Reactivity with Hydrogen
The Beryllium anomaly, Banana Bonds, and Hydrolith.
The alkaline earth metals combine with hydrogen to form metal hydrides ($MH_2$). However, the first element of the group, Beryllium, presents a massive kinetic and thermodynamic barrier, making its hydride impossible to prepare by direct combination.
1. The General Reaction
Except for Beryllium, all other alkaline earth metals ($M = Mg, Ca, Sr, Ba$) combine directly with hydrogen gas upon heating to form their respective hydrides.
- Nature of Hydrides: While the hydrides of Ca, Sr, and Ba are typically ionic (saline) hydrides containing the $H^-$ ion, Magnesium hydride ($MgH_2$) possesses significant covalent character.
- Thermal Stability: The thermal stability of these hydrides decreases down the group as the size of the metal cation increases, making the lattice less stable with the small hydride ion. ($MgH_2 \gt CaH_2 \gt SrH_2 \gt BaH_2$).
2. The Beryllium Exception (Preparation of $BeH_2$)
Beryllium does NOT react directly with hydrogen, even at very high temperatures. Therefore, Beryllium Hydride ($BeH_2$) must be prepared indirectly through a chemical reduction process.
$BeH_2$ is prepared by reacting Beryllium Chloride ($BeCl_2$) with a powerful reducing agent, usually Lithium Aluminum Hydride ($LiAlH_4$). This is a very high-yield reaction for competitive exams.
3. Structure of Beryllium Hydride ($BeH_2$)
$BeH_2$ is highly electron-deficient. The monomer $BeH_2$ only has 4 valence electrons around the central Be atom, far short of an octet. To achieve stability in the solid state, it forms a polymeric chain network.
Unlike Chlorine in $BeCl_2$, Hydrogen has no lone pairs to donate to form a coordinate bond. Instead, the polymer is held together by hydrogen bridges. Each $Be-H-Be$ bridge consists of exactly 3 atoms sharing only 2 electrons. These are formally called multi-center electron-deficient bonds, or colloquially, "banana bonds".
(Note: In this solid polymer state, Beryllium is $sp^3$ hybridized).
4. Calcium Hydride ($CaH_2$) - Hydrolith
Calcium hydride ($CaH_2$) is a highly ionic, saline hydride. It is commercially known as Hydrolith.
Hydrolith reacts violently and quantitatively with water to liberate Hydrogen gas. Because of its intense affinity for water, it is widely used in laboratories as an excellent desiccant (drying agent) for organic solvents and gases.
Historically, Hydrolith was used as a portable, solid source of hydrogen gas for filling weather balloons.
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