Hydrides

Hydrides Ionic or Saline Hydrides : NaH, CaH2 (ionic) Covalent: e.g., H2O, B2H6, ReH92- Metallic 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 Ternary compounds possible e.g., CaH2 + CaCl2  2 CaHCl Colors deepen as polarizability of anions & cations increases: BaHI is black Saline hydrides react violently with water producing dihydrogen gas. NaH (s) + H2O (aq) → NaOH (aq) + H2(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 300-700oC 2 M(l) + H2(g) 2MH(s) M(l) + H2(g) 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 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? MgH2 adopts the rutile structure CaH2 SrH2 BaH2 All take on a PbCl2-like distorted hcp array.

Reactions of Ionic metal hydrides 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. 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 Ionic hydrides are powerful reducing agents and good hydrogen-transfer agents NaH + B(OCH3)3 Na[HB(OCH3)3] 4NaH + TiCl4 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 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 : Electron-deficient, 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. (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 Form with many transition metals Nonstoichiometric M + x/2 H2  MHx M = Ti, U, Pr, Pd, Pt, etc. Stoichiometric formulas possible: TiH2, UH3, PrH2