G13 Boranes Flashcards
What is the 1H and 11B NMR Spectrum of BH4
In the 1H NMR Spectrum of BH4 the B-H coupling is 64.9Hz
In the 11B NMR Spectrum of BH4 the B-H coupling is also 64.9Hz.
What are boranes?
Boron’s equivalents of alkanes
Simplest = BH3
BH3 = Trigonal planar with unhybridised 2p orbital
Describe features and characteristics of diborane.
Assuming the boron is sp3-hybridized (as for ethane – C2H6)
* BUT insufficient number of electrons to form bonds
* Physical measurements showed two-fold symmetry
3-centre-2-electron (3c2e) bond
Describe the features of the diborane 3c2e bond
An electron deficient chemical
bond where three atoms share two electrons
The combination of three atomic orbitals forms the electron deficient bond
* One bonding orbital, one non-bonding orbital and one anti-bonding orbital
* Two electrons go into the bonding orbital
The bridging B-H-B bonds are weaker and longer than the terminal B-H bonds.
Draw MO diagram of diborane terminal B-H bonds (orbitals)
What ways can bridging sp3 orbitals combine in for diborane? Draw a diagram
In or out of phase
Draw bridging hydrogens in and out of phase
Draw the antibonding, non-bonding and bonding combinations for diboranes B-H-H bridge
Draw the diborane MO energy diagram and label anti-bonding, non-bonding and bonding energies
Is 3C2E weaker than 2c2e?
Yes, so is cleavaged more
What are the two types of cleavge 3c2e bonds undergo? Draw a diagram
- Homolytic cleavage
- Heterolytic cleavage
Describe a borohydride closo structure
[BnHn]2-
Complete closed polyhedra of n vertices
Electron pairs: n+1
Describe a borohydride nido structure
[BnHn+4]
BORON occupies n corners of an (n+1) polyhedron
Electron pairs: n+2
Describe a borohydride arachno structure
[BnHn+6]
BORON occupies n corners of an (n+2) polyhedron
Electron pairs: n+3
