Covalent bonding between the elements

Question 1

What are cluster compounds?

Answer 1

Species which have 3D shapes and have direct element-element bonds.

Question 2

What is the difference between clusters with a core and naked clusters?

Answer 2

Core clusters: cluster have a core which is surrounded by a shell of other substituents, if there are no other substituents then the cluster is said to be naked.

Question 3

How is the synthesis of borane clusters traditionally characterised?

Answer 3

Reaction occuring at high temperature, resulting in low yields with a number of different products.

Question 4

Name the 5 different reactions of borane clusters

Answer 4

Combustion, hydrolysis, electrophilic substitution, base-induced degradation, deprotonation reactions.

Question 5

What are the 3 methods of electron counting?

Answer 5

Total valence electron counting (TVEC), skeletal electron count (SEC), skeletal electron pairs (SEP).

Question 6

What is the total valence electron count and how can it be calculated?

Answer 6

The number of valence electrons which constitute the cluster.

1. Add up the number of valence electrons on the core atoms.
2. Substituents such as H, Bu^t etc all count as one electron.
3. Charge, add on one electron for each unit of negative, remove one electron for each unit of positive charge.

Question 7

What is the skeletal electron count and how can it be calculated?

Answer 7

The number of electrons which contribute towards the bonding of the cluster core. For clusters based on a deltahedron (such as an octahedron, icsohedron etc) where each atom is bonded to 3 others then:
SEC= TVEC - 2n (n= the number of cluster vertices).

Question 8

How can the skeletal electron pairs be counted?

Answer 8

SEP = SEC/2

Question 9

What is the difference between electron precise/deficient/rich?

Answer 9

1. Electron precise: all the bonds in the cluster are 2-electron 2-centre bond. TVEC= 5n, SEC= 3n.
2. Electron deficient: too few electrons for 2-electron 2-centre bonds. TVEC< 5n, SEC< 3n.
3. Electron rich: have more electrons than are needed for only 2-electron 2-centre bonds. TVEC> 5n, SEC> 3n.

Question 10

In general, what structures to electron rich/deficient clusters have?

Answer 10

Electron rich= open, electron deficient= c=much more closed.

Question 11

List at least 3 closed structures

Answer 11

Tetrahedral, trigonal bipyramid, octahedral, icosohedral etc.

Question 12

What is Wade’s rule>

Answer 12

For an n vertex cage with n+1 SEP, then a closo (closed) structure will be adopted.

Question 13

How many electrons/orbitals are involved in skeletal bonding in a B-H fragment?

Answer 13

2 electrons/ 3 orbitals.

Question 14

In general, what molecular bonding orbitals does an n-vertex B-H cluster have?

Answer 14

n-bonding orbitals by combination of the p-orbitals from the B-H fragments + 1 (only ever one) bonding combination of sigma bonding orbitals (as changing the phase of one orbitals results in an anti bonding orbital combination).

Question 15

How do you get from a closo structure to a nido and an arachno structure?

Answer 15

By removing one vertex from an n+1 CLOSO cluster, we generate the core structure of the n+2 NIDO structure with the same SEP. Removing a further vertex gives an n+3 ARACHNO cluster.

Question 16

What is the cluster with one less vertex than an arachno cluster?

Answer 16

A hypho cluster.

Question 17

How can the structure of nido/arachno/hypo clusters be determined?

Answer 17

By considering the shape of the parent closo cluster; the parent has the same number of SEP as the cluster in question and has SEP-1 vertices.

Question 18

What are the 5 steps for determining cluster geometry?

Answer 18

1. Determine vertices (n), TVEC and SEP.
2. Assess if closo, nido, arachno or hypho.
3. Determine parent closo structure.
4. Remove vertices as appropriate.
5. Add hydrpgen atoms.

Question 19

How do Zintl ions form clusters?

Answer 19

Each of the group 14 elements counts 4 towards the TVEC but instead of 2 electrons being involved in a bond to a hydrogen, like in B-H, 2 electrons act as lone pairs and so we can still remove 2 from the TVEC per vertex to generate the SEC.

Question 20

What is required (in principle) for a fragment to form part of a cluster?

Answer 20

3 orbitals available for bonding.

Question 21

Name 3 ways of characterising clusters

Answer 21

NMR, IR spec, single crystal X-ray diffraction.

Question 22

What are the 2 ways in which we can rationalise the similarity in behaviour between many transition metal and main group species?

Answer 22

The isolobal analogy and the Wade-Minogs rules (both are interrelated).

Question 23

What is the isolobal analogy?

Answer 23

2 fragments are isolobal if they have the same number of frontier orbitals, with the same symmetry, approximately the same energy and the same number of electrons. Hofmann rationalised that for any given compound then replacement of one isolobal fragment by another might give a stable species.

Question 24

How can we predict which fragments are isolobal (use examples to explain)?

Answer 24

If we (notionally) remove a hydrogen atom from methane and homolytically cleave the C-H bond we will end up with a 7-electron CH3 group with an electron in a sp3-hybrid orbital with a1 symmmetry. If we start with a similar saturated (18-electron) species Cr(CO)6 and perform the same cleavage we would lose CO+ and thus generate 17-electron [Cr(CO)5]-. In order to ensure that the charges do not get out of hand, this species is isoelectronic with Mn(CO)5 which also has 17-electrons (ie we have moved one element to the right which will have one more electron). The unpaired electron occupies an orbital which is mostly composed of dz^2 and also has a1 symmetry.

Question 25

How can we find further isolobal relationships across the periodic table?

Answer 25

By adding/removing electrons then adjusting for charge,

Question 26

When is it unusual for transition metal clusters to form large closo-type clusters? What geometries are adopted instead?

Answer 26

When n>6; in these cases metal clusters tend to adopt geometries in which additional groups are on the faces of a closo type cluster- FACE CAPPING.

Question 27

Does a capped cluster have the same/more than/less than the number of electron pairs for framework bonding as the uncapped cluster?

Answer 27

The same.

Question 28

How many SEP does a closo cluster with n vertices have?

Answer 28

n+1 SEP

Question 29

How many SEP do monocapped/bicapped/tricapped clusters with n vertices have? What cluster are they each based on?

Answer 29

Monocapped cluster= n SEP; based on a closo cluster with n-1 vertices and one face is capped.
Bicapped cluster= n-1 SEP; based on a closo cluster with n-2 vertices and 2 faces are capped.
Tricapped cluster= n-2 SEP; based on a closo cluster with n-3 vertices and 3 faces are capped.

Question 30

What does PSEPT stand for?

Answer 30

Polyhedral skeletal electron pair theory

Question 31

Why do we see metal-metal bonds and cluster compounds towards the bottom of the periodic table?

Answer 31

Bonds to transition metals generally get stronger as groups are descended.

Question 32

What are the 4 general steps for metal-metal multiple bonds in complexes?

Answer 32

1. Determine oxidation state and d-electron count of the metal (tells us how many d-electrons are available to form the metal-metal bonds).
2. Identify the correct molecular orbital overlap diagram.
3. Populate the diagram and determine the expected bond order.
4. Consider other factors (such as periodicity).

Question 33

How many electrons do each of these metal-metal bonds count for?

Answer 33

M-M = 1x extra electron
M=M = 2x extra electrons
M≡M = 3x extra electrons

Question 34

Do M-M bonds affect the oxidation state of a complex? Explain your answer

Answer 34

No as the electrons are shared easily.

Question 35

What was the double bond rule and how was it rationalised?

Answer 35

For many years it was proposed that multiple bonds (ie double/triple bonds involving pi-interactions) which involved elements with a principal quantum number (n) greater than 2 could not exist. This was rationalised on the basis that pi-bonds in heavier main group elecments are weaker than in C/N/O.

Question 36

Why does adding bulky substituents to main group compounds help stabilise double and triple bonds?

Answer 36

1. The thermodynamic data tells us that for elemetns such as P, sigma-bonding frameworks are preferred (2x sigma-bonds are stronger than 1x pi and 1x sigma).
2. If R (substituent) is very large, steric repulsion disfavours the formation of the sigma-framework due to the inhibition of the aggregation process (eg intermolecular P-P bond formation) which gives the sigma-bonds—-> the large R groups prevent the rective centres colliding.

Question 37

What are the conditions for orbital mixing?

Answer 37

The orbitals must have the same symmetry and similar energy (ie small energy difference).

Question 38

What is an intrinsic sem-conductor?

Answer 38

A material which formally has a filled valence band and an empty conduction band, but the BAND GAP is small so that electrons may be thermally promoted.

Question 39

How does the conductivity of a semi-conductor vary with temperature?

Answer 39

It increases with increasing temp.

Question 40

How big is the band gap in insulators Vs conductors? How does this affect their properties?

Answer 40

Insulator= large band gap; difficult to promote an electron into the conductor band.
Conductor= small/zero band gap hence very easy to promote an electron into the conductor band.

Question 41

What is an extrinsic semi-conductor?

Answer 41

A material which has been doped to create either a donor band near the conduction band (n-type) or an acceptor band near the valence band (p-type).

Question 42

Name 2 common semi-conductors

Answer 42

Elemental silicon and germanium.

Question 43

Describe the Czochralski process for crystallising silicon

Answer 43

1. Melting of polysilicon, doping.
2. Introduction of the seed crystal.
3. Beginning of the crystal growth.
4. Crystal pulling.
5. Formed crystal with a residue of melted silicon.

Question 44

What is the metal-organic chemical vapour deposition (MOCVD) used for? Describe the process, listing the advantages/disadvantages and properties of the precursors.

Answer 44

The preparation of thin films of III/V and II/VI materials. Gasses such as hydrogen/helium/nitrogen are used to carry volatile precursors into a chamber which is held at a high temperature (600-700°C). Under these conditions, the precursors decompose to deposit a film of the desired material.
Advantages: route is effective and there is little carbon contamination.
Disadvantages: the reagents are difficult to handle and highly toxic.
Properties of precursors:
1. Compounds must be volatile, so we can use in the CVD chamber in the gas phase.
2. Precursors must be extremely pure or the materials will be contaminated.
3. Must be stable enough to handle and prepare, but then decompose under the CVD conditions.

Question 45

List a few applications of polysiloxanes

Answer 45

Lubricants, medical implants, contact lenses, sealants.

Question 46

How does the electronic structure of polysilanes differ from classic organic polymers (such as polyethylene)? What is the effect attributed to?

Answer 46

Polyethylene is an insulator with a large band gap. Polysilanes and polystannanes are semi-conductors with small band gaps than polyethylene.
This effect is attributed to the phenomenon of sigma-conjugation.

Question 47

What is sigma-conjugation (with regards to polysilanes and polystannanes)?

Answer 47

As the orbitals are larger at Si and Sn, they may interact with atoms further along the chain- not just their immediate neighbours. This is through the delta-orbital framework and is similar to the pi-conjugation in aromatic systems. At carbon, the orbitals are too small to interact in this fashion so sigma-conjugation can’t occur.

Question 48

What is the evidence for sigma-conjugation?

Answer 48

Evidence is present in the UV-vis spectra of polysilanes where a band corresponding to the sigma-sigma* transition is often observed at 300-400nm.

Question 49

What is a deltahedron?

Answer 49

3D shape with triangle faces.