Why is MgH2 covalent

content1 Introduction 2. hydrogen3. Noble gases4. Halogens5. Chalcogens6. Pentele7. Tetrele8. Boron

More details about the individual hydrogen compounds are given for the individual elements that are dealt with in the lecture Chemistry of Metals and Chemistry of Non-metals. At this point it is only about an overview and the basic grouping of the hydrogen compounds. Depending on which binding partner is present, a distinction is made between three different types of hydrides, the salty, the metallic and the covalent Hydrides. The following figure shows the hydrogen compounds of the individual elements in the periodic table:

Fig. 2.2.1. Hydrogen compounds of the elements in the periodic table‣SVG
About the individual groups:
  1. Salt hydrides are formed with the electropositive alkali and alkaline earth metals. These are typically ionic compounds (pattern: CaH2, LiH) with simple structures of the ion crystals. Since the hydride ion H- about the ionic radius of iodide I.- has (but is significantly more variable than this), one observes the corresponding simple structures with the corresponding compositions:
    • The alkali metal hydride AI.H crystallize in the NaCl structure.
    • The alkaline earth metal hydrides AIIH2 of calcium, strontium and barium form the simple fluorite type CaF at higher temperatures2, at room temperature they crystallize in complex structures. MgH2 however, crystallizes in the rutile type. (see also chapter 3.6. of the lecture Chemistry of Metals).
    Fig. 2.2.1. LiHFig. 2.2.2. CaH2Fig. 2.2.3. Ti with TiH
    The ionic hydrides react with water to form elemental hydrogen. They find little practical use because they are insoluble in practically all solvents.
  2. Metallic hydrides: Many transition metals (apart from the elements in the so-called hydride gap and the elements of the Zn and Cu groups, which tend to form covalent hydrogen bonds) form 'hydrides' with sometimes considerable phase widths (non-stoichiometric compounds) The structures consist of simple metal packings (mostly with the closest packing of spheres ) in which all octahedral and tetrahedral gaps can be filled, so that a maximum composition of MH3 is possible. A number of ternary alloys are used to store hydrogen, as they can be reversibly charged with hydrogen (see Fig. 2.2.3. TiH). In most cases, the metallic properties are retained when hydrogen is stored, but the brittleness is often significantly increased (more precisely: also with metals)
  3. Covalent hydrides arise with the typical non-metals, i.e. with the main group elements from III. Group in an oblique course through the periodic table. The further classification of covalent hydrides is based on the polarity of the E-H bond:
    • Typical compounds with a partial positive charge on hydrogen are e.g. HF, HCl H2O etc.
      • These compounds are acids:
        HCl + H2O ---> H3O+ + Cl-
        (H+ are transferred, acidic effect)
      • They are still oxidizing agents
        2 HCl + Zn ---> Zn2+ + 2 cl- + H2
        (Zinc is oxidized, H.+ reduced to elemental hydrogen).
    • Compounds without pronounced polarity of the E-H bond (e.g. CH4, H2) are neutral and generally not redox-active.
    • A compound with a negatively polarized hydrogen atom is e.g. BH3.
      • These compounds are Brönsted bases when there are lone pairs of electrons:
        PH3 + H+ ---> 2 PH4+
        (PH3 absorbs protons, i.e. acts as a base)
      • They are also reducing agents:
        SiH4 + O2 ---> SiO2 + 2 H.2O
        (O2 will be reduced)
In addition to the simple hydrides, there are many interesting, more complex compounds with E-E bonds, e.g. in the systems P-H, B-H, but also already in the case of O-H. These connections are discussed below for the respective elements. Another special feature of hydrogen is the formation of so-called hydrogen bonds. These are considered for fluorine and oxygen.
Pictures leftovers: content1 Introduction 2. hydrogen3. Noble gases4. Halogens5. Chalcogens6. Pentele7. Tetrele8. Boron