Binary compounds of hydrogen

Binary compounds of hydrogen are binary chemical compounds containing hydrogen and another chemical element. By convention all binary hydrogen compounds are called hydrides even when the hydrogen atom in it is not an anion.^[1][2][3][4] The hydrides can be grouped into several clusters.

Binary hydrogen compounds in group 1 and group 2 are the ionic hydrides (also saline hydrides) with exception of beryllium hydride and magnesium hydride which have intermediate properties between ionic and covalent. Beryllium hydride is electron-deficient and polymeric with bridging hydrogen atoms. Group 1 and 2 hydrides are high melting solids that react violently with water.

Elements in group 3, group 4, chromium in group 5, the Lanthanoids and the Actinoids form metallic hydrides, characterised by their metallic luster and hardness, their ability to conduct electricity and their magnetic properties. They are also less dense that the metal itself. Metallic hydrides form by reaction of the metal with hydrogen at elevated temperatures. They can be thought of as a solid solution with hydrogen as interstitial element or as an interstitial hydride. Many metallic hydrides are non-stochiometric. Examples are TiH_1.7, NbH_x (0 < x < 1), LaH_2.87 and YbH_2.55. Exceptions are stochiometric compounds of uranium (trivalent) UH₃ , europium (divalent) EuH₂, plutonium PuH₂ and americium AmH₂.

The affinity for hydrogen for the other d-block elements is low. Therefore elements in this block do not form hydrides (the hydride gap) with the notable exception of palladium. Palladium can absorb up to 900 times its own volume of hydrogen and is therefore actively researched in the field hydrogen storage. In other oxidation states d-block elements again form a wide range of transition metal hydrides for example the rhenium ion in potassium nonahydridorhenate.

Another cluster of elements at the end of the d-block,^[5] copper, zinc, cadmium and indium again form intermediate or borderline hydrides and elements in group 13 to 17 (p-block) form covalent hydrides (or nonmetal hydrides). In group 12 zinc hydride is a common chemical reagent but cadmium hydride and mercury hydride are very unstable and esoteric. In group 13 boron hydrides exist as a highly reactive monomer BH₃, as an adduct for example ammonia borane or as dimeric diborane and as a whole group of BH cluster compounds. Alane (AlH₃) is a polymer. Gallium exists as the dimer digallane. Indium hydride is only known from gas-phase studies or in matrix isolation and the hydride of thallium is unknown.

In group 14 the total number of possible binary saturated compounds with carbon of the type C_nH_2n+2 is very large. Going down the group the number of binary silicon compounds (silanes) is small (straight or branched but rarely cyclic) for example disilane and trisilane. For germanium only 5 linear chain binary compounds are known as gases or volatile liquids. Examples are n-pentagermane, isopentagermane and neopentagermane. Of tin only the distannane is known. Plumbane is an unstable gas.

The periodic table of the binary hydrides

The relative stability of binary hydrogen compounds can be inferred from their standard enthalpy of formation values.^[6]

References