Lecture 17 (Boron, etc.) Flashcards
What does B have a high tendency to form?
Cluster bonds (polyhedral cages) in combination with H and, H and C
What kind of reactivity does B have?
low reactivity
How B synthesized?
reductive methods
What are exo-hydrogen atoms?
B-H bond pointing towards the center of the polyhedron
What are endo-hydrogen atoms?
Bridging hydrogen atoms
What does n represent in wade-mingos rule?
The number of skeletal atoms (vertices) in the cluster
What are closo boranes?
boranes with no bridging hydrogen atoms
How many electron pairs in a closo structure?
n+1, BnHn^2-, occupy all vertices
How many electron pairs in a nido structure?
n+2, BnH(n+4), occupy all but one of the vertices
How many electron pairs in an arachno structure?
n+3, BnH(n+6), occupy all but two of the vertices
How many electron pairs in a hypho structure?
n+4, BnH(n+8), occupy all but three of the vertices
How do you determine the # of vertices?
The number of borons
How do you determine the # of bridging hydrogens?
Example. B5H9 reference
vertices= 5
(5BH)= B5H5
H9-H5=4 bridging hydrogens
How do you determine the # of electron pairs?
[2(#verticesBH)+1(bridgingH)]/2
What is the name of the deltahedron with 6 vertices?
octahedron
What is the name of the deltahedron with 11 vertices?
octadecahedron
What is the name of the deltahedron with 10 vertices?
bicapped square antiprism
What happens with higher bonding boranes who are electron deficient?
The B in the BH unit is sp hybridized, with two empty p orbitals. On sp hybrid orbital forms the B-H bond, the other is occupied with 2 e- and participates in cluster bonding
What is the name of the deltahedron with 12 vertices?
icosahedron
Boron halides are strong lewis acids. True or false.
True
What happens to the lewis acid strength of lower group halide elements?
The strength increases
BBr3> BCl3> BF3
What lowers lewis acidity?
Pi characters, and steric replusions
What increases lewis acidity?
bond strength
What is hyperconjugation?
the interaction of the electrons in a sigma bond with an adjacent empty (or partially filled) non-bonding p orbital, anti-bonding sigma or pi orbital, or filled pi orbital
