Compounds ionic carbides

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

Ion exclusion chromatography, of ascorbic acid, 25 760 Ion hopping, 14 469 Ionic aggregates, 14 463—466 Ionically conducting polymers, 13 540 Ionic carbides, 4 647 Ionic compounds, rubidium, 21 822 Ionic conduction, ceramics, 5 587-589 Ionic crystals, 19 185. See also Silver halide crystals... 

Group 2 elements typically form ionic carbides of formnla MC2. When BeO is heated with C at 1900-2000 °C, a brick red colored carbide of formula Bo2C results. This ionic compound adopts the antifluorite structure, that is, like the Cap2 structure except that the positions of the cations and Anions are interchanged. It is unusual because it reacts with water, forming methane, and is thus called a methanide (equation 9). 

The C NMR spectra of the ionic carbides CaC2 and BaC2 have been determined by Wrackmeyer et al. (1990) who report 8iso values of 206.2 ppm for CaC2 and 232.1 ppm for BaC2, both shifts with respect to TMS. The NMR results indicate that the environment of the carbon atoms in these compounds is not axially symmetric, and that the carbide unit in these ionic carbides is not comparable with the carbon-carbon triple bond in alkynes. 

Carbon combines with metals to form ionic compounds called carbides, such as CaC2 and Be2C, in which carbon is in the form of C2 or ions. These ions are strong Brpnsted bases and react with water as follows ... 

The alkali metals form only ionic carbides, mostly simple ionic salts of acetylene, M2C2, which liberate acetylene on reaction with moisture. There has been much recent interest in permetalated and hypermetalated hydrocarbon species, or methanides . Most studied in this respect has been lithium, presumably because of its volatility and amenability to calculation. Mass spectrometric and calculational evidence has been presented for CLie, CLis, and C2Li4, but real samples of CLi4, C3Li4, and C5Li4 are preparable. All are pyrophoric powders. The heavier metals form another class of carbide , the graphite intercalation compounds, but as the electron has not been totally freed from the metal, these were considered in the previous section. 

Ionic carbides, which contain the carbide ion C" . An example is aluminum carbide, AI4C3. Compounds of this type react with water to give methane (they were formerly also called methanides). The dicarbides are ionic carbon compounds that contain the dicarbide ion C C". The best-known example is calcium dicarbide, CaC2, also known as calcium carbide, or simply carbide. Compounds of this type give ethyne with water. They were formerly called acetylides or ethynides. Ionic carbides are formed with very electropositive metals. They are crystalline. 

The binary compounds of carbon with metals, metalloids, and certain nonmetals are called carbides. The more active metals form ionic carbides, and the most common of these contain the acetylide ion (C2 ). This ion is isoelectronic with N2, and its Lewis structure, [ C=C ], has a carbon-carbon triple bond. The most important ionic carbide is calcium carbide (CaC2), produced by the reduction of CaO with carbon at high temperature ... 

Interstitial carbides, which are INTERSTITIAL COMPOUNDS of carbon with transition metals. Titanium carbide (TiC) is an example. These compounds are all hard solids with high melting points and metallic properties. Some carbides (e.g. nickel carbide M3C) have properties intermediate between those of interstitial and ionic carbides. 

Ionic Carbides Compounds composed of carbon and a low-electronegativity metal such as an alkali metal or an alkaline earth metal are ionic carbides. Most ionic carbides contain the dicarbide ion, C2, cotmnonly called the acetylide ion. For example, calcium carbide has the formula CaC2 and a structure similar to that of NaCl (Figure 22.18 ). Calcium carbide forms by the reaction of calcium oxide with coke in an electric furnace. 

Important types of inorganic carbon compounds include carbides and carbonates. Carbon can form carbides with metalhc, covalent, or ionic properties. 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

Using the periodic table as a guide, predict which of the following compounds form ionic solutions in water silicon carbide, SiC magnesium bromide, MgBr2 carbon tetrabromide, CBr chromic chloride, CrCl3. 

The four rather distinct forms of chemical bonding between atoms are metallic, ionic, covalent, and dispersive (Van der Waals). All of them are sub-topics of quantum electrodynamics. That is, they are all mediated by electronic and electromagnetic forces. There are also mixed cases, as in carbides and other compounds, where both metallic and covalent bonding occur. 

Compounds containing carbon in a negative oxidation state are properly called carbides, and many such compounds are known. In a manner analogous to the behavior of hydrogen and boron, carbon forms three types of binary compounds, which are usually called ionic, covalent, and interstitial... 

However, these compounds, or, more properly, those species with the same Mg C ratios and resulting stoichiometries are not fanciful. They are two of the best known magnesium carbides and more often written in an ionic dialect, as Mg ( 2) and (Mg +) i.e. they are the magnesium salts of totally deprotonated acetylene and... 

When discussing metal alloys (Section 4.3), we saw that atoms of non-metallic elements such as H, B, C, and N can be inserted into the interstices (tetrahedral and octahedral holes) of a lattice of metal atoms to form metal-like compounds that are usually nonstoichiometric and have considerable technological importance. These interstitial compounds are commonly referred to as metal hydrides, borides, carbides, or nitrides, but the implication that they contain the anions H, B3, C4, or N3- is misleading. To clarify this point, we consider first the properties of truly ionic hydrides, carbides, and nitrides. 

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