The Tetrels and Pnictogens Flashcards
What is an allotrope?
Allotropy or allotropsim is the property of some chemical element to exist in two or more different forms, with distinct structure
e.g. dimond, graphite, C60 buckminsterfullerence etc
How happens to the most common oxidation state going down group 14?
- For the top of the group +4 is the most common: carbon, silicon and germanium
- +2 becomes more favoured down the group
- with lead stable in +2
- This is due to the inert pair effect (the tendency of electrons in the outermost atomic s orbital to remain unionised or unshared in compounds of group 13-16 elements) + **relativistic effects
What happens to atomic radius going down group 14?
- Atomic radius gets bigger going down the group
- This effect results from the fact that electrons are being places in orbitals with increasing principal quantum number and therefore lie further from the nucleus
What happens to ionisation energy going down group 14 (kJ/mol)
- Ionisation energy decreases for the first few elements then stabilises off
- Decrease is due to the increase in principal quantum number associated with the outermost electrons, resulting in an increased distance from the nucleus (off sets increase in nuclear charge)
- Ge, Sn and Pb as all proceeded by d-block and f-block electrons which have poor abilities of shielding the outer electrons (d- and f-block contraction) + relativistic effects for Pb
What happens to electron affinity going down group 14 (kJ/mol)
- The electron affinities for C is lower than Si due to the small size of C, leading to increased repulsion when removing an electron
- Ge and Sn are proceeded by d-block electrons hence experience increased nuclear charge despite being further away
- Pb is anomalously low due to the inert pair effect + relativistic effects
Methane (CH₄), Silane (SiH₄), Germane (GeH₄), and Stannane (SnH₄), are all group 14 hydrides
What happens to the stability of hydrides going down group 14?
- The stability of hydrides decreases down group 14 (PbH₄ is extremely unstable and has not been isolated)
- This is due to the M-H getting longer and weaker going down the group due to larger, more diffuse orbitals, more polar bonds, metal not shielded and low-lying LUMOs
Carbon halides are hydraulically stable meaning?
It does not react with water/undergo hydrolysis readily
Group 14 halides (Si-Pb) are hydrolytically unstable
Why?
- Although the further down the group the M-Cl bond gets stronger due to increased differences in electronegativitiy, hence greater electrostatic attraction
- It also means the bond is more polar and more electrophilic metal(loid)
- And also due to their larger size, they are more easily attacked by nucleophiles
For Ge, Sn, and Pb we can also form MX₂, which are more metallic (bigger difference in electronegativity, why?)
Why are the MX₂ compounds more likely to form?
- Bigger difference in electronegativity (higher ΔΧ) are tetrel is become less electronegative
- Left and down on the Arkel Ketelaar triangle
- More likely to form due to the inert pair effect
Halides can also react with alcohols to form
M(OR)₄ from the halide
(with the strength of the S-O bond being the driver)
Silcones are?
Siloxane polymers
How can we form silicones
Through the hydroloysis of silylhalides
Then a condensation reaction
(way to form simple siloxanes)
Polymeric siloxane require three types of monomeric units to be formed
What are they
(the cross links are to typically add strength)
Where does the thermal and chemical stability of silicones comes from
The thermal and chemical stability of silicones comes from the strength of the Si-C bonds and of the Si-O-Si bridges
Silicones are remarkably inert
What however will they react with?
Will react with fluorinating agenets (due to the strength of Si-F bonds is very high, 582 kJ mol⁻¹
Why is the Si-O bond so strong?
- Due to pπ-dπ bonding
- The filled 2p orbtial of the oxygen donated electron density into the empty 3d of the si
- (the strength of the silicon-oxygen bonds mean that the chemistry of silicon is dominated by these compounds
Tetrahmethylsilane (TMS) is sued as a references for ¹H, ¹³C and ²⁹Si NMR
Why would you use TMS over Silicon tetrachloride for NMR
- Because the central silicon is sterically hindered by the methyl group making it more stable
- Si-Cl bonds are more polar - more likely to undergo attack (nucleophilic)
- AND Cl⁻- is a much better LG than CH₃⁻
Why are double and triplet bond favoured for first row elements but not for lower down the periodic table?
- Easy to make double and triple bonds with first row elements because the orbital overlap is good
- e.g. E=O bond (C=O yea, Si=O no)
- Orbital overlap to make π-bonds with heavier group 14 elements is possible, but σ-bonds will always be preferred
If double bonds are not preferred for silicon, how can the geometry of a compound encorage them to form?
- Sterically hindering group can block oligomerisation (bonding with another group)
- This stops the metal centres polymerising via σ-bonds and forces them to π-bond
- This can also lead to the molecule no longer being planar
What is a ‘paw-paw’ donor-acceptor bonding?
Where the same molecule will act as both as lewis acid and a lewis based all at the same time
(much weaker than covalent overlap of two triplet fragments)
e.g. these two Sn are sp² hybridised with a triplet arrangement (like carbenes)
What are Zintl ions?
- Clusters from group 14 elements which use cryptand-222 ligands to enable the crystallisation of these compounds
- In general, a Zintl phases contain a group 1 or group 2 metal, along with post-transition metals (group 12) or the metalloids from group 13, 14, 15, or 16
What happens to the Electron Affinity (kJ mol⁻¹) going down group 15
- Nitrogens electron affinity is significantly lower due to the small size of the atom, where adding an electron would lead to increased repulsion
- P and As have similar electron affinities (rather than a suspected increase for As) due to d-block contraction
- Sb and Bi have similar electron affinities (rather than a suspected increase for Bi) due to f-block contraction
What are the common oxidation states for group 15 elements?
- General trend of s² p³
- symmetrically half filled p orbital
- Fewer oxidiation states further down the group due to the inert pair and relativistic effects
What forms of nitrogen can be found?
- Predominatly N₂ due to the strong nature of the N-N triple bond (ΔH = 945 kJmol⁻¹)
- N³⁻ ion does exist, formed when Li metal reacts with nitrogen (of which the lattice energy is high)
What forms can phosphorus be found in?
- Because phosphorus is a larger atom allotropes are very common
- This is because P=P bond are less favourable
- (The P-P bond is still pretty weak however)
- Can form a tetrahedron
- Or black phosphorus which is rippled layers of 6 membered rings
- Diphosphorus P₂ does exist, but only under extreme conditions
White phosphorus (P₄) is pyrophoric
Meaning it is very reactive towards oxygen and ignites in contact with air (reaction shown below)
What is the driving force for this reaction?
- The strong P=O bond
- This bond is so strong due to pπ-dπ interaction of the oxygen donating e- density from its filled p-orbital into the empty, low-lying d-orbital of phosphorus
What forms do As, Sb, and Bi comes in?
- They form layered structures similar to phosphorus
- Elements become more metallic as the group descends
Phosphorus forms two oxides
What are they?
- Phosphorus (V): tetrahedral structure
- Phosphorus (iii): one LP on each phosphorus
Nitrogen forms 7 molecular oxides
What are the stability of them like compared to N₂ and O₂
Nitrogen molecular oxides are all thermodynamically less stable than N₂ and O₂
Why does nitrogen not form negative oxidation states for N oxides?
Because oxygen is more electronegative so always presents as O²⁻
The following compounds are group 15 hydrides NH₃, PH₃, AsH₃, SbH₃, (BiH₃ = unstable above -60°C)
What happens to the bond energy and angles going down the group?
- Bond energy decreases down the group, due to decreased stability from larger, more diffuse orbital and hence worse orbital overlap
- Bond angle also gets smaller going down the group due to bonding going from between hydrides to just between orbtials
What happens when you heat phosphorous acis up to 200°C
A disproportionation reaction
A way of forming PH₃
NH₃ is well known to be basic
Why is PH₃ not basic
- The P-H sigma bonds are mainly between the p-orbitals on the phosphorus and s-orbitals on the hydrogen
- The 3s orbitals contribute little to the bonding which is where the LP for basicity would come from
- (Drago’s rules)
Phosphines (PR₃) is a good ligand for transition metals
(R = Me, Ph, ₛBu etc)
Why?
π-bonding involved overlap between a σ’ orbital on phosphorus and a filled metal d-orbtial
MX₃ is formed for all group 15 halide but for MX₅ not all halides are formed
Explain the trend show
- No MI₅ compounds are formed for group 15 because the bonding would involve large valance orbitals on both elements - poor overlap and very weak bonds
- The larger the group 13 metal and the larger the halide, the larger the orbitals are resulting in worse overlap and weaker bonds
How is NF₃ different to NH₃
- Boiling point for NF₃ is much lower due to having no hydrogen bonding
- They bond have the same trigonal pyrmidal structure with a LP on the Nitrogen
- NF₃ has a slightly smaller bond angle due to the high electronegativity of fluorine
- Where the fluorine pulls electron density away from the nitrogen, which results in less electrostastic repulsion between bond pairs
This is what PCl₅ appears like in gas phase
How does PCl₅ as a solid
- In the solid state PCl₅ exists as a tetrahedral [PCl₄]⁺ cation and ocetahedral [PCl₆]⁻ anion
