paper 2

• Synthesis of Li3N


  Direct reaction of lithium metal with nitrogen gas at high temperatures
• Li3N is the only stable nitride of the alkali metals
• None of the other group I metals form a nitride
• Instead group I metals form azides with the N3- ion, which has a much lower formation enthalpy
• Group II metals can form nitrides (M3N2) since the higher ionic charge of M leads to a higher lattice enthalpy which can overcome the formation enthalpy of the nitride ion
• Synthesis of MeLi


  MeX + 2Li → LiMe + LiX (X = Cl, Br)
• MeLi shows high-T fluxionality in 13C NMR
• Synthesis of LiCoO2


  LiCoO2 formed by heating Li2CO3 and CoCO3 in air. Then oxidative deintercalation of Li+ using elemental halogens
• Structure is important to properties; LiMnO2 has a disordered rocksalt structure so cannot be easily deintercalated
• Li can be removed electrochemically from LiCoO2 to form highly metastable CoO2, during which the CoO2 planes shift and the oxide lattice changes from ABCA cubic stacking to ABAB hexagonal stacking – CdI2 structure
• Synthesis of Na2(2.2.2-crypt)


  2Na + 2,2,2-crypt (dissolved in ethylamine) → Na(2,2,2-crypt)+Na-
• Na2(2,2,2-crypt) is a good reducing agent, since the electron is almost free
• Cavity size of cryptand is selective for Na+ ion size
• Structure of Na2(2.2.2-crypt) confirmed by 23Na MAS-NMR study
• Synthesis of NaAl11O17


  Na2CO3 + Al2O3 → NaAl11O17
• Another example of a fast ion conductor is α-AgI. Here the Ag+ lattice is effectively molten and Ag+ ions can hop between 42 cation sites
• Synthesis of K3C60


  Intercalation of potassium into C60 (reaction with K vapour)
• Synthesis of α-AgI


  Reaction of an iodide solution e.g KI with a solution of silver ions e.g AgNO3. α-AgI is formed from β-AgI at 146oC
• Synthesis of RbAg4I5


  Partial replacement of Ag+ cations in AgI with Rb+, by reacting AgI with RbI
• The entropy change on transition from β to α AgI is roughly half that associated with the melting of a typical ionic solid e.g. NaCl, consistent with 'melting' of Ag+ lattice
• Ag+ ions distributed across 12 Td sites, 6 Oh sites and 24 trigonal sites; total of 42 cation sites per unit cell for 2 Ag+ ions
• High Ag+ mobility leads to the highest known room-temperature ionic conductivity for a crystalline solid
• RbAg4I5 has higher ionic conductivity at low temperature compared to α-AgI, but compromise overall performance
• Synthesis of TiCp4


  Reaction of TiCl4 and 4NaCp
• g+ ions


  Cations distributed across 12 Td sites, 6 Oh sites and 24 trigonal sites; total of 42 cation sites per unit cell for 2 Ag+ ions
• Crystal structure of RbAg4I5


  Different from α-AgI, but share some similarities:
• Similarities between RbAg4I5 and α-AgI

  • Rb+ and I− ions form a rigid lattice
  • Ag+ is randomly distributed between tetrahedral sites
• RbAg4I5 has higher ionic conductivity at low temperature, but compromise overall performance
• Aspects covered


  • Compounds
  • Aspects
  • Points of interest
• TiCp4


  Of interest due to its fluxionality, probed by NMR
• Structure of TiCp4

  • Two η5-Cp and two η1-Cp ligands
  • Not all 4 Cp rings η5, as that would exceed the 18VE rule
  • 16VE - poor overlap of the 3d orbitals, there is only a small stabilisation of bonding orbitals, therefore the 18 electron rule is not strongly obeyed as is common for 4d/5d metals
  • (Ti not concerned with having 18VE, just wants to lose 4 electrons to form Ti(IV) – chemistry of Ti is dominated by Ti(IV))
• NMR experiments to probe fluxionality mechanisms in TiCp4

  1. Very low T: 4 peaks, 10:2:4:4

  2. Intermediate T: 2 peaks, 10:10

  3. High T: 1 peak
• Ring-whizzing


  A series of 1,2- hydride shifts that puts the η1-Cp hydrogens in the same environment, but still different environment to the η5 hydrogens

  
• Hapticity interchange


  At sufficiently high T, all Cp hydrogens are equivalent as there is hapticity interchange between η5-Cp and η1-Cp
• TiO1+x


  Can be prepared from TiO2 and Ti at very high temperature (1500oC)

  
• Structure of stoichiometric TiO

  • Rocksalt but contains a large number of Schottky defects
  • At x=0, 15% of the cation sites and 15% of the anion sites are vacant to form an ordered monoclinic structure

  
• Change in stoichiometry of TiO1+x

  • Accompanied by change in vacancy concentrations, but total concentration of vacancies is always the same
  • Vacancies allow the lattice to contract and reduces Ti-Ti distances, maximising M-M bonding which is favourable for Ti since it is an early transition metal with extended d orbitals. The M-M bonding compensates for the loss of electrostatic stabilisation when forming a defect.

  
• VO


  Of interest due to its comparison with other 3d metal monoxides and with VO2
• Synthesis of VO


  1. Heating V2O5 in excess CO gives V2O3, then heating that with CaH2 gives V(s)

  2. V2O3 + V(s) → 3VO
• Structure of VO


  • Distorted rocksalt structure containing weak V-V bonds
  • Non-stoichiometric - stoichiometry between VO0.8 and VO1.3
  • V is an early 3d TM and its 3d orbitals are not yet contracted into the core, so there is good interaction between metal d orbitals, giving a broad band (W>U)
  • Metallic conductor due to partially filled t2g conduction band – V(II) is t2g3