Compounds of Interest Flashcards
Why is Li3N of interest?
Only stable nitride of alkali metals
What are features of Li3 synthesis?
- Li + N2 @ high T in a dray environment
- Possible as lattice enthalpy large enough due to small Li+ to overcome endo formation enthalpy N3- ion
How do alkali metals (other than Li) react with N2?
Form azides which have a lower formation enthalpy
What is the structure of Li3N?
Alternating layers of (Li2N)- and Li+
Li+ ions are 2 or 3 coord (as small), 2x different N-Li distances
What are the properties of Li3N?
Ionic conductivity in 2 dimensions, as Li+ can move between interstitial sites and causes cation vacancies
Functions as fast ion conductor
Small band gap
What is the reactivity of Li3N?
Explosive reaction with water: Li3N + 3H2O -> NH3 + 3LiOH
Spacings mean intercalation of small molecules like H2 or Li+
LiCoO2 can also do this
Why is MeLi interesting?
Due to tetramic structure and use as a reagent in organic synthesis
What is the synthesis of MeLi?
MeX + 2Li -> LiMe + LiX
X = Cl, Br
Metal-halogen exchange
What is the structure of MeLi?
Connected (LiMe)4 tetramic units, with CH3 capping each face of Li4 tetrahedron
Li-Li similar to Li2 in gas (smaller than metal so more covalent)
Cluster forms as e- deficient bonding model
How can you spec probe MeLi?
C13 NMR shows high T fluxionality
6Li I = 2
Low T: slow exchange, 7 lines (C couples to 3Li)
High T: fast exchange, 9 lines (C couples to 4 Li)
What is the MO diagram of MeLi?
t2 bond polarised on C
What are the reactions of MeLi?
Nucleophlic equivalent of Me-
Transmetallated into organocopper reagents, for use as softer nucleophiles
What can you compare the structure of MeLi to?
NaMe rockstalt structure - more polarised as poorer orbital energy match
KMe has the NiAs structure
Grignard also covalent
Why is LixCoO2 a compound of interest?
Used in rechargeable batteries
How are LixCoO2 compounds synthesised?
Li2CO3 + 2CoCO3 + heat -> Li2CoO2
then deintercalation using X2
What is the structure of LixCoO2?
Layered structure consiting of sheets of edge-sharing Co(III) octahedra separated by layers of Li cations
This can be deintercatalated
Oxide lattice has ABCA cubic stacking (CdI)
What is the structure of LixCoO2?
Layered structure consiting of sheets of edge-sharing Co(III) octahedra separated by layers of Li cations
This can be deintercatalated
Oxide lattice has ABCA cubic stacking (CdI)
Why are LixCoO2 good batteries?
Electrochem oxn is reversible so rechargable
Reductive intercalation highly favourable
What is the band structure of LixCoO2 states?
Oxidation of Co(III) to Co(IV) lowers energy of Co 3d so O 2p depleted which can lead to a fire risk as O2 released
Why is Na2(2.2.2-cryptand) of interest?
Binding of Na+ to cryptand strong enough to drive disproportionation to sodide anion, Na-
2Na <-> Na+ + Na-
Good reducing agent (not as strong as Cs equivalent)
How is Na2(2.2.2-cryptand) synthesised?
2Na + 2,2,2-cryptand -> Na2(2.2.2-cryptand)
Done in ethylamine
What is the structure of Na2(2.2.2-cryptand)?
HCP array of Na(2.2.2-cryptand)+ with Na- in octahedral holes
NiAs structure
Na+ is 8-coord
Why can Na2(2.2.2-cryptand) form?
Cryptand effect
Entropic - preorganised, so fewer dof lost on complexation, Na- not well solvated so little H2O organisation
Enthalpic - lone pair repulsion overcome in complexation
Why is Na β-alumina a compound of interest?
Very high electrical conductivity and use as a solid electrolyte in Na-S battery system
What is the synthesis of Na β-alumina?
Na2CO3 + Al2O3 -> NaAl11O7
Within a sealed vessel to avoid loss of Na2O
What is the structure of Na β-alumina?
Al3+ occupies Td and Oh holes in close-packed layers (similar to spinels)
Every 5th layer has 3/4 of oxide ions missing
Layered structure with Na within layers
Why is Na β-alumina a good conductivity?
Na+ mobility
Mobile in oxide-deficient layers as smaller than O2- and many states to occupy
Only in 2 dimensions
What occurs to Na β-alumina if you exchange for larger cations?
As radii increases they are less mobile as cannot move as freely within oxide layers
BUT Li+ has v low conductivity as so small it occupies smaller sites so higher activation energy for motion
Why is K3C60 a compound of interest?
Superconductor at temperatures as high as 40K
How is K3C60 synthesised?
Intercatalation of K vapour into C60
N-style doping
What is the structure of K3C60?
C60 by itself are fcc
With K+ cations in all Oh and Td with C603- ccp
What is the band structure of K3C60?
How is K3C60 a superconductor?
BCS theory - movement of pairs of e- (cooper pairs) coupled by lattice vibrations
Tc = ωe1/λ
λ = VN(Ef)
Tc also depends on lattice parameter a0 (as separation increases then bandwith increases)
Why is α-AgI/RbAg4I5 interesting?
Fast-ion conductor
How do you synthesise α-AgI/RbAg4I5?
KI + AgNO3 -> α-AgI
RbAg4I5 can be made by inserting some RbI
only α-AgI at higher temp
What is the structure of α-AgI?
Body-centered array of I-
Ag+</sub> ions distributed across different sties and this mobility leads to being a conudctor?
How is the structure of RbAg4I5 similar to α-AgI?
Rb+ and I- form a rigid lattice
Ag+ randomly distributed between Td Sites
Has higher ionic cond at low T but comprises overall response
Why is Rb9O2 interesting?
Fast-ion conductor
How do you synthesise Rb9O2?
Partial oxn of Rb @ low T gives Rb6I which then decomposes to Rb9O2 and Rb
What is the structure of Rb9O2?
2x ORb6 face-sharing octahedra
e- delocalisation between Rb ions, so good ion conductor (and coloured)
Short Rb-Rb distance
What is the reactivity of Rb9O2?
Melts to form Rb2O & Rb
Reacts with water to produce fully oxidised RbOH
Why is TiCp4 an interesting compound?
Due to fluxonality of protons in NMR
How is TiCp4 synthesised?
TiCl4 and 4NaCp
What is the structure of TiCp4?
2x η5 CP and 2x η1 Cp to avoid exceeding 18VE rule
16 VE - poor overlap of 3d orbitals so only small stabilisation of bodning orbitals. Means doesnt follow 18 e- rule (like 4/5d)
Makes sense as Ti(IV) is majority of Ti chemistry
What is the NMR of TiCp4?
Why can ring wizzing occur in TiCp4?
16VE so can form suitable TS
Why is TiO1+x interesting?
Abnormal structure
they didnt even give a reason
Why is TiO1+x interesting?
Abnormal structure
they didnt even give a reason
How can TiO1+x be synthesised?
TiO2 and Ti at 1500C
What is the structure of TiO1+x?
when x=0, 15% of cation and anion sites vacant, means ordered monoclinic structure
Change in stochiometry changes vacancy conc, but total conc same
Vacancies reduce Ti-Ti distances and max M-M bonding, which is good for early TM as more extended d-orbitals
What is the reactivity of TiO1+x?
Reacts with acid to form Ti3+ and H2
Because stable oxn states are 3/4, as early in period and compensated by electrostatic interactions with anions
How does TiO1+x compare to NbO/NiO?
NbO - 25% vacancies, more M-M as better 4d overlap so wider bands and more metallic
NiO - cation vacancies, Ni1-xO gives mixed valence (2/3). Mott-hubbard insulator to metallic conductor
Why is BaTiO3 interesting?
Perovskite structure
How do you synthesise BaTiO3?
React TiO2 and BaCO3 at high T
What is the structure of BaTiO3?
Cubic perovskite with corner-sharing TiO6 octahedra
Large Ba expands lattice along with the 2nd order JT, displaces Ti(IV) from centres of octahedra
Leads to ferroelectric polarisation - charge accumlates in material while thermally stable
Why is VO an interesting compound?
Structure comparison with 3d MOs and VO2
How do you synthesise VO?
V2O3 and V(s)
What is the structure of VO?
Distorted rocksalt with weak V-V
varies between VO0.8-1.3
What is the band structure of VO?
Broad band (W>U) as early 3d TM
Metallic conductor as partially filled t2g
How does VO compared to 3d MOs?
Later MOs - more contracted 3d, U>W, mott-hubbard insulators
Early MOs - more extended 3d, M-M leads to more defective structure
Why is VO2 of interest?
Undergoes Pierl’s distortion where it changes from metallic to insulating
How is VO2 synthesised?
V2O5 + CO -> CO2 + VO2
What is the electron count of VO2?
V(IV) is t2g1
Would expect metallic behaviour
What is the structure of VO2?
> 340K is undistorted metallic<340K undergoes Pierls distortion, by which V-V dimers formed and e- localised in V-V bonds
What is a Pierls’ distortion?
1D chain of equally spaced ions with 1 e- is unstable
Leads to distortion
Why can the Pierls distortion occur in VO2?
Early in 3d, orbital overlap good and V-V bonds outweighs loss of lengthening V-O bonds
How does the structure of VO2 compare to similar??
TiO2 - rutile band gap insulator, d0
CrO2 - 3d overlap not sufficient, undistrorted rutile
NbO2 - distortion occurs to higher temp as better overlap, insulating
How is CrO2 synthesised?
Thermal decomp of CrO3
How is CrO2 ferromagnetic?
Rutile structure - dyz and dxz able to mix with O 2p
Leads to delocalisation in this band
Why is [2,6-Dipp2C6H3]Cr2 a compound of interest?
Quintuple Cr-Cr
How is [2,6-Dipp2C6H3]Cr2 synthesised?
What is the structure of [2,6-Dipp2C6H3]Cr2 ?
Trans-bent with quintuple bond
1xσ - dz2
2xπ - dyz, dxz
2xδ - dxy, dx2-y2
Confirmed by X-ray, IR, and NMR (no Cr-H bonding)
What type of ligands stabilise multiple metal bonds?
Bulky ones-
Limit intermolecular reactions which yields
Also want few ligands as too many reduces VE available to form M-M
Why is Cr2(OAc)4.2H2 of interest?
Quadruple bonds
How is Cr2(OAc)4.2H2 synthesised?
CrCl3 + Zn/HCl -> CrCl2
CrCl2 + NaOAc -> Cr(OAc)2
All under N2
What is the bonding in Cr2(OAc)4.2H2?
Quadruple bonding
All e- paired in dimer, so diamagnetism (Cr 2+ is JT distorted)
Red due to δ-δ* transitions
What is the structure of Cr2(OAc)4.2H2?
Dimer with eclipsed config - as allows for quadruple bonds even if bad sterics
C4V point group
What is the reactivity of Cr2(OAc)4.2H2?
Strong reducing agent Cr(II) - Cr(III)
Reduces atmospheric O2 so can used as scrubber for oxygen
How can you compare Cr2(OAc)4.2H2?
Mo - shorter quadruple bonds as better overlap of 4d, and stronger bonding so δ-δ* gap larger so lesser thermal δ* population
Re - same number of VE and eclipsed
Os - not eclipsed as δ* fully occupied so sterics outweighs
Why is M(CO)5(C(R)OMe) interesting?
(when M= Mo, W and R = Me, tBu)
Fischer carbene
How is M(CO)5(C(R)OMe) synthesised?
What is the reactivity of M(CO)5(C(R)OMe)?
Reacts with nulceophiles as C in carbene electrophilic
What is the elec structure of M(CO)5(C(R)OMe)?
d-orbitals rel low energy due to π-acceptor co-ligands (CO)
CR2 has O/R sub which destabilises C 2p, leading to singlet carbene formation
But C 2p empty, so C is electrophlic
HOMO metal d-bsaed, LUMO is C-based
How is a Fischer carbene compare to a schrock alkylidene?
Schrock:
no π-acceptor co-ligands, d-orbitals rel high in energy, no π-donors on C
Triplet carbene
Why is Fe2Cp2(CO)4 interesting?
Interest due to fluxionality
How can you synthesise Fe2Cp2(CO)4?
Reaction between Fe(CO)5 and cyclopentadiene dimer C10H12
What is the reactivity of Fe2Cp2(CO)4?
React with Na/Hg amalgam to form v nucleophilic [CpFe(CO)2]- anion
Reacts with Br2 or I2 by OX -> CpFe(CO)2X
What is the structure of Fe2Cp2(CO)4?
Fluxional - between cis and trans via a non-bridged intermediate
NMR: 1 CO in fast exchange, 2 in slow exchange (bridged & terminal)
Why is Fe3O4, called magnetite, interesting?
Due to structure, elec conductivity, and magnetic properties
How can you synthesise Fe3O4 (magnetite)?
6Fe2O3 + high heat -> 4Fe3O4 + O2
Heat is 1400C
Also produced in the body but can be poisonous
What is the structure of Fe3O4 (magnetite)?
Fe(II)Fe(III)2O4 has the inverse spinel structure
ccp arrangement of O2- with Fe(III) in 1/8 Td holes and 1/4 Td Oh, and Fe(II) in 1/4 Oh holes
Fe(III) - hs d5, LFSE is 0 (no preference for Oh or Td)
Fe(II) - hs d6, favours an Oh coord as 0.4ΔOh > 0.6ΔTd
How does Fe3O4 (magnetite) compare to Co3O4?
Co3O4 adopts normal instead of inverse
Co(III) in 1/2 Oh holes and Co(II) in 1/8 Td holes
Why is magnetite, Fe3O4, a good electron conductor?
Above 120K: e- transfer between Oh Fe(II) and Oh Fe(III) which are in edge-sharing octahedra. This is good as transition is t2g -> t2g, so no change in σ* and small franck-condon barrier to e- transfer
Below 120K: Verwey distortion occurs which reduces conductivity
How is Fe3O4, magnetite, ferromagnetic?
Ferro - can form a naturally occuring magnet
How does Fe3O4, magnetite, react?
Decomposes with CO in blast furnace to Fe + CO2
Binds to impurities in water and sediments, for purification
Why are Fe(NCS)2(bipy/phen)2 a compound of interest?
They are spin crossover compounds
Which are compounds within which high-spin-to-low-spin transition at a TM centre, in response to a change in temperature or pressure
What are the spin properties of Fe(NCS)2(bipy/phen)2?
ls at low T, hs at high T
Due to maximising vib entropy in higher T
As in hs - weaker bonding, so vib energy levels closer together so more populated, more vib entropy
What is the ligand field strengths in Fe(NCS)2(bipy/phen)2?
Both bipy/NCS- have intermediate field strengths
Means hs/ls spin states are similar energies
Why is KFe2(CN)6, prussian blue, interesting?
Intense blue colour
This arises from intravalent charge transfer (IVCT)
How can you synthesise KFe2(CN)6, prussian blue?
K4[FeII(CN)6] + FeIII(H2O)6 3+</sub> -> KFe2(CN)6 + 6H2O
What is the structure of KFe2(CN)6, prussian blue?
FCC of FeII(CN)6 with FeIII in Oh holes and K+ in 1/2 Td holes
CN- linear between Fe ions, aligned so softer C adjacent to FeII and N next to FeIII
Why is CoCO4 of interest?
Eqm between bridged and non-bridged structures
Also used in org synthesis
How is CoCO4 synthesised?
CoIII(OAc)3 + H2 + CO @ high T and p
H2 reduces Co
What is the structure of CoCO4?
Major isomer has 2xbridging CO and eclipsed terminal CO (C2v)
Unbridged has staggered CO (D3d)
Distinguished by IR
How does CoCO4 react?
Reacts with alkyne
Also reacts with Na then acidification to give a pre-catalyst used in hydroformylation reactions (HCo(CO)4)
Why is YBa2Cu3O7 of interest?
Superconducting material
How can you synthesise YBa2Cu3O7?
Sol-gel precursor - dry and heat to form product
High T ceramic synthesis with heating and grinding cycles
What is the structure of YBa2Cu3O7?
CuO4 square-planes separated by layers of CuO3 which insert or remove e-, which partially oxidises Cu(II) to Cu(III)
This gives a superconducting metal
If just Cu(II) then would be Mott-Hubbard insulator
How is superconduction explained in YBa2Cu3O7?
Low T - BCS theory
Higher T - not clearly explained but involves stronger e-/phonon coupling
Why is ZrO2 of interest?
Reaction with Y2O3 to give fast-ion conductor
This is called yttria stabilised zirconia
How can you synthesise ZrO2?
Exists naturally
Or heat Zr(OH)4
What is the coord of Zr in ZrO2?
Low T: 7-coord in monoclinic
High T: 8-coord in dist fluorite
Show in x-ray
hat is the band structure of ZrO2?
Band gap insulator - O 2p valence filled and conduction 4d band empty
What is the conductivity of ZrO2 at different T?
High T is sig more conductive
As low T (monoclinic) has 2 distinct anion sites whereas fluorite has 1 anion sites
More randomly distributed in high symm structure, so easier hoppijng between sites and faster conductivity
How does ZrO2 react with Y2O3?
ZrO2 + (x/2) Y2O3 -> Zr1-xYxO2-2/x
Y stabilises fluorite as Y(3+) larger than Zn(4+) which leads to O2-</sub> vacancies to maintain charge neutrality
Gives good ionic conduction at high T
How is the product of ZrO2 reacting with Y2O3 useful?
Solid electrolyte in oxygen sensors in internal combustion energies
What is the structure of ZrO2 compared to other MO2?
TiO2 - rutile at all T, Ti(IV) 6-coord as smaller
SiO2, GeO2 - 4 coord system
Why is the structure of Cp2ZrCl2 interesting?
Precursor to an alkene polymerisation catalyst
How do you synthesise Cp2ZrCl2?
ZrCl4 + 2NaCp -> Cp2ZrCl2 + 2NaCl
Zr(II) to Zr(IV)
