large boron cage compounds.
borazine
Borazine

Inorganic Benzene 无机苯

slide59

Unlike benzene, borazine undergoes addition reactions:

Notice where the H+ and the Cl- end up: This illustrates how unrealistic the formal charges on the boron and nitrogen atoms really are!

Like benzene, borazine can form -complexes with transition metals:

slide60

Boron nitride BN

Isoelectronic with C-C

CO(NH2)2 + B(OH)3

BCl3 + NH3

Higher T & P can convert

BN to diamond structure,

harder than diamond.

BN layer structure

Comparison with graphite

focus on glassmaking
Focus On Glassmaking
  • Soda-lime glass.
    • The oldest form of glass.
    • Na2CO3 + CaCO3 + SiO2
    • Fused at 1300C.
  • Small amounts of impurities impart beautiful colors.
    • Fe2O3 brown
    • CoO blue
  • Adding B2O3 gives strength.
    • Borosilicate glass – Pyrex®
uses of boranes
One of the principal uses is in testing current theories of structure and bonding.

Hydroboration reactions are extremely useful in organic chemistry, e.g. in anti-Markovnikov addition reactions.

Uses of Boranes
boranes a fuels
Boranes a Fuels
  • In principle boranes are more efficient fuels than hydrocarbons (30 J/gram more energy).
  • This lead to interest in their use a rocket fuels.
  • But:
    • Too reactive with O2 to be handled on a large scale
    • Combustion gives solid polymeric ‘BO’, which clogs exhausts.
boron neutron capture therapy
Boron-Neutron Capture Therapy
  • Treatment for brain tumours (difficult to remove by excision).
  • Impregnate tumour with 10B-enriched compounds (clusters, e.g. B12H11SH2- salts, are good because they contain a high %age B).
  • Treatment with thermal neutrons releases a lot of energy
  • 10B + n 4He + 7Li + 
  • Patient trauma is high.
slide65

Usage of boron and its compounds

  • Additive to steel to enhance the high temperature strength. And used as neutron absorbent (no gamma ray production);
  • Boron fiber used in epoxy matrix, stiffer and stronger than steel and 25% lighter than Al;
  • Borax, Na2B4O7.10H2O, an ingredient in detergents, fire proofing;
  • Borane, in high-energy fuels such as jet plane and rockets.