Hydrides ionicity

D.21 E = Si silicon tetrahydride sodium silicide D.23 (a) lithium aluminum hydride, ionic ... 

A first group of hydrides (ionic hydrides) is formed with the more electropositive elements of the 5-block of the Periodic Table. This group of hydrides includes the salt-like MeH (Me+H ) NaCl-type compounds of the alkali metals and the di-hydrides (Co2Si-type) formed by the divalent metals Ca, Sr, Ba and also by Eu and Yb. The thermal stability of these hydrides decreases from Li to Cs and from Ca to Ba the chemical reactivity on the contrary increases from Li to Cs and from Ca to Ba. While the reaction of NaH with water is very violent, the reaction of LiH or CaH2 can be used for a portable source of hydrogen. 

Elements with very low electronegativeties of approximately 1.0 see Electronegativity), form compounds in which the hydrogen appears as an anion, H. These compounds have many of the properties associated with ionic substances and are sometimes called ionic or saline hydrides. Ionic hydrides, formed with alkali (M+H ) and alkaline earth (M +H2 ) metals, are colorless crystalline solids that either melt or decompose at temperatures above 600 °C. All of the hydrides can be formed by direct combination of the elements at elevated temperatures ... 

In a similar manner, the hard BFj molecule will prefer to bond to another fluoride ion, but the soft BH, acid will prefer a softer hydride ionics... 

There are basically three types of hydrides ionic, covalent and transitional metal-interstitial hydrides. 

LiH (lithium hydride) ionic compound BeH2 (beryllium hydride) covalent compound B2H6 (diborane, you aren t expected to know that name) molecular compound CH4 (methane, do you know that one ) molecular compound NH3 (ammonia, you should know that one) molecular compound H2O (water, if you didn t know that one, you should be ashamed) molecular compound HF (hydrogen fluoride) molecular compound. LiH and BeH2 are solids, B2H6, CH4, NH3, and HF are gases, and H2O is a liquid. 

When hydrogen combines with another element it forms a binary hydride—that is, a compound that contains hydrogen and one other element. There are three categories of binary hydrides ionic hydrides, covalent hydrides, and metallic hydrides. 

Hydrides. Ionic hydrides, such as CaH2, give a basic reaction in water covalent hydrides like HCl are acidic in water. Does it follow that NHs, which is basic in water, is more like an ionic hydride than is CH, which gives no basic reaction Explain. 

Since, generally, any base stronger than OH will react with water to produce OH we must use another solvent to observe very strong bases. The high base strengths of the hydride ion and the oxide ion can best be observed in molten salts as solvents, since hydrides and ionic oxides are either insoluble in ordinary solvents or attack them. 

These give ionic or salt-like hydrides, for example 2Na + H2 2NaH... 

These solid ionic hydrides (having an ionic lattice and containing the hydride ion H ) react with water, for example... 

When the molten ionic hydrides are electrolysed, all yield hydrogen at the anode, the metal at the cathode. 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

Calcium hydride is highly ionic and is insoluble in all common inert solvents. It can be handled in dry air at low temperatures without difficulty. When heated to about 500°C, it reacts with air to form both calcium oxide and nitride. Calcium hydride reacts vigorously with water in either Hquid or vapor states at room temperature. The reaction with water provides 1.06 Hters of hydrogen per gram CaH2. 

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

Hydrogen reacts direcdy with a number of metallic elements to form hydrides (qv). The ionic or saline hydrides ate formed from the reaction of hydrogen with the alkali metals and with some of the alkaline-eartb metals. The saline hydrides ate salt-like in character and contain the hydride, ie,, ion. Saline hydrides form when pure metals and H2 react at elevated temperatures (300—700°C). Examples of these reactions ate... 

Although the alkylation of paraffins can be carried out thermally (3), catalytic alkylation is the basis of all processes in commercial use. Early studies of catalytic alkylation led to the formulation of a proposed mechanism based on a chain of ionic reactions (4—6). The reaction steps include the formation of a light tertiary cation, the addition of the cation to an olefin to form a heavier cation, and the production of a heavier paraffin (alkylate) by a hydride transfer from a light isoparaffin. This last step generates another light tertiary cation to continue the chain. 

The hybridic nature of the Si—H bond is utili2ed to generate C—H bonds by ionic hydrogenation according to the foUowiag general mechanism, ia which a hydride is transferred to a carbocation. 

The so-called ionic method for hydrogenating thiophenes (78T1703) is a further illustration of the chemical consequences of protonation. Protonation of the thiophene ring renders the ring susceptible to hydride ion attack, conveniently derived from triethylsilane (Scheme 12). 

KURSANOV PARNES Ionic Hydrogenation A non-calalytK hydrogenation of C C. C O, C N bonds and hydrogenotysis of C-OH, C Hal etc, under the action of an acid and a hydride ion donor... 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

---