Transcript Ionic or Saline Hydrides

Hydrides
Hydrides
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Ionic or Saline Hydrides : NaH, CaH2
(ionic)
Covalent: e.g., H2O, B2H6, ReH92Metallic or non-stoichiometric
hydrides: PdHn, UH3
* figures are electronegativity
Ionic or Saline Hydrides
2 M + H2  2 MH (exothermic) only LiH can be
melted w/o decomposition
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Ternary compounds possible e.g.,
CaH2 + CaCl2  2 CaHCl
Colors deepen as polarizability of anions & cations
increases: BaHI is black
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 Saline hydrides react violently with water
producing dihydrogen gas.
NaH (s) + H2O (aq) → NaOH (aq) + H2(g)
Hydride
Ion
Ionic Radii:
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H- 1.30 Å (in LiH) ~ 1.54 Å (in CsH)
F- 1.33 Å
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Cl- 1.67 Å
H-: low charge/size ratio; easily polarizable (i.e. it
can be easily distorted by any nearby cation)
Strong basic character, reacts violently with H+
source
Ionic hydride preparation and
structure
Typically these compounds are prepared by direct
interaction with the metals at 300-700oC
2 M(l) + H2(g)
M(l) + H2(g)
2MH(s)
MH2(s)
The rates of these reactions are
Li> Cs> K> Na
All produce pure white solids that appear grey when
impure.
Ionic hydrides: Preparation and Structure
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Crystal Structure
Knowing that the ionic radius of H is between that
of Cl- and FWhat do you think about the crystal structure of
alkali metal hydrides, LiH and CsH?
Crystal Structures of Metal hydrides
Alkali metal hydrides and LiH and CsH take
on the NaCl Structure.
What about others?
MgH2 adopts the rutile structure
CaH2
SrH2
BaH2
All take on a PbCl2-like
distorted hcp array.
Reactions of Ionic metal
hydrides
1. All thermally decompose to give metal and
hydrogen. Only LiH is stable to its melting point of
688 oC.
 Note that LiH is unreactive at moderate
temperatures toward oxygen and chlorine.
2. Generally ionic hydrides are highly reactive toward
air and water.
MH(s) + H2O
H2(g) + MOH(s)
MH2(s) + H2O
H2(g) + M(OH2)(s)
Reactions of Ionic metal
hydrides
3.
Ionic hydrides are powerful reducing agents and
good hydrogen-transfer agents
NaH + B(OCH3)3
4NaH + TiCl4
Na[HB(OCH3)3]
Ti+ 4NaCl +2H2
Covalent hydrides
• Covalent hydrides:
 Neutral binary XH4 compounds of Group 14, like
methane
 Slightly basic binary XH3 compounds of Group 15,
NH3 and PH3
 Weakly acidic or amphoteric, binary XH2 of Group
16, H2O and H2S
 Strongly acidic binary HX compounds of Group 17,
HCl and HI
Covalent hydrides
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Covalent hydrides of boron
Hydridic complex compounds of hydrogen.
Examples are LiAlH4 and NaBH4.
Notes:
Ionic in nature
But possess tetrahedral anions containing covalent
bonds to H
Powerful reducing agents
Classification of Molecular halides
Molecular hydrides are further classified according to
the relative numbers of electrons and bonds in their
Lewis structure into :
(i) Electron-deficient,
(ii) Electron-precise and
(iii) Electron-rich hydrides.
Electron-deficient Covalent Hydrides
In fact all elements of group 13 will form electrondeficient compounds.
Electron-precise Covalent Hydrides
 Electron-precise compounds have the required
number of electrons to write their conventional
Lewis structures.
 All elements of group 14 form such compounds
Electron-rich hydrides
 Electron-rich hydrides have excess electrons which
are present as lone pairs.
 Elements of group 15-17 form such compounds.
(NH3 has 1- lone pair, H2O – 2 and HF –3 lone pairs).
Illustration: Would you expect the hydrides of N, O
and F to have lower boiling points than the hydrides
of their subsequent group members? Give reasons.
Solution
On the basis of molecular masses of NH3, H2O and HF, their
boiling points are expected to be lower than those of the
subsequent group member hydrides. However, due to higher
electronegativity of N, O and F, the magnitude of hydrogen
bonding in their hydrides will be quite appreciable. Hence,
the boiling points NH3, H2O and HF will be higher than the
hydrides of their subsequent group members.
Metallic Hydrides
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Form with many transition metals
Nonstoichiometric
M + x/2 H2  MHx
M = Ti, U, Pr, Pd, Pt, etc.
Stoichiometric formulas possible:
TiH2, UH3, PrH2