As hydrides compounds denotes the hydrogen with other elements. These can be divided into four types:
- Covalent hydrides
- Salt hydrides
- Metallic hydrides
- Complex transition metal hydrides
A sharp distinction between these types of bond is not possible, since the bond relationships do not change abruptly, but constantly. Textbooks therefore draw different dividing lines.
These are hydrogen compounds with the semimetals and non-metals of the 3rd – 7th centuries. Main group . Under normal conditions these are mostly gases or liquids. The most important group of these compounds are the hydrocarbons . The polarity of the bond here depends on the bond partners, so there are
- Compounds with positive hydrogen, in which the hydrogen has the oxidation number +1. Examples are water (H 2 O), ammonia (NH 3 ) or hydrogen chloride (HCl). The hydrogen here has an acid function and acts as an oxidizing agent .
- Compounds with negative hydrogen, in which the hydrogen has the oxidation number −1. In covalent connections, these are to be viewed as special cases. Examples are silanes or boranes . Here the hydrogen has a basic function and acts as a reducing agent .
- Compounds that contain a weakly polar hydrogen bond. The carbon-hydrogen bonds of the hydrocarbons , which are regarded as non-polar , are important here. Here, too, the hydrogen has the oxidation number +1.
Salt-like hydrides are ionic compounds that contain the hydride ion H - . Strongly electropositive metals of the 1st and 2nd main group are involved here , with the exception of beryllium. They crystallize in an ion lattice . Examples: sodium hydride (NaH) or calcium hydride (CaH 2 ). With water or acids there is an extremely violent evolution of hydrogen. The reducing effect of the hydride ion is only pronounced at high temperatures, otherwise it acts as a very strong base.
Properties of the hydride ion
The hydride ion is considered to be highly polarizable. Therefore, the hydride ions in solids are not spherical but deformed. If you still calculate an effective radius for the hydride ion, the value obtained depends even more than that of other ions on the environment. The Pauling H - radius for the free ion is 208 pm. A radius of 153 pm has been suggested for crystalline hydrides such as KH and CsH with less polarizing cations, while the value in LiH should be around 135 pm.
They also have a salt-like structure, but in contrast to the salt-like hydrides, they contain no free hydride ions, but hydrogen covalently bonded to a metal or semimetal. The best known and most important are lithium aluminum hydride (LiAlH 4 , lithium tetrahydridoaluminate, lithium alanate ) and sodium borohydride (NaBH 4 , sodium tetrahydridoborate, sodium borate). They are used as strong reducing agents in chemical synthesis because they can transfer the hydrogen as a hydride ion to suitable substrates. While lithium aluminum hydride reacts explosively with water, sodium borohydride can be used in aqueous solution (and in other protic solvents such as alcohols).
Complex transition metal hydrides
The complex transition metal hydrides are ternary compounds consisting of hydride ions, a transition metal (M) and an electropositive metal (A). They have the general form A x M y H z . There are a large number of these compounds, but they are generally divided into alkaline earth transition metal hydrides and alkali transition metal hydrides. They crystallize in a crystal lattice that contains complex anions . Example: Mg 2 NiH 4 .
In these the hydrogen is stored in the metal lattice of transition metals . Usually these are not composed stoichiometrically . The storage changes the structure and the electronic properties of the metal grid. Examples of stoichiometric structures:
- Crystallize in the NiAs structure : MnH, CrH
- Crystallize in the fluorite structure : TiH 2 , VH 2 , CrH 2 and CeH 2
- Entry on hydrides. In: Römpp Online . Georg Thieme Verlag, accessed on April 19, 2014.
- W. Bronger: The space chemistry of hydrogen in metal hydrides in comparison with corresponding fluorides and chlorides . In: Journal of Inorganic and General Chemistry . tape 622 , no. 1 , 1996, ISSN 1521-3749 , p. 9-16 , doi : 10.1002 / zaac.19966220103 .
- R. D. Shannon: Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides . In: Acta Crystallographica Section A . tape 32 , no. 5 , September 1, 1976, ISSN 0567-7394 , p. 751-767 , doi : 10.1107 / S0567739476001551 .
- DFC Morris, GL Reed: Pauling crystal radius of the hydride ion . In: Journal of Inorganic and Nuclear Chemistry . tape 27 , no. 7 July 1965, p. 1715-1717 , doi : 10.1016 / 0022-1902 (65) 80037-9 .