Covalent bonding between the elements Flashcards
What are cluster compounds?
Species which have 3D shapes and have direct element-element bonds.
What is the difference between clusters with a core and naked clusters?
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.
How is the synthesis of borane clusters traditionally characterised?
Reaction occuring at high temperature, resulting in low yields with a number of different products.
Name the 5 different reactions of borane clusters
Combustion, hydrolysis, electrophilic substitution, base-induced degradation, deprotonation reactions.
What are the 3 methods of electron counting?
Total valence electron counting (TVEC), skeletal electron count (SEC), skeletal electron pairs (SEP).
What is the total valence electron count and how can it be calculated?
The number of valence electrons which constitute the cluster.
- Add up the number of valence electrons on the core atoms.
- Substituents such as H, Bu^t etc all count as one electron.
- Charge, add on one electron for each unit of negative, remove one electron for each unit of positive charge.
What is the skeletal electron count and how can it be calculated?
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).
How can the skeletal electron pairs be counted?
SEP = SEC/2
What is the difference between electron precise/deficient/rich?
- Electron precise: all the bonds in the cluster are 2-electron 2-centre bond. TVEC= 5n, SEC= 3n.
- Electron deficient: too few electrons for 2-electron 2-centre bonds. TVEC< 5n, SEC< 3n.
- Electron rich: have more electrons than are needed for only 2-electron 2-centre bonds. TVEC> 5n, SEC> 3n.
In general, what structures to electron rich/deficient clusters have?
Electron rich= open, electron deficient= c=much more closed.
List at least 3 closed structures
Tetrahedral, trigonal bipyramid, octahedral, icosohedral etc.
What is Wade’s rule>
For an n vertex cage with n+1 SEP, then a closo (closed) structure will be adopted.
How many electrons/orbitals are involved in skeletal bonding in a B-H fragment?
2 electrons/ 3 orbitals.
In general, what molecular bonding orbitals does an n-vertex B-H cluster have?
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).
How do you get from a closo structure to a nido and an arachno structure?
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.
What is the cluster with one less vertex than an arachno cluster?
A hypho cluster.
How can the structure of nido/arachno/hypo clusters be determined?
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.
What are the 5 steps for determining cluster geometry?
- Determine vertices (n), TVEC and SEP.
- Assess if closo, nido, arachno or hypho.
- Determine parent closo structure.
- Remove vertices as appropriate.
- Add hydrpgen atoms.
How do Zintl ions form clusters?
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.
What is required (in principle) for a fragment to form part of a cluster?
3 orbitals available for bonding.
Name 3 ways of characterising clusters
NMR, IR spec, single crystal X-ray diffraction.
What are the 2 ways in which we can rationalise the similarity in behaviour between many transition metal and main group species?
The isolobal analogy and the Wade-Minogs rules (both are interrelated).
What is the isolobal analogy?
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.
How can we predict which fragments are isolobal (use examples to explain)?
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.
How can we find further isolobal relationships across the periodic table?
By adding/removing electrons then adjusting for charge,
When is it unusual for transition metal clusters to form large closo-type clusters? What geometries are adopted instead?
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.
Does a capped cluster have the same/more than/less than the number of electron pairs for framework bonding as the uncapped cluster?
The same.
How many SEP does a closo cluster with n vertices have?
n+1 SEP
How many SEP do monocapped/bicapped/tricapped clusters with n vertices have? What cluster are they each based on?
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.
What does PSEPT stand for?
Polyhedral skeletal electron pair theory
Why do we see metal-metal bonds and cluster compounds towards the bottom of the periodic table?
Bonds to transition metals generally get stronger as groups are descended.
What are the 4 general steps for metal-metal multiple bonds in complexes?
- Determine oxidation state and d-electron count of the metal (tells us how many d-electrons are available to form the metal-metal bonds).
- Identify the correct molecular orbital overlap diagram.
- Populate the diagram and determine the expected bond order.
- Consider other factors (such as periodicity).
How many electrons do each of these metal-metal bonds count for?
M-M = 1x extra electron M=M = 2x extra electrons M≡M = 3x extra electrons
Do M-M bonds affect the oxidation state of a complex? Explain your answer
No as the electrons are shared easily.
What was the double bond rule and how was it rationalised?
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.
Why does adding bulky substituents to main group compounds help stabilise double and triple bonds?
- 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).
- 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.
What are the conditions for orbital mixing?
The orbitals must have the same symmetry and similar energy (ie small energy difference).
What is an intrinsic sem-conductor?
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.
How does the conductivity of a semi-conductor vary with temperature?
It increases with increasing temp.
How big is the band gap in insulators Vs conductors? How does this affect their properties?
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.
What is an extrinsic semi-conductor?
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).
Name 2 common semi-conductors
Elemental silicon and germanium.
Describe the Czochralski process for crystallising silicon
- Melting of polysilicon, doping.
- Introduction of the seed crystal.
- Beginning of the crystal growth.
- Crystal pulling.
- Formed crystal with a residue of melted silicon.
What is the metal-organic chemical vapour deposition (MOCVD) used for? Describe the process, listing the advantages/disadvantages and properties of the precursors.
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.
List a few applications of polysiloxanes
Lubricants, medical implants, contact lenses, sealants.
How does the electronic structure of polysilanes differ from classic organic polymers (such as polyethylene)? What is the effect attributed to?
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.
What is sigma-conjugation (with regards to polysilanes and polystannanes)?
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.
What is the evidence for sigma-conjugation?
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.
What is a deltahedron?
3D shape with triangle faces.
