Water of Crystallisation, Aquo Complexes and Hydrates

1
Water of Crystallisation, Aquo Complexes and
Hydrates

• Solids that consist of molecules of a compound
  along with water molecules are called hydrates
• Examples contain water bound to cations or bound
  to anions or other electron rich atoms via
  hydrogen bonds.
• M(OH2)6n etc.
• Water of crystallization found in some crystals
  eg. CuSO4.5H2O (see structure)
• Lattice Water Hydration of a cation or anion
  requires water molecules in the lattice to fill
  the empty space to improve size compatibility of
  units in the lattice.
• Example of group 1 halides

2
Water of Crystallisation, Aquo Complexes and
Hydrates

• Zeolitic water water molecules occupy large
  cavities in zeolitic framework (zeolites are
  aluminosilicates having a framework structure
  with large cavities)
• Clathrate hydrates a class of compounds in which
  molecules occupy "cages" made up of
  hydrogen-bonded water molecules.

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Structure of the elements - Metals

• Metals - an array of cations in a sea of
  electrons
• Cations assume one of 3 basic arrangements
  hexagonal close packed (hcp), cubic close packed
  or face centered cubic (fcc) and body centered
  cubic (bcc)
• Valence electrons are completely delocalised over
  the entire structure - hence metallic properties

4
Structure - Property Relationship

• Metals are good conductors of heat and
  electricity.
• Metals are hard. Varies from metal to metal
• Metals are lustrous. This is due to the uniform
  way that the valence electrons of the metal
  absorb and re-emit light energy.
• Metals are malleable (can be flattened) and
  ductile (can be drawn into wires)

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Closed Packed Structures - hcp
REPEATING UNIT
CN12
6
fcc or cubic close packed
REPEATING UNIT
CN12
7
bcc
1st plane
2nd plane pack above the holes in the first plane
REPEATING UNIT
CN 8
8
Structure of the metals
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Boronmetalloid covalent bonding

• Boron Allotropes
• Many known - 3 structurally characterised
• Red crystalline ?-rhombohedral boron
• Lustrous grey-black ?-rhombohedral boron
• Black crystalline ?-tetragonal boron
• Based on B12 icosahedron

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Allotropes of Boron
Note B is electron deficient with 4 valence
orbitals and only 3 valence electrons
?-rhombohedral boron Simplest allotropic
form B12 icosahedra in distorted ccp
?-rhombohedral boron Thermodynamically most
stable allotrope Complex structure Repeating unit
contains 105 B atoms
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Carbondiamond, graphite, fullerenes, carbon
nanotubes
Diamond
12
Graphite
13
Fullerenes Buckyballs
C60 same form as a soccer ball

• There are 12 pentagonal and 20 hexagonal faces
• Each C atom is sp2 hybridised and bonded to 3
  others
• They are prepared by electrically heating a
  graphite rod in He

C70,C76,C78,C80, C82,C84 also known
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Carbon Nanotubes

• These are fullerene-related cylindrical carbon
  molecules with tube diameter in the nanometer
  range.
• Structure consists of walls of graphite sheets
  rolled into a cylinder and capped at the tips
  with a portion of the fullerene molecule (ie
  pentagonal rings).
• All carbons are sp2 hybridised
• Nanotubes align themselves into ropes held
  together by Van Der Waals forces.
• Extraordinary strength and unique electrical
  properties have found them applications in
  nano-electronics, optics and other materials
  applications

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Group 15 elements
?H 945 kJ/mol
Phosphorous At rt 3 main allotropes - white
phosphorous (P4 molecular), red phosphorous (Pn
polymeric), black phosphorous (P? layers
polymeric)
White P soft, waxy, toxic solid least stable
most reactive P-P 221pm angles 60?
Red P heat white P at 300 ? for several days
less reactive
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Layered structure of black phosphorous
Black P flaky solid graphite-like
appearance Thermodynamically the most stable
form Least reactive Obtained by heating white P
at 12,000 atm or at 220-370 ? for 8 days in
presence of Hg
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Zintl phases

• Group 15 elements react with more electropositive
  metals to form a variety of polynuclear anion
  polyanions
• These materials are formed by (i) solid state
  reaction of an electropositive metal and the
  group 15 element (ii) direct reduction of the
  group 15 element in liquid ammonia containing the
  appropriate metal.

• Polyanions were extensively investigated by
  Eduard Zintl and are often referred to as Zintl
  Ions
• Zintl phases are intermetallic compounds formed
  between strongly electropositive metals (alkali,
  alkaline-earth) and more electronegative ones.

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Zintl phases Structures of common group 15
Zintl ions various rings and cages

• E42- (E As Sb and Bi )

• E113- (E P As and Sb)

• E5- (E Sb)

E73- (E P As and Sb)
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Zintl phases
These anions are obtained as crystalline solids
by complexation with ligands such as cryptands or
crown ethers e.g. K(2,2,2,-crypt)2Bi4

• Stability of the polyanion compounds arises out
  of stabilization by the large
• cryptand or crown ether ion.
• Structures are determined through X-ray
  diffraction studies.