Molecular hydrides and complexes derived from them

  Covalent hydrides with molecular structures are formed by the p-block elements in groups 13 to 17 with the exception of Al and Bi; BiH3 is thermally unstable, decomposing above 198 K, and little is known about PoH₂.

   Hydrides of the halogens, sulfur and nitrogen are prepared by reacting these elements with H₂ under appropriate conditions the remaining hydrides are formed by treating suitable metal salts with water, aqueous acid or NH₄Br in liquid NH₃, or by use of [BH₄]^- or [AlH₄]^-, e.g. reaction 9.34. Specific syntheses are given in later chapters.  Most molecular hydrides are volatile and have simple structures which comply with VSEPR theory. However, BH₃ although known in the gas phase, dimerizes to give B₂H₆ and GaH₃ behaves similarly.

   Anionic molecular hydrido complexes of p-block elements include tetrahedral [BH₄]^- and [AlH₄]^-. Both LiAlH₄ and NaAlH₄ slowly decompose to give Li₃AlH₆ and Na₃AlH₆, respectively, and Al. Because it is difficult to locate H atoms in the presence of heavy atoms it is common to determine structures of deuterated analogues. Both Li₃AlD₆ and Na₃AlD₆ contain isolated octahedral [AlD₆]^3-  ions. Molecular hydrido complexes are known for d-block metals from groups 7–10 (excluding Mn) and counter-ions are commonly from group 1 or 2, e.g. K₂ReH₉, Li₄ RuH₆, Na₃RhH₆, Mg₂RuH₄, Na₃OsH₇ and Ba₂PtH₆. In the solid state structures of these compounds (the determination of which typically makes use of deuterated analogues), isolated metal hydrido anions are present with cations occupying the cavities between them. The [NiH₄]^4- ion in Mg₂NiH₄ is tetrahedral. X-ray diffraction data have confirmed a square-based pyramidal structure for [CoH₅]^4- (Figure 1.1a), and [IrH₅]^4-  adopts an analogous structure. These pentahydrido complexes have been isolated as the salts Mg₂CoH₅ and M₂IrH₅ (M= Mg, Ca or Sr). Alkaline earth metal ions have also been used to stabilize salts containing octahedral [FeH₆]^4- , [RuH6]^4-  and [OsH₆]^4- (Figure 1.1b). Isolated H^- and octahedral [ReH₆]^5_ ions are present in Mg₃ReH₇. However, in the solid state, Na₃OsH₇ and Na₃RuH₇ contain pentagonal bipyramidal [OsH₇]^3- and [RuH₇]^3-  anions, respectively. The reaction of Na[ReO₄] with Na in EtOH yields Na₂ReH₉, and the K‏ and [Et₄N]‏ salts have been prepared by metathesis from Na₂ReH₉. The hydrido complex K₂TcH₉ can be made from the reaction of [TcO₄]^- and potassium in EtOH in the presence of 1,2-ethanediamine.  Neutron diffraction data for K₂[ReH₉] have confirmed a 9- coordinate Re atom in a tricapped  trigonal prismatic environment (Figure 1.1c); [TcH₉]^2-  is assumed to be similar to [ReH₉]^2-. Despite there being two H environments in [ReH₉]^2- , only one signal is observed in the solution 1H NMR spectrum indicating that the dianion is stereochemically non-rigid on the NMR spectroscopic timescale. Palladium(II) and platinum(II) form the square planar [PdH₄]^2- and [PtH₄]^{2- .} The salt K₂[PtH₄]  is made by reacting Pt with KH under H₂ (1–10 bar, 580–700 K). ‘K₃PtH₅’ also forms in this reaction, but structural data show that this contains [PtH₄]^2- and H^- ions. A high pressure of H₂ is also needed to form Li₅[Pt₂H₉] but, once formed, it is stable with respect to H₂ loss; the structure of [Pt₂H₉]^5- is shown in Figure 1.1d. The Pt(IV) complex K₂[PtH₆] results if KH and Pt sponge are heated (775 K) under 1500–1800 bar H₂; neutron diffraction confirms that deuterated [PtD₆]^2-  is octahedral. The linear [PdH₂]^2- ion is present in Na₂PdH₂ and Li₂PdH₂, and contains Pd(0). The reaction of KH with Pd sponge at 620K yields a compound of formula K₃PdH₃; neutron diffraction data show that this contains isolated H^- and linear [PdH₂]^2- ions.