Hydrides

Hydrides

  

Ionic or Saline Hydrides : NaH, CaH 2 (ionic) Covalent: e.g., H 2 O, B 2 H 6 , ReH 9 2 Metallic or non-stoichiometric

hydrides: PdH

n , UH 3

* figures are electronegativity

Ionic or Saline Hydrides

 2 M + H 2  2 MH (exothermic) only LiH can be

melted w/o decomposition

 Ternary compounds possible e.g., CaH 2 + CaCl 2  2 CaHCl Colors deepen as polarizability of anions & cations increases: BaHI is black  Saline hydrides react violently with water producing dihydrogen gas.

NaH (s) + H 2 O (aq) → NaOH (aq) + H 2 (g)

Hydride Ion

Ionic Radii:



H

-

1.30 Å (in LiH) ~ 1.54 Å (in CsH) F

-

1.33 Å 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 300-700 o C 2 M (l) + H 2(g) M (l) + H 2(g) 2MH MH (s) 2(s) The rates of these reactions are impure. Li> Cs> K> Na All produce pure white solids that appear grey when

Ionic hydrides: Preparation and Structure

  

Crystal Structure Knowing that the ionic radius of H is between that of Cl and F What 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?

MgH 2 adopts the rutile structure CaH 2 SrH 2 BaH 2 All take on a PbCl 2 -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 o C.



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) + H 2 O H 2(g) + MOH (s) MH 2(s) + H 2 O H 2(g) + M(OH 2 ) (s)

3.

Reactions of Ionic metal hydrides

Ionic hydrides are powerful reducing agents and good hydrogen-transfer agents NaH + B(OCH 3 ) 3 Na[HB(OCH 3 ) 3 ] 4NaH + TiCl 4 Ti+ 4NaCl +2H 2

Covalent hydrides

• Covalent hydrides:    

Neutral binary XH 4 compounds of Group 14, like methane Slightly basic binary XH 3 compounds of Group 15, NH 3 and PH 3 Weakly acidic or amphoteric, binary XH 16, H 2 O and H 2 S 2 of Group Strongly acidic binary HX compounds of Group 17, HCl and HI

      

Covalent hydrides

Covalent hydrides of boron Hydridic complex compounds of hydrogen. Examples are LiAlH 4 Notes: and NaBH Powerful reducing agents 4 . Ionic in nature But possess tetrahedral anions containing covalent bonds to H

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 electron deficient 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. (NH 3 has 1- lone pair, H 2 O – 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 NH 3 , H 2 O 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 NH 3 , H 2 O and HF will be higher than the hydrides of their subsequent group members.

Metallic Hydrides

   Form with many transition metals Nonstoichiometric M + x/2 H 2  MH x M = Ti, U, Pr, Pd, Pt, etc. Stoichiometric formulas possible: TiH 2 , UH 3 , PrH 2