Covalent Bonding Between Elements

Question 1

What are cluster compounds?

Answer 1

Cluster compounds are species which have three dimensional shapes and have direct element-element bonds.

Clusters typically have a “core” which may be surrounded by a shell of other substituents. If there are no other substituents then the cluster is said to be “naked”.

Question 2

What do the lines between elements represent in cluster compounds?

Answer 2

The lines represent connectivity, not 2 centre 2 electron bonds.

Question 3

Describe the structures of cluster compounds.

Answer 3

The structures of cluster compounds are based on the platonic solids. The shapes are called deltahedra because they have triangular faces. They could also form elemental clusters.

Question 4

How can borane clusters be synthesised?

Answer 4

The synthesis of borane clusters occurs by reactions occuring at high temperatures, which result in low yields with a number of different products.

Question 5

What types of reactions can borane clusters do?

Answer 5

1. Combustion
2. Hydrolysis
3. Electrophilic substitution / Freidel-Crafts
4. Base-induced degradation
5. Deprotonation reactions

Question 6

How do you calculate the total valence electron count (TVEC) and what is it?

Answer 6

TVEC: the number of valence electrons which constitute the cluster.

1. Add up the number of valence electrons on the core atoms.
2. Substituents all count as one electron.
3. Add on one electron for each negative charge or remove one electron for each positive charge.

Question 7

How do you calculate the skeletal electron count (SEC) and what is it?

Answer 7

SEC: the number of electrons which contribute towards the bonding of the cluster core. For clusters based on a deltahedron where each atom is bonded to three other atoms then:

SEC = TVEC - 2n

n = number of cluster vertices

Question 8

How do you calculate the skeletal electron pairs (SEP)?

Answer 8

SEP = SEC / 2

Question 9

What does it mean if a cluster compound is electron precise, electron deficient or electron rich?

Answer 9

Electron precise: all the bonds in the cluster are 2 centre 2 electron bonds.

Electron deficient: to few electrons for 2 centre 2 electron bonds.

Electron rich: more electrons than needed for 2 centre 2 electron bonds.

In general, electron rich clusters have open structures and electron deficient clusters are much more closed.