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.