BeH2 is a covalent polymer, magnesium hydride is partially ionic and the hydrides of calcium, strontium, and barium are very ionci and hydridic in their properties.[]
Ion Sizes
Electronegativities
As you go down the alkaline earth metals, the hydrides go from forming covalent molecules to forming very ionic species at the bottom of the group due to size and electronegativity differences.
Electronegativity decreases as you go down the group, while size increases as you go down the group, as shown in the figures to the left. The higher electronegativity of beryllium (1.6) is closest to the electronegativity of hydrogen (2.2), allowing the metal hydride species more capable of sharing the electron wave function evenly over the molecule. Also due to the uniqueness principle, which says that the 2nd row elements are very small and highly charged in relation to the 3rd row elements, beryllium is able to accomidate the small hydride species. This ability to share and accomidate forms multiple 3 center 2 electron bonds, creating a polymeric species as shown below.
While the lower electronegativities of magnesium (1.3), calcium (1.3), strontium (1.0) and barium (0.9) cause the metal hydride species to ionize due to the higher electronegativity difference between the two species. This characteristic of these metal hydrides also allows them to form ionic lattices. Also simply due to the size difference between these lower alkaline earth metals and hydrogen, s orbital overlap is greatly reduced. This size differences causes instability within the species, and allows the hydrogen to dissociate as an ionic hydride species, H-. This ability to ionize hydrogen is used reduce many species in organic and inorganic synthesis’.
CaH2 + H2O 'à' H2 + Ca(OH)2
(CaH2 ionizes to Ca2+ and 2H =, H = pulls a proton off of H2O forming hydrogen gas
'SrH2 + H2O 'à H2 + Sr(OH)2
Sources[]
http://www.meta-synthesis.com/webbook/35_pt/radius_data.jpg